Alternator overvoltage ?

[dblake]

My alternator is intermittently going into overvoltage mode. I recently had it rebuilt and it worked fine for a few weeks. When it returns to normal it still reads a bit high at around 15 volts. It’s not getting warm or giving any other symptoms. Battery cables and connections are clean and batteries are not warm to the touch. Unfortunately I didn’t get a chance to check it with my voltmeter the last time it did it. I’m hoping it’s just my alternator and not my PCM. I have a mean green on order.

 

[Ozymandias]

Check the voltage drop through the cross over cable.
Probably you have a bad conductivity there. Also the grounds from both battery's going to the engine block.

 

[dblake]

Check the voltage drop through the cross over cable.
Probably you have a bad conductivity there. Also the grounds from both battery's going to the engine block.

Thanks. Any idea what my readings should be?

 

[Ozymandias]

Identical on both ends of the wire, almost zero voltage drop.

 

[dblake]

Identical on both ends of the wire, almost zero voltage drop.

Thanks

 

[Ozymandias]

PCM is reading voltage from the drivers side battery, that's why a bad cross over cable, or bad grounds, can guide it into an overcharge situation.

 

[dblake]

PCM is reading voltage from the drivers side battery, that's why a bad cross over cable, or bad grounds, can guide it into an overcharge situation.

Thanks for the info, but I’m leaning towards my alternator being the culprit. The guy that rebuilt mine took parts from like 5 others to make it. He didn’t even have the right voltage regulator so he showed me how to rewire it..

 

[GCroyle]

Maybe innocently take it to local parts store and ask to have it tested.

 

[petersonj]

There is also a battery temperature sensor in the battery tray under the driver side battery. The PCM uses this information to determine battery voltage. Example: 90° day battery voltage may show 13.8 volts, 30° day battery voltage may be 14.4 volts. This would be normal.

This is probably not your problem, but you should be aware that it does significantly influence the charging voltage value showing on the dash gauge or the display on a multimeter connected to the battery.

A few years ago I had removed my driver side battery and battery tray. I forgot to reconnect the temperature sensor when I re-installed everything. I drove the truck for a couple of weeks that way and I noticed considerable fluctuation in the dash voltage gauge before I finally figured it out.

- John