john j oliver

OK This is really weird. My 95 LT1 head lights started going crazy on me. They were working fine then they started back feeding to the fog lights and high beams (with the fog lights off). When I turn on the high beams, only the driver side highs turn on, they back feed to the fogs and the check water level and right turn signal (inside) come on. I pull the fog relay and fuse and they still come on. In Low beam they feed back to the high beams (very dim). Its like they took a life of their own. I've pulled all the lights, highs and low, and fogs and checked for corrosion and checked and cleaned the to two large grounds near the radiator. Also checked the ground lug near the battery. Checked the fog relay and fuse. They all look good so far. Any clues? Any help? Please and thanks. John

GaryDoug

I think the big clue here is that the fog lamps are somewhat lit even when the fog lamp relay is removed. Is that true? I know you said that, but lets specifically address that again. The only way they can be lit with the relay removed is if there is a difference in voltage between the two chassis grounds G105 (right) and G106 (left) on each side of the radiator. You said that you checked those, but there must be something you missed. Use a voltmeter to check the voltage at those two points while the headlamp switch is on, preferably at the wires themselves. Both should measure zero volts. I suspect an open connection at the lugs or somewhere upstream to the lamps, maybe at splice S113.

p.s. focus on the ground near the high beam headlamp that does not work (passenger side?)

john j oliver

Thank you for replying. Yes that is correct.... with the fog light relay removed and the fog fuse removed they are dimly lit. when you say look "Use a voltmeter to check the voltage at those two points while the headlamp switch is on, preferably at the wires themselves. Both should measure zero volts."... I am assuming remove the ground from the can and test voltage from that wire to ground...Is that correct? Also you said "I suspect an open connection at the lugs or somewhere upstream to the lamps, maybe at splice S113." ...I will try to find that splice but if you answer before I can find it.. that would be a great help. As an fyi this afternoon, I am replacing the high beam dimmer switch on the steering column. I really don't think that is the problem but it was $10. Thanks again. I'm still working on it. This has been such a great car. It has been garage keep and this is the very first electrical issue in 180k miles.

GaryDoug

"when you say look "Use a voltmeter to check the voltage at those two points while the headlamp switch is on, preferably at the wires themselves. Both should measure zero volts."... I am assuming remove the ground from the can and test voltage from that wire to ground...Is that correct?"

You could do that and use an ohmmeter setting instead , making sure the reading is under one ohm. A better test is to leave everything in place and probe the wire through the insulation at the passenger-side ground (G105). Measure voltage with the headlamp switch on, it should be zero or very close. I am creating a bit of controversy by suggesting you push through the insulation on a wire not inside the cabin, but you could reseal it afterwards with a bit of clear lacquer (nail polish works) to prevent corrosion of the wire.

"Also you said "I suspect an open connection at the lugs or somewhere upstream to the lamps, maybe at splice S113." ...I will try to find that splice but if you answer before I can find it.. that would be a great help. "

That splice is probably way under the harness wrapping, maybe to the side of the battery. It should be a last resort and only if you find the voltage to be ok (0v), but the resistance to ground to be not ok at G105.

john j oliver

Roger That... and again thanks for replying. I will finish installing the steering column today and concentrate on the grounds and try to locate c113. I searched the web and could not find that physical location.... Will continue searching. Like you said I suspect its near the battery as that is the only thing I haven't pulled the wire loom on. I also found shbox.com head light fog light diagram... http://shbox.com/1/headlight_chevy_1995.jpg and cross-referenced to pages 8A 14-7 and 8 in the GM service manual showing G105 to S112 and G106 to S113 which like you said is on the left and right side of the radiator which leads me to a question. If these are in series; junction S112 going to G105 (ditto for the other) and the lights are upstream of those, wouldn't the short be upstream of the lights as when the low beam are on the highs are dim as well as the fogs? maybe down stream S151?

GaryDoug

Try to imagine this possibility: G105 is still connected together with everything on the G105 node, except that it is not connected to ground. What happens to the current flow in this case, when the high beams are on? Obviously it cannot travel to ground at G105, so instead it goes through the low beam lamps and the fog lamps to get to G106 on the other side. Because of the multiple devices in series that it has to go through, those devices (low beams and fogs) get less voltage each and are only dimly lit.
But then how does that explain the Low coolant indication lighting also? Look at where the sensor gets it's ground.....G105. What isn't obvious by looking at the diagram is the design of that sensor. It is not a simple switch as drawn, but rather an electronic device that closes an electronic switch. A missing ground at the sensor can mess up the functionality of the electronic circuitry in that device and cause abnormal operation.

john j oliver

Success It was the dimmer switch. After replacing the dimmer switch I check the high and low beams and all was good. Them I re-connected the fog lights and success. All is good with the world. I think the clues were what you said about the c112 connection and me thinking the short was ahead of that by looking at the wiring diagram from shbox showing the high beam dimmer switch connecting to the fog light. Success ! Is it too early to open that first beer? Thanks buddy, I owe you a cold one.

GaryDoug

Congratulations! Looks like you were on a better track than I ;-)