alternator charges on bench but not in 95 XJ6

[doxjag]

Original and new alternator put out 14.5 volts on bench but only 11 volts in running car and the battery dies. I also tried a new battery. Battery does not discharge with the ignition off. Any ideas are welcome!

 

[vettedog72]

If the alternator is without fault, the problem is with the wiring. Recheck all connections, if they are good, then a wire is partially broken.

How are you checking the voltage in the car and how did you check the voltage on the bench? While on the bench, did you check the alternator for max current flow?

 

[doxjag]

Thanks.
I had an auto electric specialist check the alternator. Everything was within factory specs. I even tried a new one with the same results. I check voltage in the running car at the instrument panel gauge, the lighter socket and at the battery, all with the same erratic, mostly low readings. It seems I'm getting readings that don't correlate with engine rpm's or with AC or lights being on or off.

 

[maynard]

The altnerator needs to be excited before it can charge. This is done by voltage to the alternator that runs through the warning light on the dash. If yours is burnt out, the alterntor won't charge.

 

[jessebogan]

There are 2 things that I can think of. First, check that the rubber bond has not failed on the crank pulley. This is not uncommon, and will lead to the Alt not charging, or charging intermittantly. Chasing this can make you crazy!. The inside part will turn at engine speed, and the outer will turn much slower. Since the other belts are driven off of bolt on pulleys, nothing else will be affected. The second thing is the "dump module" I cannot remember if they kept it after the XJ40s, but if they did, it is a small black box with a small wire that connects to a stud on the alternator. This is not to be confused to the heavy power wire. If it has a dump module, you can disconnect it and run the car. If it is bad, the car will charge normally. Replace it, do not leave it off forever, it is there for a reason. HTH

Jesse