Voltage Drops Cause Inadvertent Circuit Interactions!   
Jun 2014  R. Kwas (changes on-going)  [Comments added.] 


Example of Power Side Inadvertent Circuit Interaction
How does a fuse work?
Example of Chassis Side Inadvertent Circuit Interaction


Reference Information: 
Headlight / Tachometer Interaction on 1800ES Solved!

Example of Circuit interaction on a non-Volvo


An example of the most typical circuit interaction one might experience, is the flickering or dimming of house lights when something else in the house, or even down the street, is turned ON.  This is a sure indication of an in-line resistance which results in voltage drops, and this ties multiple, normally isolated and independent circuits together...this can happen on a car electrical system as well...

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...talk about whacky symptoms and weird circuit interactions...  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.  Update:  He cleaned up a poor connection on Ign Coil and installed new radio noise suppression capacitors, also at the Ign Coil, so it looks like there was Ign Noise on the vehicle elec buss, and getting into the overly sensitive Tach, by way of its power line...!  See also Reference InformationHeadlight / Tachometer Interaction on 1800ES Solved! 


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), power supplied to Tachometer is clean from the vehicle power buss, and operates normally and without symptoms, 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 (both 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 result in a significant Voltage Drop, and so 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! 

How does a fuse work?  A fuse consists of a conductor, strategically sized to allow a normal current magnitude to flow, but that melts easily beyond that, when an over-(fault)current flows.  If the current going through the fuse is greater than its intended normal operating magnitude, the wire heats up beyond the point of it being able to shed this heat, causing it to melt and break the circuit, stopping over-current flow.  This is the simple explanation which applies when everything is normal...things do get more complicated when there is an additional heat source...like from the inadvertent Resistance of a poor connection, which also makes heat as it passes current (it's called I2R heat)...then things can get interesting...see:  I2R Heating.


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

[For a practical Volvo-example. see also:  Mounting screws of the 122S taillight assembly are also in the current carrying path.]

[Refer to FIGURE 2]  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 can result in an entirely different set of symptoms!  In the first place, this will result in lamps getting less than full voltage, making them put out less than their best light, but it will also result in interaction of circuits which normally would be completely separate, and 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 (Refer to Figure 2!).  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.   

Figure 2.

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 (intentionally shorted!) with zero R...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, or maybe simply a loose fastener, and reconnected with ACZP...that should keep the whackiness away for a long time! 


Another somewhat different circuit interaction is the electrical noise (either Charging System or Ignition), getting into the audio system, affecting the program material.  See:  https://www.sw-em.com/Radio_Notes.htm#Radio_Interference 



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: 

Headlight / Tachometer Interaction on 1800ES Solved!
Craig K has fed back details for his ignition noise mitigation, and implementation.  It seems that two Ignition type capacitors, connected to the Ign Coil positive terminal, quieted down the noise which was being coupled into his Tachometer.  Again, I believe at least part of the problem is due to the increased sensitivity to noise as the Tach ages and it's internal filtering is reduced, and this is covered on the SW-EM Tach Info page. He didn't mention it, but I expect it also quieted down the radio! 

Shown here are the two ignition type condensers connected to the Ign Coil positive terminals.

Craig Keller image and used with his kind permission.

Shown here are the two ignition type condensers connected to the Ign Coil positive (+) terminal. 
These were successful in quieting down electrical noise coupled into Tachometer.    

In rereading this page, I recall that a contributing factor could certainly be that the chassis return current for the 1800 Tach is not carried by a separate wire, but by the Tach mounting hardware!!  See also:  https://www.sw-em.com/Two_Wrongs_Make_a_Right!.htm


Example of Circuit interaction on a non-Volvo:  

Vintage Volvo owners are not the only ones who might encounter a circuit interaction with wacky symptoms! 

In this thread on the VW site:  https://www.thesamba.com/vw/forum/viewtopic.php?p=9646155#9646155  ), an owner experienced the following: 

"...honked when backing up but nothing happened. At first I thought it was something with the horn or the steering wheel switch, but putting it back in neutral and the horn worked fine. In reverse again and no horn."

This excerpt from the factory Wiring Diagram of this VW Vanagon, shows the two circuits share the same Fuse (S15)...that's a big hint! 

15 on the Wiring Diagram is the Ign power buss, so at first glance, this is likely be a Power Side Inadvertent Circuit Interaction.  My betting money is on the current of the Backing-Lights causing a VD across an R Which has developed in the common power feed somewhere around the Ign power buss or Fuse S15, which leaves insufficient V, to operate the Horn Relay (J4).  If owner reports back with findings and what he corrected, I'll add it here...


Sources of external materials are attributed.  Otherwise, this page is Copyright © 2014-2022,  Ronald Kwas. The term Volvo is 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! 


B A C K ! . . .to Tech Articles Index Page