Gas gauge/headlights - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

 
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post #1 of 12 (permalink) Old Feb 27th, 11, 3:31 PM Thread Starter
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Gas gauge/headlights

The gas gauge on my '69 is pretty accurate, until you turn on the headlights. It then dips down by about half of what is accurate. Turn the headlights back on, and the gauge returns to normal.

I realize this is probably a bad ground, but maybe someone who has had this problem before can give me a head start before I go through the one-by-one check on connections.

The battery is correctly grounded to the engine and there are ground straps to the firewall on each side.

Any pointers? Thanks.
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post #2 of 12 (permalink) Old Feb 27th, 11, 3:56 PM
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Re: Gas gauge/headlights

If the headlamps have a good ground, you may have a low voltage issue.

I've had low voltage affect oil pressure, and temp readings.
May have even affected the fuel, but I was not paying attention to that at the time.

Anything lower than 13 volts running, can cause issues like this

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post #3 of 12 (permalink) Old Feb 27th, 11, 4:03 PM Thread Starter
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Re: Gas gauge/headlights

With headlights off, the volt meter reads 14. With head lights on, it reads 13.5. With headlights, blower motor, windshield wipers, and stereo on, it will read 13.0. The only time it dips below that is with all the accessories on, the blower motor at max, and idling at a stop light.

The dip in the gas gauge is only affected by turning on the headlights. If I turn only the running lights on, then the gauge dips only half as much.
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post #4 of 12 (permalink) Old Feb 27th, 11, 4:15 PM
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Re: Gas gauge/headlights

Might look into dirty fusebox terminals and fuse clips feeding the gages.
Maybe even the ground from the sending unit to frame.

Stab in the dark>>>>>>>>>>perhaps the resistor in the gas gage is bad???

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post #5 of 12 (permalink) Old Feb 27th, 11, 4:16 PM
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Re: Gas gauge/headlights

Someone here posted that the sending unit wire should have 5 volts with the key on.
Might go back under the car and check for that.

With lights on and off

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post #6 of 12 (permalink) Old Feb 27th, 11, 5:17 PM
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Re: Gas gauge/headlights

True that headlights are a huge load on these older electrical systems. A few years ago when I had some lighting issues like, dim gauge backlighting and dime headlights etc..I checked into the dash harness. Not trying to send you in to deep but, GM used solder joints with 10-14 wires joined at one point. I cut them all and connected them to terminal blocks so they each had a nice clean path. Never had any issues since. Will post pic of that joint I cut out. Original harness to car.
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post #7 of 12 (permalink) Old Feb 27th, 11, 9:25 PM
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Re: Gas gauge/headlights

had to break out the diagram for this one. reading from a 69 diagram, the fuel gauge runs through the gauge(s) fuse circuit. the headlight switch runs through Inst light fuse circuit. they are on different circuits. however, as u suspected, the grounds are common to all. i suspect a bad headlight switch(not likely) or bad ground connection somewhere(likely) behind the dash. the fuel gauge loop is not the issue unless its got a bad ground somewhere (likely). i would first test the grounds @ headlight switch, then test for 12volts and ground @ fuel gauge and sending unit. cant rule anything out though.
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post #8 of 12 (permalink) Old Feb 28th, 11, 2:38 AM Thread Starter
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Re: Gas gauge/headlights

Thanks for all the leads! Gives me an idea where to start.
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post #9 of 12 (permalink) Old Feb 28th, 11, 5:50 AM
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Re: Gas gauge/headlights

had the same problem on my 68. Dashes are similar. I had loose connections on the little earth links on the back of the dash. The gauges were getting an earth thru to dash lights until I turned on the lights, than the earth the gauges were seeing become a positive dash light feed. run an earth lead from the back of the dash to a bolt on the pedal bracket. problem gone.
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post #10 of 12 (permalink) Old Feb 28th, 11, 9:51 PM Thread Starter
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Re: Gas gauge/headlights

Col,

I see you're in Australia. I assume that by "earth lead" you mean a "ground wire". Is that correct? Where on the back of the dash do you connect? Is it to the gauge?

Thanks for the advice! I'll give it a try.
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post #11 of 12 (permalink) Old Mar 1st, 11, 5:43 AM
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Re: Gas gauge/headlights

sorry for the aussie talk. Yeah I mean ground lead.

On the back of the dash there are some 1/4 inch head screws that hold the metal backing on. I put a ground lead under one of them. My dash light bulbs in the 68 ground thru the metal backing of the dash. On the back of the dash where your gas gauge is you should find 3 studs. If you connect a volt meter from the ground terminal of the gas gauge to the ground terminal of your battery and turn your lights on, you should not see a reading any higher than 1 volt. If you do than try connecting a ground lead from the gas gauge ground to chassis ground.

Hope this helps. Col.
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post #12 of 12 (permalink) Old Mar 1st, 11, 2:11 PM Thread Starter
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Re: Gas gauge/headlights

Thanks again!
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