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High voltage alternator

Wolfe

NAXJA Forum User
My alternator just pumped out 39 volts blowing the main fuse. This is the second time this has happened. The first time it happened I replaced the regulator and loom and its been good for almost a year. Could it be a faulty regulator again or should I be looking for another reason why I'm blowing the regulator?
 
I'm guessing we are talking about the 96 here? The regulator is in the PCM (main computer).

The alternator regulates on the ground side of the alternator. so if the wire from the alternator to the PCM shorts, you get a runaway alternator. That would be my first check, wiggle the harness with the PCM unplugged, the dark green wire at the alternator unplugged and see if the dark green wire is shorted to ground someplace between the alternator and the PCM.

When you disconnect the battery be careful. not good to wiggle the battery clamps, this has been know to ruin a regulator. Disconnect the battery before disconnecting the PCM.

I guess anything is possible, but the typical regulator failure is very low or no voltage.

You might want to test the alternator itself, pull the fuse disconnect the green and orange wire and the dark green wire, check one of these connectors at the alternator to ground with an ohm meter. Possible the field winding is shorting to the case.
 
Its my sons 1995 Xj. I've changed alternators twice and changed the loom twice. I thought the regulator was the black rectangular box of the back of the alternator but you are saying the regulator is in the pcm? is this correct? This seems to happen after the battery has been disconnected for some time, just changed faulty fuel pump. Could it be a fault in the pcm? I'll look for a fault to earth then reset pcm again failing that I have access to some second hand PCM's so I may check them.
 
The black rectangular box on the back of the alternator is either a connector (if it bolts onto the outside) or just a rectifier pack and wiring if it's the internal part. On OBD XJs (I don't know how it's set up on RENIX ones) the regulator is built into the ECU.

What he is saying is that if the wiring harness wire for the switched field terminal (there are two - one is wired to the output of the ASD relay and is 12 volts at all times while the vehicle is running, the other goes to the regulator output on the ECU, this is the switched terminal) gets shorted to ground somewhere between the ECU and the alternator, the alternator will provide max output until something releases the magic smoke. The short to ground is probably an intermittent coming from a worn spot in the wire shorting against something, that's why it seems to happen at a random.
 
The black rectangular box on the back of the alternator is either a connector (if it bolts onto the outside) or just a rectifier pack and wiring if it's the internal part. On OBD XJs (I don't know how it's set up on RENIX ones) the regulator is built into the ECU.

What he is saying is that if the wiring harness wire for the switched field terminal (there are two - one is wired to the output of the ASD relay and is 12 volts at all times while the vehicle is running, the other goes to the regulator output on the ECU, this is the switched terminal) gets shorted to ground somewhere between the ECU and the alternator, the alternator will provide max output until something releases the magic smoke. The short to ground is probably an intermittent coming from a worn spot in the wire shorting against something, that's why it seems to happen at a random.

Thanks there's nothing obvious but I'll continue to look for the fault, you don't think it could be the ECU? do you?
 
It could be, but I'd put my bets on wiring. ECUs are usually pretty hard to kill. It does happen, just not frequently.
 
The black rectangular box on the back of the alternator is either a connector (if it bolts onto the outside) or just a rectifier pack and wiring if it's the internal part. On OBD XJs (I don't know how it's set up on RENIX ones) the regulator is built into the ECU.

What he is saying is that if the wiring harness wire for the switched field terminal (there are two - one is wired to the output of the ASD relay and is 12 volts at all times while the vehicle is running, the other goes to the regulator output on the ECU, this is the switched terminal) gets shorted to ground somewhere between the ECU and the alternator, the alternator will provide max output until something releases the magic smoke. The short to ground is probably an intermittent coming from a worn spot in the wire shorting against something, that's why it seems to happen at a random.

It's a moulded rubber block, no electronics enclosed. It's there for the "convenience" of the technician - apparently, ChryCo things wrenches are too dim to figure out which wire goes where.

If you're genuinely blowing out high voltage, that probably means that you've got a fault somewhere that is over-energising the regulator and causing the high voltage condition (feeding the full 12VDC to the field coils on an alternator usually nets you a few hundred amps at 90VAC or so - AC, because the diodes short through in short order, and stop rectifying the AC voltage into DC (supplying rich, chunky AC volts to electronics that don't like AC voltage!)

Stupid question - how do you know you're getting 39V when it fails? How have you determined this? (I only ask out of interest - I know the IP voltmeter won't read that high, and I don't trust it for diagnostic purposes anyhow.)
 
It's a moulded rubber block, no electronics enclosed. It's there for the "convenience" of the technician - apparently, ChryCo things wrenches are too dim to figure out which wire goes where.

If you're genuinely blowing out high voltage, that probably means that you've got a fault somewhere that is over-energising the regulator and causing the high voltage condition (feeding the full 12VDC to the field coils on an alternator usually nets you a few hundred amps at 90VAC or so - AC, because the diodes short through in short order, and stop rectifying the AC voltage into DC (supplying rich, chunky AC volts to electronics that don't like AC voltage!)

Stupid question - how do you know you're getting 39V when it fails? How have you determined this? (I only ask out of interest - I know the IP voltmeter won't read that high, and I don't trust it for diagnostic purposes anyhow.)

