While driving my truck, the radio goes off for a few seconds and then comes back on. I noticed my dome light dims at the same time.

So, I hooked up a voltage meter to my cigarette lighter with the adapter from Radio Shack and while idling the voltage is stable (a little over 12 volts), but once I put the truck into gear, the voltage starts to drop some. While driving, it will bounce around and bottom out around 9 volts and go back up right away to about 12 volts; all the while turning my radio on and off.

It appears it only happens while driving the truck, not while it is idling in park.

I put a voltage meter on the green and red wires on the alternator, while the engine was running, and the voltage was around 6 volts. I am not sure whether this really tells me anything.

Is my alternator going bad or is my voltage regular going bad?

What is the best way to test a voltage regulator?

Thanks!

 

Sounds like a bad connection, turn the radio up and start jiggling wires.

 

Thanks Seal, but that is actually what I did in the first place. I pulled the dash and checked all the connections at the radio. The radio runs fine until I drive. Same with the dome light. The voltage at the lighter drops down to 9 volts while driving, doesn't when idling and then goes back up to 12/13 volts.

 

have you check what the voltage is at the alternator when this happen?

 

I replaced the voltage regulator, it did not fix the problem and returned it to Advance Auto. Took them my alternator to check, the employee said it failed, I don't think he did it right. Bought a new alternator at NAPA and put it on. Same problem, voltage dropping when using any combination of electrical devices, brakes, lights, blinker, radio, etc, bottoming out at times at 8.52v if more than one or two devices were in use. But, not as often with the new alternator.

Remembering that last week I took off a frayed "auxillary" ground that went from the negative post to the grounding bolt near the post, (above the grill and under the hood, the one that the front lights use), I replaced it. It seems to have fixed the problem. I am guessing that the bolt doesn't give a good enough ground and the previous owner ran this extra wire from the bolt to the negative post. Last week it had continuity, so I decided to take it off. So, it seems that a bad ground was causing the problem of the voltage drops when electrical devices were used. Does this make sense?

 

Makes perfect sense to me that a bad connection would be the problem.