Voltage Drop When I Accelerate

[Rosedog]

So I noticed when I accelerate and it puts a load on the system (automatic downshift) I noticed my voltage drops quite noticeably but when lay off the accelerator / back to normal driving conditions the voltage goes back up to normal.

 

Has anyone experience this before? I called the dealer and they sounded perplexed by this without bringing it in. They suggested it might be my belt and they could be right. I don't know when the last time it was changed or if it has been changed (I'm over 11X,XXX miles now on the truck and I bought it used).

 

I thought it could be the alt and have given it thought to putting a new (reman.) one in there (higher output for those times when it rains, heater, lights and the stereo is on - yay for living in the Puget Sound convergence zone). Either way that requires the belt to come off, which at that point I don't know where to take it off at.

 

Your thoughts and input are most welcomed.

 

2003

4.8L

[Rosedog]

Okay well I ended up replacing the Alt. to a reman. 145amp, new belt and new tentioner. While I had the K&N kit off I cleaned the MAS and the TBV.

 

Took it for a test drive with the stereo cranked, wipers on full (its raining outside), headlights - foglights on and the heater cranked and the voltage stayed pretty consistant. Even when I floored it and it downshifted to 3rd and it started pulling, the voltage stayed consistant again ~ YAY!

 

However I noticed that the volt gauge (while sitting in the drivers seat normally) looks to be in/on the 14 mark, when I look at the gauge straight on its actually just on the 14 mark even with all the accessories off.

 

The battery is fairly new (Optima Yellow top), bought maybe just under a year ago.

 

Now that the old 105amp alt is out (before I take it in for the core) I'm going to have it bench tested and see if that was the problem or not. Oh and my milelage is actually closer to 130,000. At least two parts of this ordeal with maintance.

 

 

Again, your thoughts and input are most welcomed.

[TexasAl]

It might be a bad cell on your battery. Take the old alternator to get tested. Also, most auto parts stores can check your battery off the vehicle. Make sure it is not the battery, as it may end up killing your alternator.

[nickvrebel]

your vehicles charging system charges at 14.2v when the motor is running.... sounds normal

 

edit:

and don't trust the instrument cluster gauge, use a volt meter on the back of the alternator to confirm

[Matt_]

I've seen alternators go bad like that, more specifically the voltage regulators.

 

A tech at the shop I worked at couldn't figure it out... the alternator even "tested" good using our machine. I put the gauge on it and revved the engine up to a higher RPM than is necessary for the "test" and the alternator effectively changed polarity and started to drain the battery. Replaced the alternator and all was good.

 

Very very rear... but it happens.