I've used my multimeter to measure it of the back of the alternator and at the fuse terminal or what's left of it. I know this is out of left field but could loose terminals contribute to the ECU sensing a high load and pumping out the high voltage to overcome the high resistance? The condition of the battery clamps leave a lot to be desired. I've just bought new ones to replace them.
 
I've used my multimeter to measure it of the back of the alternator and at the fuse terminal or what's left of it. I know this is out of left field but could loose terminals contribute to the ECU sensing a high load and pumping out the high voltage to overcome the high resistance? The condition of the battery clamps leave a lot to be desired. I've just bought new ones to replace them.

Just checking.

I suppose that's possible - I'm not sure where the PCM picks up its voltage sense signal to feed the regulator anyhow.

It's just weird that you're getting high voltage rather than high current - I've not seen a fault like that before. At least, not without identifying a wiring fault that shorted the field to the +12V line, bypassing the regulator. Weird...
 
I had high output resulting from a loose + battery terminal. My guess is that the ecu sensed a lack of load current and upped the voltage. Tightened the loose terminal clamp and that fixed it. I dont recall the voltage level as i only saw the guage was close to pegged.
 
I had high output resulting from a loose + battery terminal. My guess is that the ecu sensed a lack of load current and upped the voltage. Tightened the loose terminal clamp and that fixed it. I dont recall the voltage level as i only saw the guage was close to pegged.

I had the same when a cell went bad in my battery (96 XJ), the regulator tried to compensate. But it stopped at 15-16 volts, guesstimate. I've seen batteries blow form overcharging. The old Delco alternators could have a failed internal regulator and charge to around 36 volts, seen it happen on numerous occasions, mostly in Chevys. You should have a Denso alternator with an external regulator. Seems like the engineers would have some sort of fail safe in the regulator (learned from past mistakes). A grounded field winding seems like it might allow a runaway alternator. Or like 5-90 suggested, your meter is reading AC voltage and you have a failed diode and/or your meter is measuring peak voltage and not RMS voltage.

In my experience most anything is possible, some things are more likely than others.
 
The failsafe is actually there, it's just a fusible link or fuse in the B+ wire from the alternator to the battery. Tough to really do anything else without overcomplicating things.

Since the B+ supply from the alt goes directly to the battery, as long as you leave the battery connected, a bad alternator, regulator, or shorted-to-ground field control lead should simply blow the B+ fuse. If the battery isn't connected, it can smoke all kinds of other important things in your electrical system - basically, anything solid state (ECU, TCU, ICM if present, instrument panel, ABS module, radio, ACM... plus any borderline parts such as nearly dead ignition coils) if you get unlucky. Yet another reason to not use the old carbed vehicle trick of disconnecting the battery once the vehicle is running to see if the alternator works.
 
Changed ECU fault continues, tightened battery terminals faulty continued. Disconnected the alternator loom at junction plug and reconnected and system worked fine then checked with multimeter it read 14 volts had a friend rev the engine up and the fuse blow checked alternator voltage back up to 39 volts and noticed that when the RPM's increased so did the voltage to as high as 90volts !!!. I plan to try a different alternator next.
 
I've seen a few cases of voltage regulator portion of the ECU failing. You can wire in a cheap voltage regulator from an old Mopar and get by:

pic2.gif


I actually have one of those regulators in my red Jeep now, I have an HO alternator with a Renix system. (Renix alternators were internally regulated).
 
I've seen a few cases of voltage regulator portion of the ECU failing. You can wire in a cheap voltage regulator from an old Mopar and get by:

pic2.gif


I actually have one of those regulators in my red Jeep now, I have an HO alternator with a Renix system. (Renix alternators were internally regulated).

Electrically, it will work.

However, you'll end up with a persistent CEL/MIL you won't be able to get rid of. This won't impact the charging function at all, but it can mask other issues that would trip the CEL, so I'd advise pulling codes about once a quarter just to make sure.
 
I don't really know, you got me. You have the feed voltage to the alternator from the ASD relay. You seem to be OK there. You have the ground side of the field winding which controls the voltage/amps, If the windings are grounded it can cause a runaway alternator.

The regulator in the PCM, ups the voltage when it detects a low voltage condition. Is it possible the voltage feed to the PCM is flaky and you are constantly getting low volts? It will work with around 9 volts going in, not well but it will work, maybe the regulator is constantly getting a low voltage reading?

Maybe the field winding wire to the regulator is shorting inside the harness, to another wire. Two wires shorting together? By now I think I'd cut the alternator to regulator wire out of there and try a new wire.

I'd also try another meter.

I'm out of ideas.
 
I still think it is an intermittent short to ground on the field winding wire to the ECU. Why? Here's my theory.

Everything is fine. Then the field winding control wire shorts to ground, causing the alternator to put out max power. This blows the fuse. Now the ECU sees no charge output because the fuse is blown, so it cranks the alternator to max output with no effect. By the time you shut the thing off and measure it, the field winding control wire short has vibrated enough that it comes loose again. So you scratch your head and put in another fuse and the cycle repeats itself.

Intermittents are a real bastard to track down because of things like this.
 
I think the idea of running a replacement wire is a good one i'll try that. Am I on the right track if I cut the green wire at both ends an replace with a new wire in between?
 
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