Need help with Guage problem

[dennisr]

I have aftermarket VDO guages in my 69. I have been having problems such as when I start turning on accessories, the volt meter drops to 10.5 volts. However when I put a tester on the battery, it reads that it is still at 13.8 Volts. The electronic odometer on the speedo has gone blank, but blinks on occasionally. The electronic oil pressure works intermittently. And the led read out on the stereo has gone out. All these gremlins lead me to believe I have a ground problem. I have checked the ground from the engine to the frame, and checked the battery ground. I have also used a set of jumper cables to ground the truck anyway I could think of with no changes. It all seems strange because there are no charging problems and it always starts and runs all accessories just fine. (vintage air and electric fan)

Anyone have a clue what might be causing this?

Thanks in advance,
Dennis

[1971LT1SWBPOSI373]

Sounds like your getting a voltage drop, weather it's through a bad ground, a corroded contact, or an accessory that is pulling the voltage down. I think what might help you is to take voltage readings of your power (Ringing the wire) comming from your battery as it travels into your cab to the fuse block. (the 13.8 vdc). Start your truck and take some voltage readings with a fluke meter (digital)starting at the battery, the firewall fuse block (engine side), fuse block cab side, ignition switch, ect. (start at the battery and work your way torwards the firewall) I don't have a print to reference right now to give you locations, but you can trace it out and do it, it's not a complex wiring diagram. Somewhere your getting a drop, and the way you describe all of your digital guages and radio I wouldn't be suprized if you found a problem at the fuse block. Take several ground continuity checks at the same locations. If you can find where the voltage drop is occurring then you can back up and figure out what is causing it. Take some readings and message me back.

[dennisr]

Thanks TJ,
Unfortunately you getting over my head. I don't know a thing about the electrical side of a vehicle. I printed you post and I'll take it to a friend or maybe a shop. I did however move my electric fan power off of the fuse block and wired it directly to the ac switch so it comes on with air. That helped my volt gauge read more accurately and doesnt take it down to 10.5 when running. I did notice I don't have an outside fuse block, it appears to have an aftermarket harness. That may be my whole problem, an incorrect install.

Thanks again,
Dennis

[Blue Beard]

I had elect problems..............

Used an old taillight socket with bulb, disconnected Neg lead from the battery. With alligator clips I hooked one end of the socket to the neg cable and then touched the other wire to the battery terminal (this is done with the ign off) found there was power at the light..............

Disconnected the wiring harness at the fire wall and found the short was under the hood.

That could help you get started..........

[1971LT1SWBPOSI373]

Dennis, Go to FAQs above and scroll through that board. About 10 entries down you'll find wiring diagrams that you can print out and take with you. Tape the 4 pages together and the circuits will be easier to trace. I traced your fan comming from the engine compartment and it goes through the heater switch to the fuse block, although this wiring diagram doesn't have A/C, I would think one that does would be similar. It sounds like you have some sort of aftermarket wiring harness, which are perfectly fine. The standard harness comes with a black fuse block that screws to the interior cab firewall. In the engine compartment there is a second 1/2 to this harness... the engine side, the engine harness plugs into the fuseblock that is attached to the interior firewall through a hole in the firewall. There would be lots of contacts where these two harnesses plug together, possibly one of them being corroded, a bent contact, or a bad crimp or a frayed wire at the contact. If you do have an aftermarket harness, the fan may have been bussed improperly, dragging the voltage down..... does the condition get worse with fan speed? Also, Your instrument lighting should have it's own dedicated fuse, along with the radio. There shouldn't be anything else tied in on those two circuits. If you do have a aftermarket harness you can compare it with the wiring diagram just to see how the circuits are being brought in and fused. Hope this helps.... TJ