Reverse Gear Affects Ignition!?!...or Whacky Ignition Function (WIF)

Article in progress!

Dec 2023 R. Kwas  (Comments Added)


Fault Condition Explained
Solution and Recommendations
Reference Information


...another case of vintage Volvo inadvertent circuit interaction...this time featuring WIF!

In a 1970 1800E, the symptom of "... Taking it out of reverse causes an engine stall. ..." is reported on :

Elliot Lucas name and video frame capture, posted with his kind permission.


Helpful posters suggest an Alternator dropout, or even that the mechanical load on the engine is varying (draggy Pilot Bearing?), but the author, (an "electrical guy"), proposes that that there is an inadvertent (and obviously unwanted!) electrical interaction between the reverse gear and the Ignition...and knowing that when we shift into reverse gear, electrically, we also cause electrical things to happen, which turn ON the backing lights, the reader can maybe already guess where this will be going... 

I'll call this another episode of CSI Göteborg, brought to you by SW-EM...stay tuned!

First, we extract the power routing from the Wiring Diagram to see if there is common power or return wiring...: 

Wiring Diagram Excerpt of circuits involved in WIF.


Carefully inspecting the above Wiring Diagram Excerpt, common routing of Ignition Power and Reverse Light power is clearly evident, by way of the Fuseblock (they may be separated by a Fuse, but they are still on the same Ign Power Buss!).  The author therefore proposes that the momentary perturbation of the ignition power is caused by the power-spike at the moment of deactivation (and even possibly during activation, if there is contact-bounce!) of the inductive current of the Reverse Light Relay, on the same current path as the Ignition, is causing a momentary "abnormality" in the Ignition Power, resulting in the symptom of a momentary drop-out of the engine [...we know that breaking the current in an inductive circuit like a relay results in a reverse voltage spike!  See:  Reference Information], and since the Main Relay for Fuel Injection is also on this circuit, I propose that the spike is momentarily interfering with the Ignition OR Fuel Injection

Since the Ignition circuit is a fast circuit able to recover quickly, it is relatively unaffected by the spike, whereas the FI ECU is a much more complex circuit, it is not able to recover nearly as quickly.  In normal operation, Ignition Power can be applied to the RLR and MRFI without any such an what causes the interaction?  

The Question must be asked:...What is actually wrong to cause this interaction, and what has changed from the factory wiring to make the current of one circuit affect the current of the other circuit?  [Does this sound familiar to readers of the SW-EM site?  See:  Reference Information]  The simple  Answer:  It is our old nemesis! In-line Resistance has developed in the common circuit path, which was not there originally, but which now allows the interaction of circuits.  [But where...? on!]

To locate the In-line Resistance, I suggested the owner remove Fuse6 to prevent the generation of the spike when breaking the Relay current...[of course this means loss of Reverse Lights and Horn also!]

To also confirm the author's hypothesis, observing the Tachometer while shifting into and out of Reverse, we might expect the electrical perturbation in the Ignition power path to show up as some sort of glitch of the Tach Needle...the Solution would then be clear.  Watching his video, a drop-out not unlike switching the Ign OFF, when shifting out of Reverse (and dropping the RLR) is observed.


Below, I have introduced the proposed Resistance R, upstream of the Fuse6 into the WD Excerpt, which would explain the interaction of these normally unassociated circuits.  Introducing this R electrically separates both Ignition and MRFI circuits from the vehicle Main Ign Power buss (backed up by the vehicle Batt and Alt), which previously was able to easily ride out such a momentary disturbance (Normal Condition!), but now, with the presence of the electrical separation, the Ign and MRFI are on the same side of the separation of the Main Ign Power Buss because of this mean-old R A (Fault Contition!) results.  

This circuit actually now very much practically resembles the power side interacting circuit considered here theoretically and simplified: 

Wiring Diagram Excerpt with R and fault condition added.


Fault Condition Explained:  A negative spike on Ign Power Buss, which occurs when shutting OFF Reverse Light Relay (T0) is electrically separated from the low impedance power buss, which would normally suppress the spike, by an inadvertently occurring Resistance (likely at the Fuseblock!), effectively increasing the magnitude of the spike.  The Resistance also subjects the Main Relay for Fuel Injection to this increased spike, which causes it to immediately drop out, effectively shutting OFF injection ECU completely(!!).  ...but the condition is only momentary and quite brief, so Relay and FI Sys recovers (thankfully quickly!) before engine totally shuts OFF, but not before a noticeable stumble of the engine occurs. 

