Truck alternates between 15v and 12v??

[kylen]

This last month I’ve been getting an awful noise from my alternator but never had time to replace it.. A few weeks ago the pulley locked up pulling into work. I replaced the alternator with a reman from the local parts store, but now I’m having an issue with voltage!

My truck will start and run fine but randomly dip down to 12v, dim the headlights, and eventually the battery light comes on. Randomly it will shoot back up to 15v.

Is this a voltage regulator problem? Alternator is rated the same as the one I got out of the truck, battery cables are on tight.

 

[Wade]

What year is your truck? I bet you that is the regulator, if you have any sort of warranty on the alternator, just get it tested and try to get a replacement. I've had brand new alternators before that are bad out of the box.

 

[Eveready]

Could be a diode not seated properly or it could be the regulator. In either case I would try swapping it out for another reman and see.

 

[low4x4]

make sure you tensioner and belt is not worn out

 

[kylen]

make sure you tensioner and belt is not worn out

Could a bad belt cause this issue? I had to drive about a mile to work with my previous alternatoe locked up, definitely burnt the belt up a bit but it looked fine visually..

 

[low4x4]

looked fine but might be stretched due to heat or the grooves worn to the point that the belt is not efficent

 

[Schurkey]

Could a bad belt cause this issue? I had to drive about a mile to work with my previous alternatoe locked up, definitely burnt the belt up a bit but it looked fine visually..

1. If you ran the engine with a LOCKED-UP alternator, the belt is trash.

2. If the engine ran with a locked-up alternator, the tensioner may be trash. I'd expect the engine to stall, or the alternator was not actually locked-up until you shut the engine off. I had that happen to me--Typical CS130 alternator failure, shut engine off, could not crank engine due to locked-up alternator.

3. Low voltage can be caused by a slipping belt. Erratic voltage can be caused by a belt that slips...sometimes. See #1 and #2. It can also be caused by poor electrical connections and by a faulty regulator.

If I were you, I'd be looking for a belt, maybe a tensioner, and then re-test the alternator.

 

[Oldblue98]

Could a bad belt cause this issue? I had to drive about a mile to work with my previous alternatoe locked up, definitely burnt the belt up a bit but it looked fine visually..

Combination of a bad tensioner or stretched belt or both could for sure. In all reality if the tensioner has never been changed or the belt, it would be a good idea to change them both out with the new alternator.