Bosch D-Jetronic Fuel Injection
10/2015 R. Kwas changes/additions ongoing
last changes (or the last time the spirit moved me): Aug 2018
D-Jet in 73 1800ES
D-Jet Fuel Handling Components
Measuring Fuel Rail Pressure
Throttle Position Switch (TPS)
Volvo Fuel Injection Fault Tracing
In Progress, and not yet on-line: Bosch D-Jetronic TPS and Miss at Cruise (MaC) Investigation
The material compiled here is in addition to the comprehensive documentation and troubleshooting information in the green Volvo factory manual as well as the Volvo Fuel Injection Fault Tracing booklet.
All Volvo and D-Jetronic owners everywhere, particularly those of us who are electronically qualified and know what we're looking at, at the circuit level, owe a huge debt of gratitude to Frank Kerfoot, for the (enormous) time and effort that he undoubtedly expended reverse engineering the circuit of the Bosch Electronic Control Unit (ECU). It is published by Porsche 914 specialist Paul B. Anders who presents it, along with theory of operation and scope waveforms on his excellent page: http://members.rennlist.com/pbanders/ecu.htm ...and in checking the ECU connector pin numbers, they are the same as was used the Volvo application, so for a very large part, the info is similar and applicable. However, some variations in the system exist, including by model year.
It's dedicated enthusiasts and privateers who help us all with their efforts and who prevent the big companies from having a monopoly on the information of what is inside their boxes. It's not that we're about to take the information and make competition for them, but sometimes we need to know details for service and troubleshooting purposes!
Bosch D-Jetronic (Model 039.906.021A) Circuit (credit: F. Kerfoot):
Sheet 1. Timing Logic (TL), Pressure-Sensing Loop (PL), Over-Run Shutoff (OS), Injection Logic (IL), Switching Logic (SL), and Output Drivers (D1 & D2).
Note that output to Injectors at pins 3, 4, and 5, 6 run through 6Ohm 5Watt Resistors. These serve to limit the current applied to Injectors (See also: Dropping Resistors) and effectively drop the applied voltage to 3V. They are then tied together internally to the transistor switches, therefore, Injectors fire in pairs! 1, 3 together, and 2, 4 together. By deduction (and a careful application of Ohms Law), I therefore calculate the actual DC resistance of the Injectors to be: 1.3Ohms.
These values are good enough for most service and troubleshooting work. For those doing more detailed work, or just interested, the Injectors are really solenoids with moving armatures and complex impedance characteristics. Mr. Anders has modeled these in pSpice and presents this on his site: http://members.rennlist.com/pbanders/ecu.htm#D1-D2
Sheet 2. Throttle Position Switch (TPS), Acceleration Enrichment (AE), Fuel Pump Control (FPC), and Cylinder Temperature Compensation (CTC)
TPS: (Additional info in Reference Information, TPS below!)
The TPS tells the ECU what driver throttle inputs are! There is a lot happening in this component(!)...an Idle switch, a "Drag Switch", plus sliding contacts which are fed back to the ECU and result in "Enrichment Pulses" during acceleration. The sliding contacts on the Printed Circuit Board (PCB) conductors eventually result in wear of the PCB contacts. Eventually, this wear results in symptoms of an occasional misfire during otherwise steady RPMs (like at highway cruise). ...or maybe not! See Update below!
More modern TPS eliminate this weakness with non-contact (magnetically coupled), technology, but we D-Jet owners are kind of stuck with them...and they are No Longer Available (NLA) from Volvo/Bosch. Luckily, and since there are a significant number of vehicles out there which were fitted with the D-Jetronic System, some sharp, enterprising individuals are producing quality reproduction PCBs, allowing TPSs to be rebuilt. See below Reference Information, TPS below!!
