SW-EM D-Jet Injector Pulsetrain Notes

The following is an overview to help non-technical individuals better understand their D-Jet System for the purpose of keeping it operating and if necessary, light troubleshooting...it's not like we can go to the neighborhood service place and ask their technicians to help with this. They wouldn't have a service comm port to plug their computer into!

I have tried to keep this article at a somewhat high level...but the subject is a bit more complicated and involved than threading a nut onto a bolt...so a certain level of technical detail is necessary.

Note: The author will use the correct terms Throttle Valve Switch TVS and Manifold Pressure Sensor MPS from this point forth, to be consistent with original Volvo and Bosch documentation, but the reader may still find the old terms (Throttle Position Switch TPS and Manifold Absolute Pressure MAP) in some SW-EM material which has yet to be updated and corrected.

Origin and timing of the Base Injection Pulses in the D-Jet injection, are determined and generated by the Distributor Contacts Assembly (DCA). This assembly consists of two grounding contacts in the Distributor base which provide the synchronous engine operating frequency to the ECU as the single lobe they are activated by alternately closes them thereby supplying the ECU with engine synchronous Distributor Contact Pulses (DCP).

DC Pulses are produced any time engine is turning and Ignition is ON...either being turned by the starter or subsequently when engine is self-sustaining.

Distributor Contact Assembly, viewed from Distributor side,
Distributor Contacts 180Degrees apart, with their respective Rubbing Blocks.

Distributor Contacts Assembly is located in the Distributor, in a separate compartment below the Ignition Points, secured by two screws into the side of Distributor body, and contacts are activated by their own (single) lobe on the Distributor Shaft Assembly, resulting in 180 Degrees of pulse separation of the signals going to the ECU. The Distributor is driven off the Camshaft, and since the Camshaft is synchronous with the Crankshaft by way of the Timing Gears, the DC Points are therefore synchronous with the engine at all times.

Best pic I could get of the lobe on the lower distributor shaft, which acts on the Distrib Contacts.
Viewed from below, so direction of rotation would be clockwise, the edges of the lobe are
highlighted and it can be seen they are 180Deg separated.

The circuit the Distributor Contacts provide their pulses to (actually, contact closures to chassis, see circuit excerpt) is a Flip-Flop circuit not unlike that which the contact closures of the Throttle Valve Switch interface with, only simpler, since the DCs have no "special conditions" to accommodate, like the TVS has. Further, it should be noted, that the timing of the Injection pulses is nearly not as critical, as the Ignition Points (which on the other hand, must be very accurately timed!)...the subsequent "Edge Detector" and "NOR Gates", sections of the circuit, process the pulses and eventually send them to electrically power Injector Pair 1 (Cyl 1/3), and Pair 2 (Cyl 2/4)! If the energization pluses are a bit early or late, the fuel they are injecting into the intake track would still get drawn in with the the air-charge...in fact, to show how non-critical timing of the Injector activation is, since the Injectors are energized in pairs, one in each pair will actually be squirting fuel behind a closed Intake Valve(!)...but since things are happening pretty fast, it is not a big deal...those Intake Valves will be opening very shortly...!

On a design note, here as on the contacts of the TVS, an Edge Triggered Flip-Flop circuit is used for its high immunity to typical contact issues, to bring the contact signals into the D-Jet ECU...

Distributor Contacts continuously pulse when the engine is running, and edge trigger a Flip-Flop
(comprised of T251 and T252), not unlike the Flip-Flop of the Throttle Valve Switch. This makes
the circuit extremely forgiving and impervious to contact bounce and wear! Even though it is a
mechanical assembly with rubbing contact, it is not uncommon for the DC to go one hundred
thousand miles with no service beyond lubrication as maintenance!
Extract from the Kerfoot ECU Wiring Diagram.

Not to oversimplify or minimize all that is going on in the D-Jet ECU, but the signal flow from DC to Injectors is actually fairly simple as can be seen well in the circuit excerpt below...the additional complexity occurs in the other circuitry which modifies the pulse-width energizing the Injectors, to optimize it for all of the possible engine operating conditions.

