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No charging: Alternator or voltage regulator?

W

Wow

NAXJA Forum User
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Over there!
My '96 XJ only reads about 12v at the battery when it's running. I know the voltage regulator is built into the ECM, is there any way to confirm the alternator is broken rather than the voltage regulator? I REALLY don't want to replace the alternator and then find out the voltage regulator is bad. The bearings on the alternator still seem fine. The battery and cables have been replaced recently, so it's not them.

Also, how long would you think it would take a novice (me) to change the alternator? It looks like I just need to remove the belt and two bolts, and it's out. It can't be that easy, can it? :)
 
It can be that easy...but if it is your first time make sure your level of patience is high. My first alternator (on something harder than my XJ) took 4 hours out and in. The second one I did took only like 2.5-3 hours. Not too tough for a novice like me, either.

You can go to a number of places and have your alternator checked, but I'm sure that someone will have a home remedy for you.
 
Also make sure the belt is tight, real tight.
Also read the voltage with the engine off and the lights on.
That should be 12 v to test your meter and battery.
New batteries go bad too.
 
Last edited:
OR2000XJ said:
You can go to a number of places and have your alternator checked, but I'm sure that someone will have a home remedy for you.
Click to expand...

Yeah, I was thinking about taking it to Autozone for a test, but is their test any more in depth than a voltage check? I think they have a machine to test the alternator off of the vehicle, but if I've got it off already I might as well put a new one back in...
 
Wow said:
Yeah, I was thinking about taking it to Autozone for a test, but is their test any more in depth than a voltage check? I think they have a machine to test the alternator off of the vehicle, but if I've got it off already I might as well put a new one back in...
Click to expand...
Yes because it also test the battery at the same time under load. New batteries fail a lot.
 
But testing under load won't tell me if it's the regulator or the alternator either. It will just tell me if it's charging or not, and I already know it isn't. It's definately not the battery, it runs fine right now on battery power alone. (I just drove it a couple miles)
 
Wow said:
But testing under load won't tell me if it's the regulator or the alternator either. It will just tell me if it's charging or not, and I already know it isn't. It's definately not the battery, it runs fine right now on battery power alone. (I just drove it a couple miles)
Click to expand...
Trust me it does.Why do you think theres a problem?
If your batteries full charged you won't get 14.5 volts after it's full.
 
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I don't recall which is which, but if you have a meter, check the field terminals for voltage. You should see anything from, say, half a volt to four volts there.

This would mean that the regulator is actually putting out power to the field coils (its job,) and that it is therefore likely the alternator is shot.

The sole redeeming quality of the PCM regulator is that it is away from the heat of the alternator, and therefore not subject (mostly) to heat-related failure - the bane of solid-state electronics.

5-90
 
Wow said:
Care to explain how?
Click to expand...

The regulator circuit should detect the drop in voltage, and increase field current accordingly.

The primary job of the voltage regulator is to, obviously, keep alternator output voltage within a specified range. The regulator has nothing to do with output current - it simply detects drops in output/system voltage and increases current/voltage to the alternator field coils to increase alternator output.

I'd not consider testing under load to be fully reliable - but it's better than nothing. I'd sooner directly monitor field coil voltage, and make sure it's keeping pace with system draw (as you've probably already guessed, the field coil voltage should go up alongside system load - no load, no voltage. High load, full field voltage.)

5-90
 
langer1 said:
Why do you think theres a problem?
If your batteries full charged you won't get 14.5 volts after it's full.
Click to expand...

There's a problem because the voltage is only 12.22v and dropping while the engine is running. A load test will only further reinforce the fact that it isn't charging, but it won't isolate the problem to the alternator or the regulator. If I take the alternator out they can test it seperately from the vehicle and the regulator, so it would definately tell me which is broken, but like I said above, if I've already got it out I might as well put a new one back in.
 
Wow said:
Care to explain how?
Click to expand...
Placing a known load across the battery with the engine off and watching the voltage will test the battery. Placing the same load across the battery with the engine running will test the ALT output by forcing it to charge to raise the voltage.
 
Wow said:
There's a problem because the voltage is only 12.22v and dropping while the engine is running. A load test will only further reinforce the fact that it isn't charging, but it won't isolate the problem to the alternator or the regulator. If I take the alternator out they can test it seperately from the vehicle and the regulator, so it would definately tell me which is broken, but like I said above, if I've already got it out I might as well put a new one back in.
Click to expand...
Do you have a yellow top battery?
 
Wow said:
I have a 2 week old Odyssey AGM battery. It is, without a doubt, functioning perfectly.
Click to expand...
Ok same thing, I ask because the SLA batteries never charge high. They recharge so easly you don't see 14.75 volts only for a brief time, if they're full voltage will drop back to 12.50 +-.25.
Do this test also.
Turn your headlights and blower motor on for 5 min's engine off.
Read the battery voltage off. Then start the engine and read it again, it should be at least 3 volts higher.
 
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