Update: A separate detailed study of the TPS, and how it can cause the Miss at Cruise MaC symptom, particularly including interaction with the ECU Enrichment circuit, is in progress. Besides being misnamed Throttle Position Switch (it doesn't even sense Position(!)...only Idle and when Acceleration is called for...it should therefore more correctly be named: Idle and Acceleration Sensor) it is already clear from preliminary findings, that wear of the PCB conductors is much less of an issue (and may not even be the cause of MaCs) than everybody (including the author) has previously thought! [In progress and not yet on-line: Bosch D-Jetronic TPS and Miss at Cruise (MaC) Investigation ]
Sheet 3. Engine Speed Sensor (ES), Engine Speed Compensation (SC), Pulse Width Multiplier (PWM), Idle Mixture Adjustment (IM, aka Idle CO adjustment Potentiometer, after '71), and Idle-Cold Mixture Compensation (ICM)
D-Jet Wiring Interconnection Diagram as integrated into the vehicle: An excerpt of the D-Jetronic Wiring Diagram from a '73 1800ES: Components of the Fuel Injection System are labeled, and correspond to the nomenclature in circuit diagrams above. All wiring to Injection ECU is white and numbered. Other, colored wires shown below are not into ECU per se, but related Battery Power and Ignition Power, so are included here because they are necessary for function of Injection System.
Of note also is that the early D-jet had a separate Relay for powering the Cold Start Injector. This Relay was under control of the ECU. Shown below is the later arrangement, there is no ECU controlled Relay, but a Thremo-Time-Switch, located on the Cylinder Head and also sensing Coolant temperature, which determined how long Starting enrichment occurs.
D-Jet Fuel Handling Components: The observant will notice a continuous circulation of fuel through the Fuel Return Pipe.
Troubleshooting D-Jetronic issues: One of the most important parameters of the D-Jet (and any other Fuel Injection System for that matter!), is the system Fuel Pressure. When pressure is constant, as well as the (injector) orifice it is being metered through, the duration Injector is open is the remaining variable by which fuel amount can be very accurately controlled. Pressure is set and regulated by the Fuel Pressure Regulator. This component keeps the pressure in the supply rail (Fuel Rail in graphic) to all Injectors at a constant 29.8PSI even with varying fuel demand. The FPR does this with a spring controlled diaphragm routing unused fuel back to the Tank. This type of FPR is sometimes referred to as a "dump type regulator"...this is in contrast to a more typical Regulator which dead-heads the supply stream (low flow) during low demand. Bosch engineers preferred to use this type of Reg so they could have continuous flow through the fuel supply system. This allows continuous filtering as well as cooling of the "wet" fuel pump.
Because the Fuel Pressure is so important, it should be verified to be correct as one of the first items to be checked after Fuel Pump is verified to be functioning, and this is also how it is presented in the referenced Fault Tracing Manual.
Measuring Fuel Rail (Fuel Pressure Regulator output) Pressure: I have used an Oil pressure gauge...it is a bit rustic, and not particularly accurate and I would certainly not use it to adjust FPR output pressure, but it was plenty accurate enough to show fuel pressure was dropping into the cellar under demand (later confirmed to be a result of a rust-blocked in-tank pre-filter, but that's a story for another day...).
Caution: The Fuel Rail will hold pressure even after the Ign is turned OFF and Fuel Pump stops charging it (the pressure will then only slowly discharge back to zero)...the point is...when troubleshooting injection issues which require opening up the Fuel Rail to insert the Pressure Gauge, use extreme care to be ready for and catch possible squirting fuel, and observe fire preventing practices! ...and having a fire blanket and extinguisher at hand when working with fuel in the open can't hurt!
Since the Cold Start Injector is out of the picture and unused in a warmed up engine, the Fuel Rail Pressure can be measured by plumbing the CSI supply line into a pressure gauge (manometer).
Link to very good troubleshooting guide: http://volvo1800pictures.com/document/fuel_injection_fault_tracer/fuel_injection_fault_tracing.pdf
Another excellent source of general info on the Bosch Injection System from the early D-Jetronic (electronic) to K-Jetronic (mechanical - constant Injection) all the way to the later variations of the Motronic engine management with Lambda exhaust sensing to allow the system to function closed-loop, is the book: Bosch Fuel Injection & Engine Management by Charles Probst, published by Robert Bentley Publishers SB ISBN 0-8376-0300-5. The book contains theory of operation, development, and practical troubleshooting information. An excellent reference for D-Jetronic, K-Jetronic and later vehicles.
Part Numbers: Source?
