Voltage Drops Cause Inadvertent Circuit Interactions   
Jun 2014  R. Kwas (changes on-going)


Example of Power Side Inadvertent Circuit Interaction
Example of Chassis Side Inadvertent Circuit Interaction


I put these notes together in response to a Brickboard posting of an 1800ES owner who was experiencing strange interaction of his Light Switch with his Tachometer.  LINK to Brickboard Thread: https://www.brickboard.com/RWD/volvo/1590735/1800/73_es_tach.html  )  He didn't post back if he ever resolved the issue, but as the reader can see, he had some pretty unusual symptoms, and I gave some suggestions for things to check...I hope he lets me know what the final fix was so that I can add it here as a practical example...in the meantime, these notes are intended to offer some explanation of strange circuit interactions the vintage Volvo owner might encounter. 

Example of Power Side Inadvertent Circuit Interaction due to corrosion (or other) caused resistance in a power connection:  “The Daisy-Chain Effect”. 

Under normal conditions (with no significant R, refer to FIGURE 1. following), power supplied to Tachometer is clean, but with increased R*, a daisy chain effect causes a voltage drop V by any current drawn by Load1 or Load2 (Link to Dropping Resistor info), to be seen by the Tach (as a dip in its supply voltage) – particularly a problem if this current is a step function (like that drawn by Voltage Stabilizer!), because Tach circuit is, by design, very sensitive to pulses on its RPM sensing input.  Unfortunately, increased sensitivity to pulses occurring on its supply power as its internal filter capacitors age, dry out and become less effective, is the common aging affect for the Tach.  (Link to 1800 Tach info:  http://www.sw-em.com/Smith%27s%20Tachometer.htm  ). 


Figure 1. 

So what we have here is a fault condition resulting from two contributing factors…both age related… 1)  the additional resistance in the power supply line, and  2)  old electronic components in the Tach itself.  My recommendation to owner is to start by reburbishing the FuseBlock to get rid of any increased series (but inadvertent, and unwanted!) R...if the root cause is an increased inline R, the symptoms will likely go away!

*  Some low R is inherent in fuses and unavoidable...in fact, it is required for the fuse to function...but this resistance is typically so low that it does not affect the load circuit function in any way.  Additional R, occurring due to poor connections at fuse ends, or riveted terminals, or crimps, which increases RTotal to the point of affecting the load circuit function in these ways, must be avoided or eliminated/minimized  Clean and use ACZP when installing fuse and push-on terminals! 

Example of Chassis Side Inadvertent Circuit Interaction due to resistance at a chassis connection: 

[Refer to FIGURE 2]  Mounting screws of the 122S taillight assembly are also in the current carrying path.  If these are loose and making less than a perfect connection so that Resistance is present, we have the situation of a Resistance in series with multiple circuits, but on the chassis side as shown below, not on the voltage supply side, as covered above.  This will result in interaction of circuits which normally would be completely separate, but because of the R, have become linked together...some pretty whacky symptoms can result!   

Consider the situation of nighttime driving with Marker lights on…some VD will develop across R due to Marker Light current.  This will not only decrease the voltage on the Marker Light lamp itself, making it dimmer, but may even cause the Marker Lights to go out entirely when also applying Brake and Directional Indicator current, because those Lamps draw even more current, resulting in more VD and even less across the Marker Light. 

Placeholder for calculations comparing Marker Light, Brake Light Currents.   


Troubleshooting advice:  When whacky symptoms are encountered, as in the example above, a Voltmeter can be used between a known good negative point (chassis, or even Battery negative post), and the low side of the Load exhibiting the symptoms.  This measurement must be performed under load (controlling switch turned ON!) and while the symptoms are being exhibited, but if any significant voltage is measured, it confirms an R, and resulting V is present on a point which would normally have no Voltage (because it not possible to develop a voltage across two points which are tied together..Voltage by definition, can only occur across electrically separated points!).  Connection to chassis should be inspected, cleaned of whatever is the source of the series R, be it corrosion, that beautiful new paint-job, maybe simply a loose fastener, and reconnected with ACZP...that should keep the wackiness away! 



Link to Thread Headlamp Fault with very good explanation from Simon of alternate current paths and unusual circuit interaction, when there is a poor chassis connection:  http://www.volvoforums.org.uk/showthread.php?t=257267


Reference Information: 



Sources of external materials are attributed.  Otherwise, this page is Copyright © 2014-2016,  Ronald Kwas. The term Volvo are used for reference only.  I have no affiliation with this company, other than to try to keep their vintage vehicles working for me, and to help other enthusiasts to do the same.  The results and highly opinionated thoughts presented here are from my own experience, and carefully considered opinion, 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 knuckles, and future! 

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! 


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