So after much careful consideration, I would say the title of this investigation should be corrected to:

Reverse Gear Affects Ignition Fuel Injection!?!...or Whacky Ignition Fuel Injection Function (WFIF)



Feedback from Owner:  "Hey Ron! Pulling the fuse for reverse lights and horn solved the stalling problem when changing to reverse!..." 


[This is no surprise...and it confirms the suspicion of the general circuit area where the issue is...although it doesn't precisely locate it.  It does locate it enough through the above test and deductions, that the following Solution and Recommendations can be made with a high degree of certainty that they will cure the issue!]


More Whacky Ignition Fuel Injection Function (WFIF): 

Having another look at the Wiring Diagrams, the observant reader will notice that the Horn Relay is actually a very similar circuit with similar currents, and also on Fuse6, so I further propose that blowing the Horn would also result in a similar momentary interruption of Ignition Injection!  I have asked owner to try this test also, and report results! about WFIF!




Solution and Recommendations: 

Reduce the inadvertent in-line Resistance R, which is causing the Reverse Light Relay power line perturbation to affect other circuits!

The precise location of R, could actually be anywhere in the current path from Ign Sw Term 15 to the terminal of Fuseblock which the 4.0 YEL or the internal terminal stack are connected to, or even internal to the Fb.  This cannot be definitely known without further tests, so my suggestion to owner is to check ALL of the likely possible locations for loose or corroded connectors [The fact that owner and car live in Hawaii, with an apparently unlimited supply of nice weather, but also corrosive ocean air, is a big clue here!], and if no issues are obvious, starting corrective measures of preventatively cleaning to shiny metal, and reconnecting with ACZP ALL connections involved (including the conical end contacts of the Eurofuses!), and retesting. 

If symptoms remain, this particular Fb (of the same stacked and riveted construction as the 122 Fb) should be reworked to minimize R occurring internally.  I have described this work here, but it does require a gentle, careful hand, and somewhat special techniques and tools, so is best left up to the SW-EM Special Services and Weiβbeer Inspection Division (SW-EM SSaWID)!

An observation and comment:  This is a somewhat unusual finding,...generally, injected 1800 Fuseblocks, which live in the relative cozy passenger compartment, are not particularly susceptible to corrosion issues, but maybe the proximity to the ocean and its corrosive atmosphere, changes all that!  I think Elliot should embark on a comprehensive corrosion prevention/ACZP application program on this entire car, as he has reported fuel injection issues also, and these are often a result of poor electrical connections!  [I am also working through these with Elliot, and will document any interesting or significant findings.]   

See also:

Reference picture of an 1800E Fuseblock (not of the vehicle exhibiting the issue):

Heavy gauge Yellow wire visible below Fuse4 supplies the Ignition Power Buss from Ign Sw. 
Allan H picture used with his kind permission. 


What the rear of the 1800E/ES Fb looks like.  Highlighted rivets and
terminal stacks of Ignition Power Buss, of particular interest in this case!


Reference Information: 

Learn about inductive spikes at the popular on-line reference site :

Learn more about inductive spikes in your vintage Volvo:'s%20Tachometer.htm#sensing_polarity_matters

Learn how "Voltage Drops Cause Inadvertent Circuit Interactions!": 


External sources attributed when possible.  Otherwise this information is Copyright © 2023 Ronald Kwas.  The term Volvo is used for reference only.  I have no affiliation with this company, other than to try to keep their fine vintage products working for me, and to help other enthusiasts to do the same, and this includes minimizing perturbations of the on-board electrical system [That will make three times that I used that word in this article...I'll stop now!]   The results and information presented here are my own experience, and highly opinionated impressions, super-groovy troubleshooting analyses, and can be used or ridiculed and laughed at, or worshipped, as you see fit.  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 SwEm 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 odd metaphor, or analogy (see:  ABS!) and probably wise-a** comment. 


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