Throttle Position Switch (TPS)
including working notes:
Early type TPS easily recognized by the radial angle of comb contacts, and separate drag and Idle (a) switches. If this was a picture of an actual switch, discrete wires between I/O connector and innards would be evident. There are no discrete wires in the later switches...this makes them simple to differentiate.
Translation (Credit?...looks like one of those
on-line translators...lots of gobbledygook, and tough to understand...I should
rework this sometime to improve understandability):
3.4 Throttle Valve Switch
Fig. 7 shows the throttle valve switch in the plan view with cut cover. The throttle idle micro-switch of forming contacts a by the lever c connected rigidly with the throttle valve wave with closed throttle valve are operated. The disk b is concentrically arranged to the hub of the lever c. One wave feather/spring between the disk and the housing bottom provides for a defined friction opposite the housing. When opening the throttle valve (in the picture against the clockwise direction) the disk b is held by the friction first, the lever c moves against the contact record d drags switch and closes this. After a small overflow route for the contact record dashed a drawn driver of the lever c carries the disk forward b. Thus the two slide with the disk b firmly connected pickup shoe on the contact plate e. With closed contact record d the teeth of the two edge contacts in the change are connected electrically with the continuous pickup shoe path. Switching a comb on the other hand has the advantage in relation to easy switching on and off that during uncertain contacting or bouncing the contact no additional enrichment impulses in the controller are released.
Adjustment of the TPS
The following good info is copied from Volvoniacs, and still needs translation: http://www.networksvolvoniacs.org/index.php/D-Jetronic_-_Beschreibung_und_Fehlersuche [My highlighted comments and clarifications added.]
Er sitzt an der Drosselklappe und gibt ein Leerlaufsignal (für Schubabschaltung) und Drosselklappenstellungssignale (nicht absolute Stellung, sondern nur Änderung der Stellung, und das bei weiterer Oeffnung des Drossel, bei Schliessung des Drossel sind die Kontakte des Schleppschalter geoeffnet) in über ein Potentiometer (falsch! kein Poti!) an das Steuergerät.gleichzeitig auch Drosselklappenstellungsgeber .ZV kontakt 1 (9 1/2 Impulse) .ZV kontakt 2 (10 Impulse)
Das Ohmmeter zwischen die Klemmen 14 und 17 anschließen. Der Wert soll 0 Ohm (Durchgang) betragen.
Bei "unendlich" den Drosselklappenschalter neu einstellen, sonst gibt es kein Leerlaufsignal und damit keine Schubabschaltung. Wenn trotz korrekter Einstellung kein Leerlaufsignal kommt, muss der Drosselklappenschalter erneuert werden. Bei etwas Gasgeben muss das Leerlaufsignal weg (anzeige unendlich) sein.
Achtung! CO im Leerlauf kann nur eingestellt werden, wenn das Leerlaufsignal durchgeschaltet wird.
Beim Fehlen des Leerlaufsignals kommt es auch zu erhöhtem Kraftstoffverbrauch, da die Schubabschaltung nur mit Leerlaufsignal und über 1800 U/min arbeitet.
Drosselklappenschaltersignal 1 (9 1/2 Impulse) und Drosselklappenschaltersignal 2 (10 Impulse) können nur mit der "Blauen Bosch Kiste" geprüft werden.
Der Schließwinkel der Auslösekontakte unten im Verteiler ebenfalls.
Als letztes CO einstellen.
Sollwert: 2% CO
Eingestellt wird ab Baujahr 1971 am Poti am Steuergerät, vorher (1800 E mj 1970)ist das Poti nicht vorhanden.
Das Poti ganz nach rechts und dann zwei Klicks nach links drehen, die Kontrollmessung muss jetzt 2% CO ergeben. Wenn nicht: Nach rechts = fett, nach links = mager.
B20: Die Kontermutter der konischen Schraube unter dem Lufteinlass vorne am Ansaugkrümmer unten aufkontern.
Schraube reindrehen = niedrigere Drehzahl
Schraube rausdrehen = höhere Drehzahl
Wenn die Einstellschraube bis zum Anschlag reingedreht ist und die Drehzahl trotzdem zu hoch ist, kann es folgende Ursachen haben:
Den Drosselklappenschalter lösen, die Anschlagschraube entlasten und die Drosselklappe schließen. Dann die Anschlagschraube beidrehen, bis die Drosselklappe anfängt,sich zu öffnen. Dann die Anschlagschraube eine HALBE Umdrehung reindrehen Dann den Drosselklappenschalter einstellen.
