Voltage Problem 1977 C20


LawnSite Member
I have a tough bug to find, I tried replacing the alternator ( built in regulator) and still have the same problem. The voltage reads good when the truck is sitting still at a red light or in park/neutral but as soon as it starts moving the voltage drops down into the red. After driving a while it some times goes back up to the center position, but always goes back up when you stop.<p>It is worse when the truck is cold, when you hit the brakes and let off the voltage goes up a little, but usually only goes above center when you are at a complete stop.<p>Shaking the truck, jumping on the bed etc, doesn't affect the voltage, bumps in the road don't seem to affect it, nor does speed.<p>I I put the truck in neutral while it's moving and rev the engine it does not affect the voltage. However, once stopped you can see the voltage guage move a little when you rev the engine.<p>Any suggestions?


I'd look for a bad ground strap<br>between the engine and body, <br>not the negative batery cable going to a alternator mount. without that strap power has to travel through rubber mounts and grime. (not the best conductor)


LawnSite Member
Yes, sure enough it was the ground strap.It looked secure, but close inspection showed it was not fastened to the engine. When the truck was sitting still the ground strap rested on the engine, but once moving it would pull away from the engine. I removed it from the firewall and put it in the front, fastened to the frame after filing the paint off for a good connection. As a side benifit I got my windhield washer pump operational again, apparently there was not enough voltage to energize it with the bad ground strap connection.