Colorado_Al

After doing a tune up, 44K and seafoam my 1999 Cherokee is running like a clock... but last night while driving home the voltage pegged the meter for a few mins then went back to normal... does the 4.0 six have a voltage regulator or is it built into the alternator..

Also has anyone else seen this problem ?

Thanks,
Al

FL Denmark

it is build into the alternator

Colorado_Al

Thanks... I never saw this before... normally when I've had an alternator fail it's been low or no voltage. Is it new alternator time or is the regulator part of the alternator worth trying to repair.

Another strange thing is it was just a brief problem... normally when solid state things fail they stay failed. It wasn't just a gage problem... I have a high voltage alarm in the back and it went off.. also the gage lights ( it was at night ) got very bright.

Al

xj^REMY

no, pre-91 alternators were internally regulated (within the alternator)... post-91 the voltage regulator is incorporated within the PCM

could be a handful of things... how old is the battery?... check ur connections, loose connection could cause intermittent spikes... also, get ur alternator tested

Colorado_Al

Looks to me that it's not built into the alternator but controlled by the ECU / or what ever the black box is called. Looks like some people sell outboard voltage regulators to bypass the ECU control... maybe that's the answer here.

Al

xj^REMY

dont know what black box u'r talking about... the ECU/PCM is next to the air filter box, cast aluminum box with 3 sets of wires plugged into it

y are u so sure its the regulator?? have u cancelled out other culprits & still verified (with mutlimeter or voltmeter) high voltage readings at the battery

lngliv3

Checking the voltage at the battery only tells you the voltage after the alt to the battery... For a more correct reading compare the voltage at both points alt and battery.....
I am a heavy truck technician at a peterbilt dealer I have seen intermitant issues with alt's overcharging the diode trio takes a crap at higher rpms thus causing an overcharging condition.....

Colorado_Al

The "black box" I was talking about is the PCM not ECU... sorry. The batt, connections and battery are all perfect..

The high voltage issue did happen at "relatively" high RPM ( over 3k going around 75 on I70 ). So far it has only happened twice and both were at highway speed.

I've seen advertising for outboard regulators that bypass the control coming from the PCM... have any of you seen these? I don't know what a 1999 PCM costs but I can imagine.

Al

Colorado_Al

So you think replacing the diode trio would fix it... or the PCM is causing the problem... It has only happened at higher rpms so I think you're on the right track.

FL Denmark

i stand corrected next time i do my homework, before i answer

lngliv3

Most alt's made these days have internal voltage regulators hence the diode trio I bleave thats how the old timer explained it to me... the alt's have a bat + terminal a f field terminal and another terminal cant recall what that does. most alts used today have a plug for a 12v source from the ignition key on to energize the field winding to start the charging process.... In my opinion I wouldn't try to repair a alt with out help not having done this b4... U could take it to a rebuilder and let them rebuild it or just replace it with a new reman unit....

Colorado_Al

Where did I give you the idea I would replace the diode pack myself?

Anyway, after doing some of my own tests... I'm tending to believe it's a fault in the PCM... it's kind of a toss to either replace the PCM or add an outboard regulator... I'm tending toward the outboard regulator since I may go with a high output alt when I need to replace it.

WildChicken

I am currently having a similar issue. I replaced the PCM and it didn't fix the issue. So I put the old pcm back on and replaced the battery temp sensor. Still having problems and now it seems to be causing a problem with my transmission.
This issue is a major aggravation as there doesn't seem to be a single fix for it. It could be a combination of things or one simple thing.

boxburn

Welcome to the forums. You'd be best creating your own thread on this as not to confuse your problem with the OP's. Even if it's the same, it's easy to mix up advice otherwise.

dave1123

Anybody think about the battery temp sensor? Underneath the battery there is a temp sensor that tells the PCM to cut back the charge when the battery gets hot. There's probably why your voltage came back after spiking. The alternator is overcharging as you say, but the temp sensor saved the system.

Rebuilding an alternator is easy. All there is are bearings, brushes, the diode trio, and 6 diodes. The diodes are usually okay and can be tested if you want to disconnect them from the matrix. A good multimeter will detect AC in the output which shows at least one is bad. The diode trio is the same of any rate output alternator, the amps are regulated by the size of the windings.