Ruckeln bei Konstantfahrt Konstantfahrruckeln liegt normalerweise am Drosselklappenschalter, alte Ausführung mit geschraubtem Deckel. Diesen dann erneuern. Es kann aber auch mit der neueren Ausführung (Deckel nur geclipst ) auftreten. In diesem Fall sind die Widerstandsbahnen [...this is poor use of terminology and could lead to misunderstanding...Widerstand means Resistance, and there is no variable Resistance here like a Potentiometer, only Contact/No-Contact areas. Ron] durch den Läufer und durch Spiel der Drosselklappenwelle verschlissen. Vor der Erneuerung des Drosselklappenschalters die Buchsen und Drosselklappenlagerung erneuern.
More Links at the Volvoniacs on the TPS: http://www.networksvolvoniacs.org/index.php/Spezial:AWCforum/st/id1928/limit:0,15/#last
D-Jetronic DKS Drosseklappenschalter: http://www.networksvolvoniacs.org/index.php/Spezial:AWCforum/st/id4878/drosselklappenschalter.html
Reproduction Parts for D-Jetronic: (links/availability not checked as of 10/2015)
Neue Stecker und Gummitüllen für D jetronic B 20 E/F Robert Bissler bei repro-parts.de
Nachproduzierte Platine Drosselklappenschalter . bei office bei v 1800.org
|Throttle Switch||039 906 111 A||0 280 120 032||2.0L||1973 - 1976||No longer available new. If you go to http://www.914world.com , and look up user "davesprinkle", he's been fabricating a kit to replace worn TPS circuit boards, so that you can rebuild your TPS.|
Replacement PCBs for TPS from:
I have not had one of these in my little hands for a critical inspection, but they look good from where I'm sitting.
Lube with Deoxit D5 at installation to prolong life and reduce wear like this (this is a representative picture showing what happens when a PCB conductor gets worn/scratched...but looking at the ratio of conductor to insulator width, it doesn't look like it's from an actual TPS):
Picture Source: http://members.rennlist.com/pbanders/TPS%20Contacts%2003.jpg
Here, although not perfectly clear,
at Red, damaged gold plating and metal displaced by continuous wiping of the
moving contact is evident. Lubrication would delay/reduce such
displacement! I'll try to get an even better picture:
Comb contact, showing evidence of wiping contact, but practically no wear.
De-Oxit D5 would go a long way to lubing and protecting this area to delay wear.
Link to D. Farrington pictures: https://drive.google.com/folderview?id=0BwOnJ74Oi_8xY2dKV1FVYnBBMXc&usp=sharing
Throttle Position Sensing Switch
In exercising the search function, detailed info on the TPS is sparse, and in the case of this publication, even highly imaginative...and incorrect, and not elementary my dear Watson! It is simply not possible for the sliding contact or alternate comb contacts to "begin to arc as the throttle is held at a steady position"...resistive voltage divider networks for T904, T905 (Throttle Flip-Flop, see sheet 2 above) limit current and voltage to a miniscule level which would never be able to produce an arc! Lack of lubrication between the board conductors and never-ending motion of the sliding contact can cause wear (this is where the correct lubrication comes in), but no arcing or carbonization ever takes place!
TPS related Excerpts from the Anders circuit analysis ( http://members.rennlist.com/pbanders/ecu.htm ) my highlights :
Acceleration Enrichment (AE) - Sheet 2
The AE circuit provides immediate and delayed enrichment when the throttle is opened for acceleration. Throttle opening is signaled by the TPS through alternating ground signals from two inter-digitated traces that are each connected to the inputs of a flip-flop (see sheet 2 of the schematic for a drawing of the switch details). A drag switch in the TPS prevents these signals from being sent when the throttle is closing. The outputs of the flip-flop are sent to two edge detectors, whose outputs are combined and sent to pulse shaping and narrowing circuits. The pulse shaper provides immediate injection pulses to the IL (Injection Logic). The width of these pulses is independent of engine speed and load, and from Kerfoot's schematic, they are about 1.5 ms in duration.
