1990 Escort - L/R Brake Light Voltage Differences

Hey all,
I'm chasing down a minor electrical problem in my car. Seems like the right brake light circuit always tests about 11.6 volts, which is about
0.2 volts lower than the left brake light circuit that tests about 11.8 volts. Both lights work but everytime I press the brakes, signal a right turn or turn the hazards on, the red LED on my graphic display lights up. Obviously not a critical issue, just gets me thinking... and tinkering.
Using some test leads I crossed the L/R brake light signals to the graphic display and make the LED light up using the left turn signal, but that exercise didn't really tell me too much. Testing resistance is tough because I only get a reading with my foot off the brake pedal, and it seems to be fluctuating, so hard to peg either side.
I have schematics and just wondering if there are any probable causes, like resistance, shorts to ground, bulbs, etc. Other than a light being out, I'm not sure what the graphic display is designed to sense. I have confirmed all bulbs are operational.
Thanks, Matt
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The system watches voltage drop. On any circuit if you know the resistance of the load ( bulb ) and source voltage the voltage drop before and after the load can be easily calculated. The bulb monitor in your car is looking for a voltage drop range, if the voltage drop goes out of range a LED comes on indicating which circuit. What I do not know is what side of the circuit it is watching, a schematic should tell you. You most likely have a bad ground different resistance values on the bulbs or dirty connections at the bulb. It takes very little extra resistance to cause a higher than normal voltage drop.

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Thanks for the tips, I was thinking about trying some things with the light sockets, like switching the bulbs out from L to R, etc. Now I'll also check the resistance to ground and clean the contacts in the sockets.
I'm no auto electrician, but from what you've described I think the side of the circuit that is being watched is the load side, before the bulb. Naturally, the schematic just indicates a ground after the bulb. All the graphic display circuitry, which consists of a couple of splices and a resistance wire between them, is before the bulb inline with the hot wire.
It helps a lot to understand how something was designed, at least in theory, when it starts blinking at you. Thanks!
Matt
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You will most likely not find anything by checking resistance. You should be checking the grounds with a live circuit and using a volt meter. Like I said, a VERY small change in resistance can cause the problem, small enough that you will not see it on a OHM meter, but you would with a volt meter.

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I have another dvm that will give me 0.000 precision on the resistance measurement, but to your point, I am curious as to where the best places would be to measure the voltage with the brake lights on.
Options I am considering are probing the hot wire to ground wire just before the bulb socket, hot wire to body ground, and ground to body ground. Does this make sense?
Any sense in checking the resistance across the bulb?
Thanks, Matt
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It means nothing on the precision of your resistance meter for making a resistance check. Do a web search on voltage drops as opposed to measuring resistance.

?????? In reality the best way to go about this is to install the voltmeter ground wire directly to the battery, yes that would entail making a very long test lead. Probe with the (+) lead anywhere you like on the circuit. Before the load you will see the highest volt readings after the load is the largest drop. You would not want to see a voltage any higher than .02. Any higher the ground path has high resistance.

That is one of the valid use of a resistance check, checking the resistance of a single component.

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Take a look at this http://www.federal-mogul.com/vgn/images/portal/cit_776/167494891519.doc

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Hey, thanks for the link! That's a keeper
Matt
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Ok, but do you understand it?

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Yeah, it makes sense now, what you were explaining. I.e., a dirty connector causing a voltage drop due to increased resistance, one side of the connector being .02 volts or something greater than the other side.
I will pick up on that shortly, but now I noticed my dash lights were dimmer and blinkers were slower. Checked the battery voltage with engine off, 11.6, then with engine on after a few minutes, 11.6. Looking at the alternator/voltage regulator stuff and circuit tomorrow, looking for 2.4 ohms between A and F terminals on the IAC. Funny thing is that the battery light never came on the dash. Probably means a new rebuilt alternator is on the way.
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opined in

I suggest measuring the voltage from the alt output to its own case, first!
--
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -

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I'm getting 3.4 ohms between A and F. Checking voltage later.
Thanks, Matt
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opined in

I'm a little surprised you STILL havent gotten the message.
I was an EE in past life and i almost NEVER use the Ohmmeter on a car or any OTHER application, unless I'm looking at an isolated component with a critical value.
Really only pertinent for gross continuity type troubleshooting and i pay very little attention to the actual values.. CERTAINLY not to "tenths"
--
- Yes, I'm a crusty old geezer curmudgeon.. deal with it! -

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Chill, man.
I got a new alternator in this morning and have charge going to the battery again.
On this model car the IAC is only a little over an inch away from the exhaust header on the #1 cylinder and is prone to failure due to heat. I've gone through a few alternators on this car over the past 10 years and so far my luck has always been the IAC failing.
FYI - ONE of the troubleshooting procedures as indicated in Haynes, Chiltons and alldata for checking the internal voltage regulator on this model, is there should only be 2.4 ohms resistance between A&F terminals. Since I've been checking them they've always been over 2.4 in failure situation.
Seems like a critical value to me.
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