Synchronous engine pulses come from the Distributor Contacts, into the Flip-Flop, get processed by Edge Detector and Nor Gate Circuits, modified by the Temperature and Manifold Vacuum Sensing, then are applied to the Injector Pairs.

Distributor Contacts, Flip-Flop, Edge Detector and NOR Gates, Drivers, Injectors circuit blocks.
Extract from the Kerfoot ECU Wiring Diagram, author's mark-ups.

The magic and real complexity happens when the Base Pulses are modified by the various environmental, engine load, operating conditions, and driver inputs which it must function under, but that's about it! Magic?...what magic?...just what I would call some thorough and thoughtful engineering!

is an output signal to the Fuel Pump control circuit, where Injection pulses are sensed to assure the activated FuPu is sending fuel only to a running engine! This circuit times out and shuts OFF Fuel Pump Relay when no DC pulses are sensed as a safety provision (Reference: https://www.sw-em.com/bosch_d-jetronic_injection.htm#Fuel_Pump_Control ).

In order to have a good Throttle response, Enrichment Pulses must be added to the regular Injection Pulses. The MPS and Temperature Senders I and II are sufficient to provide the ECU enough info with which to run the engine, but their response alone are not nearly fast enough to give a fast Throttle response, so EPs are added, much like and accelerator pump would in carbureted engines. EPs are not synchronized to the base pulses, and they can occur at any point during the injection cycle...it just depends on when the driver pushes the right pedal as seen in the following wafeform!

Since the Enrichment Pulses are added to the regular Injection Pulses, it should be recognized by the reader, that if the TVS is disconnected entirely and therefore NONE are generated and added to the regular Injector activations, the engine will still run just fine at steady state, and even slow Throttle openings, which the MPS is perfectly capable of following!

It should also be understood that EPs are only added when the driver increases the Throttle (Throttle shaft rotation CCW). When driver releases the Throttle pedal (Throttle shaft rotation CW), no Enrichment Pulses are generated, because closing of the Throttle is ignored, due to the construction and function of the Throttle Valve Switch (See Reference Info: TVS).

Thanks to Aaron Walsh, who captured this injector waveform and also provided Idle Timing Details.

Injector Pair 1 Pulsetrain only. (Injector 2 Pulsetrain would be halfway in time, between Inj P 1 pulses.)

Throttle Fully Closed (Throttle is held CW against Throttle Stop by Throttle Return Spring) - TVS (must be correctly adjusted!) Pin 17 is grounded and with Idle Adjustment Screw (only) allowing induction air to make/allow an idle RPM (of 945, in this particular case). Note: All Induction Air is controlled by Idle Adjustment Screw (none is entering via other paths!), this is why an elevated Idle not being able to be brought down to normal levels when Throttle mechanism is properly adjusted, is indicative of a Vacuum leak elsewhere.

Throttle in the Process of Opening (so Throttle shaft rotation is CCW) - TVS 9, 20 ("Comb Contacts") of the Throttle Valve Switch alternately making contact to ground, resulting in Enrichment Pulses being added to Injector Pulsetrain.

New (more open) Throttle Position (engine under higher load/higher RPMs) - A shorter time between Injection Pulse results with higher RPMs, and pulse-width still varies as a function of engine load, so for instance, if this new, higher RPM was attained in the driveway by simply opening the Throttle, or while on a rolling-road dyne at simulated speed and under load, the pulse-width would be different, having been adjusted as a function of manifold vacuum sensed by the Manifold Pressure Sensor (MPS)!

The Edge Triggered Flip-Flop in the Enrichment Circuit. Using this circuit configuration is brilliant engineering by Bosch, and why it is therefore not possible for the comb contact to be causing the "Miss at Cruise" issue!

Bosch was indeed clever in their system design!...their use of an Edge Triggered Flip Flop (T904,T905 of the Kerfoot drawings) has complete and total immunity to false triggers from the Comb Contact [also Distributor Contacts]! Looking at the Comb Contact specifically, and the fact that each side triggers one input of an edge triggered Flip-Flop, it is clear that a worn, and intermittent (or a bouncing multiple contact), CANNOT be the cause false inputs ...the author can say this with a high degree, no...absolute(!) certainty, because of the nature of an edge triggered Flop.