The IM sets the mixture (adjustable with IA) during idle operation. The output of the IM is combined with the output of the SC to control the voltage threshold in the PL for turn-off of T201. When the throttle is open, the IM appears as an open circuit and the SC controls the PL threshold. Note that the IM is effective ONLY when the IS on the TPS is closed!! Adjustment of the IA has no effect on mixture when the throttle is open.
Throttle valve switch full-load contact (only for VW T3's from 1972)
d: TPS Idle switch, full-load switch (if present), and immediate accelerator track switches (3-way switch to accommodate all TPS types)
Adjustment procedure for TPS: http://www.pelicanparts.com/techarticles/914_cooley/throttle_position_calibrate_17-18.jpg
Numbers from Ebay, Dutch seller: peuver
Early Type: PCB Type-17 for Bosch: 0 280 120 018 / 0 280 120 021 / 0 280 120 039 this is probably an error...039 is for the late version.
Later Type: Type-03 for Bosch: 0 280 120 007 /0 280 120 011 / 0 280 120 012 / 0 280 120 026
Questions for Anders:
What is difference between 9,20,14,17 connector on Volvo and 9,20,12/47,17,2/14 of VW and -,17,12,-,9 of Porsche when PCB is identical? Answered: 2/14 is WOT output Terminal, not used in Volvo application. 5 Terminal TPS (PN 028 120 047) can be used in Volvo application if 4 Term connector of vehicle is placed not to connect to 2/14 Term of switch. Ref: Rolling VCOA Magazine Jul-Aug 2016, Solutions for a faulty Throttle Position Switch by Bill Arey
TPS adjustment procedure calls for Throttle to be fully closed, but it would seem that Idle switch closes any time shafts turns CW. Answered: This is not the case due to TPS PCB layout, and interaction of Wiper Contacts! See detailed dwg of TPS internals.
Does he have drawing which shows the connections of WOT switch of VW versions? Answered: Terminal 2/14 OF 5 Term TPS Info added to TPS detailed drawing. How does WOT switch interface to early ECU, or does it not?
During deceleration, above 1500 RPM, throttle switch cuts fuel supply off and below 900 RPM, fuel supply is turned on.
E-mail to Anders:
I have seen your excellent work on the D-Jetronic (for years now) and have to compliment you on your dedication and work, and efforts to present it on your site!
I am involved with the Volvo clubs, and as you are undoubtedly aware, the D-Jet was used in several models from this manufacturer. I am also an EE (with heavy hands-on factor as yourself) and am currently very interested in understanding the TPS and really getting to the root-cause of its' well known "cough at cruise" symptom. I have some thoughts on this I would like to discuss with someone who I can really "talk turkey" with, and who knows the difference between a negative edge triggered Flip-Flop and a hole in the ground...so I took a chance and googled your name for Phoenix, and came up with a number which I left a message on this past Sunday...I don't know if you got the message, but out of the blue like that, I might not respond to a message from a non-Porsche guy either, but I ask again if you might be so kind to allocate a few minutes of your precious time to discuss the finer points of the TPS with me, a fellow vintage car guy and D-Jet friend, I would very much appreciate it...and you might just get something out of it yourself...with the Cough at cruise symptom being pervasive, I'm surprise at the lack of info and root-cause and corrective action info out there...
...and just plain BAD information doesn't help...I had a real laugh when I ran across this:
Having studied the switch at length (including the VW version with WOT contact), I actually don't think the comb contacts have much to do with the cough at cruise at all, because according to my initial inspection, the sliding contact which supplies the comb conductors with ground and thereby a Neg edge to trigger the Flop is smaller in OD than the insulated space between combs (comb-wear and conductor smearing notwithstanding)...and even IF an extra trigger occurred, the resulting mixture enriching is unlikely to cause a misfire...I believe it is instead the hysterisis (or lack thereof) of the drag-switch which causes the cough (I've seen "2degrees" shaft rotation) because of a momentary switching to Idle (which would result in a lean stumble). I think this is rather the root-cause (not "arcing" or "comb-conductor wear"), but again, this is what I would dearly like to discuss in depth with you. My goal is to generate the complete and definitive documentation on this issue, including tests, adjustment and maintenance recommendations. I think ALL D-Jet users, Volvo, Porsche, and VW alike could greatly benefit from such a documentation, and you are clearly the expert I would like to talk to...
