Hey guys i have a 91 xj and i recently swapped the motor and now the alternator wont charge. It says it is giving out 14.1 volts i think but my battery is not getting charged, its a good brand new interstate battery. All the wires are hooked up and connections are good. I do see a skinny red wire with a fuse in it that is not connected but i am not sure what that is for. Im guessing fog lights that were taken out. I see the black wire going out of the alternator under the relay box but nothing going to the battery from the alternator. If you can help me out that would be great, Thanks!
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Bad Alternator?
- Thread starter dannemec
- Start date
88-xj-laredo
NAXJA Forum User
- Location
- New Hampshire
Have you tried disconnecting the negative cable to your battery? If you disconnect it and the engine dies your altenator is bad, if it keeps running your altenator is good.
Hope it helps a little.
mark
Hope it helps a little.
mark
5-90
NAXJA Forum User
- Location
- Hammerspace
DON'T DO THAT!
While it's true that this was an acceptable test years ago, it isn't now. Rather, it's a great way to fry your electronics...
Put a voltmeter on your battery (not the one in the IP - a good DMM) and check voltage with the engine OFF. Note it.
Start the engine, and then check again. Compare the two.
Typical "rest" voltage should be 12.0-12.6VDC with the engine off. Alternator output voltage is up around 12.8-13.5VDC. If the voltage is not higher with the engine running, then it's a charging problem.
If the voltage is higher and you're still seeing a problem, kill the engine and get a helper. Pull the coil lead, and watch the DMM while the helper cranks the engine for 15 seconds. Battery voltage should not drop below 9.6VDC under crank - if it does, it's a battery problem (just because it's new doesn't mean it's not possibly defective.) If it doesn't, then look into the alternator and regulator.
Since it's agreeably rare for Chrysler regulators to fail (good thing - they're built into the PDC!) I'd be inclined to think it ends up being an alternator issue. However, while the regulator can be back-checked (using the voltage at the field coil terminals on the back of the alternator case,) that's one diagnosis I don't want to make by "remote control."
88-xj-laredo said:
While it's true that this was an acceptable test years ago, it isn't now. Rather, it's a great way to fry your electronics...
Put a voltmeter on your battery (not the one in the IP - a good DMM) and check voltage with the engine OFF. Note it.
Start the engine, and then check again. Compare the two.
Typical "rest" voltage should be 12.0-12.6VDC with the engine off. Alternator output voltage is up around 12.8-13.5VDC. If the voltage is not higher with the engine running, then it's a charging problem.
If the voltage is higher and you're still seeing a problem, kill the engine and get a helper. Pull the coil lead, and watch the DMM while the helper cranks the engine for 15 seconds. Battery voltage should not drop below 9.6VDC under crank - if it does, it's a battery problem (just because it's new doesn't mean it's not possibly defective.) If it doesn't, then look into the alternator and regulator.
Since it's agreeably rare for Chrysler regulators to fail (good thing - they're built into the PDC!) I'd be inclined to think it ends up being an alternator issue. However, while the regulator can be back-checked (using the voltage at the field coil terminals on the back of the alternator case,) that's one diagnosis I don't want to make by "remote control."