Since it has the ability to latch, it�s also used as a bounce eliminator or in circuits where electromechanical switches are used as inputs to logic circuits. If the contacts bounce when the switch is closed, the logic device will only see the first contact closure. - See more at: http://www.industrial-electronics.com/Advanced-Solid-State-Logic/38.html#sthash.xnoQfqqu.dpuf

Since it has the ability to latch, it�s also used as a bounce eliminator or in circuits where electromechanical switches are used as inputs to logic circuits. If the contacts bounce when the switch is closed, the logic device will only see the first contact closure - See more at: http://www.industrial-electronics.com/Advanced-Solid-State-Logic/38.html#sthash.xnoQfqqu.dpuf

A Flip-Flop, also called a Latch, is a circuit configuration often used in control circuits as a bounce eliminator where electromechanical switches are used as inputs.

When electro-mechanical contacts are closed, they typically bounce microscopically, giving multiple inputs to the circuit, which, if fast enough to respond to these (electronics certainly are!), can cause errors, if not allowed for, and accommodated...and the Flip-Flop does this perfectly! By using this circuit configuration at the input, the downstream control circuit will see only the first contact closure, ignoring any and all subsequent closures, contact bounces or intermittents caused by PCB wear, because these are effectively "locked-out" and ignored, thus control errors are prevented.

Detail of Flip-Flop Operation as it applies to the TPS: Notice on circuit of Flip-Flop, because of the cross-coupling of the two transistors, once the FF has changed state as a result of the first valid low going edge to occur, presume closing switch at A (caused for instance in the Bosch D-Jet, by the first continuous connection through Comb Slider Contact, Comb Contact, Continuous Slider, and finally Continuous Pressure Contact), it doesn't care (or react to...is therefore totally insensitive and immune to!) additional triggers on the same input...because as soon as this first trigger causes QA to turn off and to assume that stable state (and results in one single enrichment pulse due to the function of the downstream circuit in the ECU), it effectively locks out that input and renders any and all further negative edges on it as ignored (called a "Don't Care" operating condition). [Once the door is closed, it cannot be closed again!]

Therefore, the only, and next trigger which can result in a Flop state change must occur on the other Flop input (B, since even if it were to occur again at A, it would not be valid due to the ignored input condition)...and to get a valid input to the B Flop input would take a large physical shaft rotation to the next and alternate comb connection, compared to the small wear area that the moving contact is in at the time. Again...a brilliant design with superior tolerance to wear! Once the next valid triggering Edge comes at the B input, the FF changes to the other stable state, and this results in the next enrichment pulse due to the function of subsequent parts of the circuit...and so on and so forth up the 20 possible edges of the Comb Contact area.

Interesting to note is that the Distributor Contacts of the Bosch D-Jet, Trigger connect to an identical edge triggered Flip-Flop (T251,T252) as the Enrichment Flip-Flop, so this ECU input is therefore equally robust in its immunity to false triggers. This circuit understandably has no provision for a Reset as Distributor contacts are constantly pulsing when engine is running...they do not have a resting or Idle or Reset position.

Extract from the Kerfoot ECU Wiring Diagram Sheet 1, showing a Flip-Flop input circuit for the Distributor Contacts.

T552 completes circuit to ground when Ign is ON, but absence of

signal will inhibit this after time-out period.

Scope-capture of waveform from both Groups 1 and 2. Note the presence of a significant negative spike due to breaking the current in the inductive Injectors, which remains, even with an R-C Snubber Network (6.8Ohm - 6.8uF) present. The ringing energy in the spike has been controlled and reduced, but the SN does not eliminate the high negative turn-off spike. The use of RC Snubbers is typical with cyclic and recurring events of the Injectors (also Ignition Points, for instance). With non-cyclic, occasional and unpredictable turn-offs of relays, Snubber Diodes are more commonly used in preference...sometimes they are even used in an RCD combination. Snubber design and control of current in inductive circuits is an engineering branch and subject on which volumes have written, and careers have been based!