Greets from Connecticut and Regards,
Working Diagram tying all TPS info together:
My detailed consideration of the function of TPS in support of an explanation of the "cough at steady speed/throttle" also referred to as a MaC symptom:
1. Diameter of the slider contact which sweeps comb conductors is smaller than the spacing between comb conductive teeth...this means that slider can never make contact across the two combs, or even that it makes contact with either comb at any one time. Sliding contact can actually be between the two combs also, not making contact with either! This is also consistent with the negative edge triggered Flip-Flop inputs the two combs are connected to at the ECU, where the two inputs would be triggered alternately in operation (Note 1). Therefore, three contact conditions can exist: Contact with one comb (term 9), OR contact with the other comb(term 20), OR between the two, and not making contact with either comb(open). Resulting circuit function: Flip-Flop T904,T905 remains un-triggered, and in a stable condition, holding last state.
Note 1. As the Flip-Flop is an edge triggered circuit, it changes state at the first valid comb contact input, locking out ANY subsequent contacts (and in this manner exhibits high immunity to repeated or false triggers) , also, the only time Flip-Flop changes state is during CCW (throttle opening) rotation because Comb Selector Contact of Drag Switch is only closed during CCW rotation! In this way TPS is direction sensitive.
2. Drag Switch (internal to TPS), provides ground connection to Comb Slider when shaft is turned CCW (only).
3. Idle Switch (grounds terminal 17, when Throttle shaft is closed turned CW).
Note 2. Since Dragswitch powering comb slider is in essentially a SPDT configuration with Idle Switch, providing ground to either is exclusive of the other. In other words, only one of the two circuits is enabled, at any time, as a function of Throttle Shaft direction. Also there is a deadzone or Hysterisis between the connection of the two directions. When a direction change occurs, the contacts of the SPDT do not change instantaneously, because in the first place it is not a snap-action mechanism, and secondly the shaft moving them is turning slowly.
I therefore hypothesize that it is not the wearing of the PC board in the comb area which is responsible for the "cough" symptom, but normal ECU action in the deadzone during a (slow) switchover which is responsible. It might be as simple as the idle contact being engaged at cruise through failed mechanical hysterisis (cutting off fuel) then the idle contact opened again (Update: This is not possible!), the engine reaction might be a momentary lean stumble. My research will concentrate on trying to substantiating this, as well as supporting the use of Deoxit D5 as a lubricant and preserver of the internal contacts.
Link to FI troubleshooting manual: http://volvo1800pictures.com/document/fuel_injection_fault_tracer/fuel_injection_fault_tracing.pdf
Links and Thread Excerpts [Yellows are addition Comments/Corrections which were not part of the original posting]:
[From www.Brickboard.com]... in response to: "'73 p1800 es throttle position switch 1800" posted by posted by "flaps" on Wed Feb 25 18:18 UTC 2004
On the earlier TPSs, the plastic cap just snapped on, and just as easily snaps off to reveal the innards...and IF you have on of those, go ahead and remove it from the throttle housing (make witness marks to help getting it back in the same place on reassembly), then carefully pop the top off, blast clean with spray solvent, and inspect:
1. PC board foils (especially the area where the slider sits almost forever at cruise throttle openings) if edges of conductors show excessive wear at the edeges, that's the classic cause of hiccups at cruise. If worn away, there's not much to repair for even the most ambitious Brickboarder...replacement is about the only option...that's why I've considered making an optical (ZERO contact) replacement for those damned switches... (cost?...but it's got to be better than those damned things... who's interested? If it doesn't look too bad, lubricate liberally with Deoxid D5 *, pop cap back on and reassemble, using witness marks as a guide (lubing the shaft couldn't hurt).
* see also: http://www.intelab.com/swem/gastight.htm
2. Mechanical switch contacts of idle sensing switch.
I have no experience with late, replacement TPS, which I understand it is not possible to open (maintenance free?...BS!...more like unmaintainable!...just like the car batteries)...somebody needs to be slapped for that!).
