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79 GMC Sierra C-30 Camper special
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Discussion Starter · #1 ·
Hi,

On my 79 Dualie: Hooking a digital meter up to the lighter socket, I lose about half a volt just from the load of the dome light. The headlights drop it around 2 volts. The starter cranks fine. Took the main firewall connector apart, spayed in contact cleaner and chased it with silicone grease and no change. Any hints on making it a little easier to find where the drop is? Thanks.
 

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79 GMC Sierra C-30 Camper special
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Discussion Starter · #4 ·
Let me start over with just the brake lights, which is important since I will be towing a trailer in the future.

This test was done with the trailer lights connected to a wired-in aftermarket connector. So, including the truck brake lights, the load was the high filaments of four incandescent 1157 bulbs. Per one site, that would be a total load of 8.4 amps at 12.8 volts, but of course because of the voltage drops the bulbs aren't getting 12.8 volts. All voltages were measured with the negative probe of the digital meter connected to the negative terminal on the battery.

Battery voltage with brake lights on: 12.5
Voltage at stop light fuse, both sides 11.8
Voltage at stop light switch, both sides: 10.8
Voltage going into turn signal switch: 10.4
Voltage at left truck brake light: 9.5
Voltage at left trailer light: 8.5

I read on another site that even the worst of stranded #18 copper wire is supposed to have a resistance of only about .006 ohms per foot. So are the above voltage drops anything like reasonable? Thanks.
 

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No, they are pretty bad. Could be bad wiring, or connection somewhere or bad ground. You'll have to do more testing to find the problem/s.
 

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maybe a fusible link partialy burned?davester's right, start at the battery and work toward the cab with voltage drop tests. Look at Utube for proper way to do the tests. The + cable goes to the starter where there are fusible links that power every thing in the truck
 

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Don't forget all of the grounds. I would also expect to have corrosion in your main battery cables.

Check your voltage at the back of the alternator post and compare it to the battery readings. Use the same ground location if you can.
 
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