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Electrical problem... Alternator?

4WDlifeform

NAXJA Forum User
Location
Beaver Falls, PA
I searched this, and just was working on the jeep... I am at a complete loss on this, but I think I have narrowed it down to the alternator... My jeep is 97 Cherokee Sport, 4.0. Battery is a red-top, and good (IMO, not tested).

My dash volt meter at cold start is reading 9v (or less). It has been "warming up", and reaching 14 volts. Lately, I have not been so lucky. It is holding 9 volts, UNTILL I shut her down, then do a warm start, its running at 14v. That doesn't always happen either... Engine off, 12v on the dash. No CEL.

This thread was pretty good for some direction:

http://www.naxja.org/forum/showthread.php?t=1078451&highlight=alternator

To follow my inspection tonight, I have checked all my grounds, related to engine/alternator. All tested well, but, cleaned them all anyhow. Battery terminals reading 12 and change... Engine running, terminals reading 13.8 when gauge showing 14v.

The only thing I did not check, is what the terminals measure when the dash is showing 9v, engine running. WHEN that happens (probably tomorrow morning), I will hit it with the volt meter. What should I see at the terminals? Probably 12ish?? What will that tell me... Alternator not charging?

Also read something about lose brushes? I am going to carry a hammer with me tomorrow as well, and give it a nice love tap if she acts up! The dash is either a solid 9 or 14; not fluctuating as if something is lose.

... and what is a PCM?

Thoughts?? Any advice for a new alternator!? If I am forced into a new one, I'm upgrading!!
 
if its 12 or less at the terminals with the engine running you got a bad alternator 99% of the time. The pcm does regulate the voltage on these.

if you checked all the grounds and cleaned everything and the battery tests good, your weak link is the alternator.. unless the pcm is sending bad input to the regulator but i doubt that.

Always check the fuses though for everything. Also on the 97+ sometimes the cluster says weird things from a poor connection, remove it and clean the terminals and put some dielectric grease in there to play it safe.
 
more than likley its an alternator. Disconnect the battery. measure the resistance of the battery cables should be a few tenths of an ohm end to end, postive to alt, postive to starter, ground to block. If thats ok- I would look at the alternator, take it out and have it tested at advance auto- if its bad you got the core out already- if it isn't then its well a PCM. My buddies 93 just needed a PCM for a regulator issue uncommon but not unheard of.
 
PCM = Pulse Code Modulation. It is the way in which (the) computer/s interprets and communicates with analog devices. On the XJ, the alternator is an analog device in that it sends a DC (Direct Current) to the battery after it is switched by the rectifier/s. The computer sends a pulse of electricity of a very short duration depending on the load demand on the alternator possibly nano seconds to turn on and off the alternator output. The longer the pulse duration, the more the alternator put out.

My XJ is a 1996 so it do not have the same connection at the dash but based on the experiece of others on this forum your problems could be the dash connection. You should check the voltage at the battery just before starting and immediately after starting in addition to the suggestions given in the other posts. The results should give you a good starting point.

if its 12 or less at the terminals with the engine running you got a bad alternator 99% of the time. The pcm does regulate the voltage on these.

if you checked all the grounds and cleaned everything and the battery tests good, your weak link is the alternator.. unless the pcm is sending bad input to the regulator but i doubt that.

Always check the fuses though for everything. Also on the 97+ sometimes the cluster says weird things from a poor connection, remove it and clean the terminals and put some dielectric grease in there to play it safe.
 
PCM = Powertrain Control Module... You are thinkink PWM whicn is Pulse Width Modulation. Commonly used to drive Dc Motors at less than full speed whilst retaining torque.

The alternator on the 97 is controlled by the PCM in the same exact manner as an external regulator would. From what the OP is reporting, I suspect worn brushes. Problem with tacking it off and having it tested is that the brushes may be making full contact and so the alternator will test good.
 
PCM = Powertrain Control Module... You are thinkink PWM whicn is Pulse Width Modulation. Commonly used to drive Dc Motors at less than full speed whilst retaining torque.

The alternator on the 97 is controlled by the PCM in the same exact manner as an external regulator would. From what the OP is reporting, I suspect worn brushes. Problem with tacking it off and having it tested is that the brushes may be making full contact and so the alternator will test good.

I have confirmed, that it IS in fact the brushes!! Started it cold this morning, running at 12ish.. Tapped it with a hammer, right to 14! Did it twice today... Doesn't even need hit that hard, just a nice firm PING!

So, how long do I have? I have at least a 3 hour trip planed tomorrow, and a small trail ride in the middle of that...

I guess I should start shopping around for a new alt. Any suggestions? Willing to upgrade...

Thanks all for your suggestions!
 
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