[From www.Brickboard.com]... posted by John Mc on Fri Nov 28 20:30 UTC 2003 [used with permission, because I couldn't have explained it better myself!]. Link to Thread throttle 1800 [E]: http://www.brickboard.com/RWD/index.htm?id=714766
PS - those contacts in the throttle switch. The
computer doesn't really need to know the actual position of the throttle - it
gets all the info it needs to know about the air going into the engine through
the air temp sensor and the manifold air pressure sensor. That switch does two
1) Idle circuit: To squeek by early emissions laws the computers had a special idle circuit which was individually tuned on the line with an adjuster on the box. This fine tune adjustment is ignored when the throttle is anywhere but closed. So there are a couple of contacts in the switch to indicate this.
2) 'Accelerator pump': When you suddenly open the throttle on any car the engine needs just a bit of extra gas or it will stumble slightly. To do this they have the throttle switch send some pulses to the computer - which just adds an extra injector firings for each pulse (typically injectors are fired only when the trigger contacts in the base of the distributor pulse). There are two independent injector circuits (they fire in pairs) and if you look in the switch a single contact slides up between a sort of interlocking zipper sort pattern of contacts, so as the contact slides up it first hits one side, then the other. Of course you only want this to happen when the throttle is opening, so that portion of the switch is sort of friction loaded, and when the throttle opens it rotates slightly on the shaft and closes a couple of contacts to complete the circuit, when closing those contacts are pulled open so you don't get the extra injector fires as the throttle closes. This 'electronic accelerator pump' action works even if the engine isn't running, turn the key on and if you listen very carefully you should hear the injectors clicking as you open the throttle, not when you close it. If it isn't working the engine will run just about as well as with it working, in fact you can totally unplug the throttle switch plug and barely tell the difference. I think the engine is a little slower to respond to quick throttle openings, but it's not something you'd particularly notice unless you were looking for it.
Good day everyone,
I hope someone can assist here. My 71 1800 has been having a problem. She has the original engine B20E. What is going on is, there is a HARD hesitation while driving at a steady speed/rpm. We have not been able to recreate in the shop. Seems it is only while under load (driving). It feels like a flapper or whatever in the air intake is closing off all airflow for a split second. The hole car then "bucks" hard. Has any one had this condition, and knows what to do? She runs good, and is at 248K miles. P.S. The condition goes away with a slight increase or decrease in RPMs. Only happens at steady rpm under load. We have replaced the Throttle Control Switch with a new unit. I have preformed the adjust check. I count 15 clicks than a large gap (around half to 3/4 throttle) then almost full open, I get a few more clicks. This gap makes me think it would be in the area of 50-65 mph on the road, and that's the area where the bucking is hardest. Am I doing doing something wrong in the adjustment of the switch? Could it be elsewhere? This was slightly happening before I added items like the fireball elec ignition, and replaced the injectors. So I don't think that's it, but on these system, I just don't know. I was even thinking timing chain slop?
turn the car [Ignition] on, don't start. as you SLOWLY move the linkage, the injectors should click once every time a new spot on the TPS is found. there are supposed to be 20 separate spots, but 17 or 18 will do. if you don't get a click, move the engine slightly, the trigger points may not be open. good luck, chuck.
Disclaimer: Sources of external materials are attributed. Otherwise, this page is Copyright © 2015-2018, Ronald Kwas. The terms Volvo, Porsche (a family name, and A TWO SYLLABLE WORD!) and Bosch are used for reference only. I have no affiliation with any of these companies, other than to try to keep their products working for me and reliably, and to help other owners do the same. The results and usually highly opinionated thoughts presented here are from my own experience, and should be used in conjunction with normal, careful shop practice, or can be ridiculed and laughed at, or worshipped, at your discretion. Remember, you alone are in control of your future, your knuckles! As always, if you can supply related additional objective info or experience, I’d appreciate hearing it, and will consider working it, along with the odd wise*** comment, into the next revision of this article.
You are welcome to use the information presented here in good health, and for your own noncommercial purposes, but if you reprint or otherwise republish it, you must give credit to the author or link back to the SwEm site as the source. If you don’t, you’re just a lazy, scum sucking plagiarist...and the Boston Globe wants you!