A Distributor Shaft from a late production Distributor from a D-Jet equipped car was selected from the Embassy parts horde... This assembly is a bit different and evolved from the earlier assembly considered here: https://www.sw-em.com/Volvo%20Ignition%20from%20Scratch.htm#Detailed%20CA%20Servicing It includes the lobe necessary to activate the D-Jet Distributor Contacts, but is still essentially similar, with the upper lobeshaft (with four lobes) acting on the Ignition Points, and the Centrifugal Advance mechanism below that. The reason this DS was retired from service was quickly determined...the Centrifugal Advance was completely frozen and inoperative!!

...so into the soft-jawed vice it went...the felt Reservoir was removed and replaced with a few drops of Liquid Wrench, and these were left to work their way down to the joint, which was presumably gummed up with cooked/coked oil (that's why I prefer and recommend synth-oil for this job, with it's higher operating temp and associated improved stability!)...after a day of soaking, some high (but judicious and limited) back-and-forth force was applied (also with a soft interface, of course!) to free things up...first a little, then a bit more, then a bit more, then a lot more...until the Lobeshaft could be moved through its entire 15Deg of rotation...

...once the Lobeshaft was moveable by hand from the lower DS, it was time to fully separate and disassemble the two. The standard method shown in the service manuals was used: Lobeshaft held in soft-jawed vice, and lower shaft tapped out with a drift... being careful not to loose the washer and circlip which are liberated in the process!

DS lower section partly separated from the upper Lobeshaft. One can only hope the owner of the car this Distributor came from replaced it with one which had a well-lubed and freely operating Cent Adv, else the symptoms [like the engine probably not being able to get the car over 60mph!] he was experiencing were not corrected!!

Two precision sections with a lube reservoir between come to the daylight for the first time in decades, and the reason this Distributor was in the "Exchanged and Temporarily Retired Part" department becomes perfectly clear!

Cooked oil residue will lock a Centrifugal Advance just fine...then you can say goodbye to any Cent Adv Curve, like in the graph!...what a mess! ...but mechanically, under that red (is it rust, or deteriorated oil?...the next steps will tell!!), everything looks to be in very decent shape!

It looks like that red glue was just gummed up oil! [I wish I knew what kind, so I could recommend AGAINST its use, given the result!] When initially cleaned, some discoloration is evident on the precision, polished bearing areas, but a fingernail test detects no discernable wear...

The discoloration is still stubborn oil residue and not damaged metal, so further cleaning with carb cleaner does remove most of it, and a fingernail still does not detect any discontinuity in the surface...this Distributor would go back together and back into service well...but I would be lubing it with a synthoil like Mobil 1 or similar which more able to endure the service conditions, and less likely to turn into that red glue!!

Discoloration of precision bearing area is able to be mostly removed with carb-cleaner and area-specific scrubbing.

The ID of upper lobeshaft is also cleaned well with carb-cleaner until the cotton swab comes out clean.

Cleaning and lubing Lobeshaft ID. Note differing lengths of Cent Adv Stop Pins. This means that Lobeshaft can only be assembled with lower shaft in one orientation, so marking this on disassembly is a good idea!

Reassembly is with Mobil 1 synthoil for the increased-stability-at-elevated-temperature-benefits, including a generous soak of the felt "reservoir".

[I don't mind doing this job, but don't really want to repeat it for a long time, if ever!].

The washer and Circlip are simple to replace because of the tapered shaft-end ("Lead" in the machining world). A 1/4" socket which just fits over the OD of the shaft will push the Circlip back into place with little drama.

External material sources are attributed. Otherwise, this article is Copyright � 2024 Ronald Kwas. The terms Volvo, Bosch, and names of other suppliers shown here are used for reference only. I have no affiliation with any of these companies other than to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here. The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion. As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future!

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SW-EM site as the source. If you don�t, you�re just a lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the unique metaphor and possibly also wise-a** comment.