Hot Engine Bay + Idle = LOW Voltage

Well guys, I searched and searched. I've done everything I can, and the problem remains.

Situation:

When the engine bay is warm, voltage gauge drops to 10v. Dash lights dim substantially. Blower motor slows down, and jeep idles very low (about 500rpms).

What I've done:

New Batt
New Alternator
All new cables, 2 gauge, custom. Including alternator to PDC. Battery to chassis ground.
Aux fuse panel wired with 0 gauge. (Which powers blower motor too)


What could this be? When the engine bay gets warm, the voltage drops and every dims/slows down.

Voltage at the battery appears to be fine, and I know the dash gauge is very inaccurate, but clearly there is a significant reduction in power because of the symptoms.

Any ideas?!?!?!

5-90! This is your territory!

What about increased RPMs and driving, do you get your power back, what about when/if you drive and come to idle again?
Does that mean you got a new alt. to fix this problem or you bought a new one recently? What kind? Rebuilt or new?

What is the voltage reading at the battery posts when idling with all lights and a/c/fan on? And what year is your xj?

Kyung

Yeah, a slight increase in RPMs (to say, about 850rpms) brings everything back to normal.

I bought a reman from Autozone. No chance whatsoever. Ammeter shows it is functioning properly.

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Last time I checked, it is fine, about 13.7VDC with heavy load. So clearly the power is there, just not getting where it should.


90XJ 4.0L Auto

Just for fun, try this...

Your RENIX XJ has a Delco (internally-regulated) alternator, and the regulator is in the rear case of the alternator. Let your rig idle, and watch a DMM connected to the battery. When it drops, aim a fan (or some sort of fresh air blower) at the back of the alternator case. See if that brings your voltage back up. DO NOT ADJUST THE THROTTLE WHEN YOU DO THIS - just let the engine idle.

Just because you "just bought a new alternator from AutoZone" doesn't mean it's really any good - I wasted three years at Kragen's, and I know what "mass chain" parts are like. For starters, they're bought for about 15 points, by the Conex box - from the Pacific Rim.

Anyhow, you may be getting a heat-related failure with your regulator. I've seen it happen - it's agreeably rare, but it does happen (I think I've seen it about five times in the best part of 30 years.) When you bring your idle up, you spin the fan for the engine faster, and you spin the alternator (which has its own fan) faster - so you move more air, and the problem "goes away."

Heat does have an effect on solid-state electronics - like your alternator regulator.

And, now, you probably understand why I don't buy from VatoBone, or Advance, or Kragens - I'd rather deal with smaller shops.

If that test brings your idle voltage/current back up, then you've got a failing regulator. Take the thing back, get your money back, and just go to Rod this time and get the job done right. I can count his failures on the fingers of one nose...

I'll definitely try that tomorrow. I'll post my findings. Thanks as always.

I should also mention, the problem was the same before with the old alternator. I only changed the alternator because I got it free from autozone.

5-90 said:

Just because you "just bought a new alternator from AutoZone" doesn't mean it's really any good - I wasted three years at Kragen's, and I know what "mass chain" parts are like. For starters, they're bought for about 15 points, by the Conex box - from the Pacific Rim

Click to expand...

Saw that coming from a mile away:roll:

Chances of two alternators with the exact same problem? I'd start checking my connections at the PDC, check clean and bolt down well any grounds...

How'd you get a free alternator?

Blaine B. said:

How'd you get a free alternator?

Click to expand...

He stole it, what he failed to realize though is that he was actually doing them a favor...

EMSJEEP said:

Saw that coming from a mile away:roll:

Chances of two alternators with the exact same problem? I'd start checking my connections at the PDC, check clean and bolt down well any grounds...

Click to expand...

The PDC is immaterial in this case - and it's under the IP in his case, anyhow.

RENIX uses an internally-regulated Delco CS-130, they didn't start with the Nippondenso until 1991. His is the last year for the internally-regulated job, so his regulator is still in the back of the alternator.

EMSJEEP said:

He stole it, what he failed to realize though is that he was actually doing them a favor...

Click to expand...

:yelclap:

EMSJEEP said:

Saw that coming from a mile away:roll:

Chances of two alternators with the exact same problem? I'd start checking my connections at the PDC, check clean and bolt down well any grounds...

Click to expand...

Adtually, better odds than you'd think. In automotive electronics, bad parts tend to run in small batches, rather than just being singles.

5-90 said:

Actually, better odds than you'd think. In automotive electronics, bad parts tend to run in small batches, rather than just being singles.

Click to expand...

I was under the impression that he replaced the stock unit with the free one just because, not that they were related in any way or manufactured together or even within 10 years of eachother?

EMSJEEP said:

I was under the impression that he replaced the stock unit with the free one just because, not that they were related in any way or manufactured together or even within 10 years of eachother?

Click to expand...

True - but volume rebuilders tend to dump parts in bins, and they're picked out at random when they're working on things. I'm not saying it's at all likely, but that is is possible. For all I know, I could be talking through my hat...

However, the original check is borne of experience - I have seen that particular problem before. Typically happens when a cheap regulator is used - I've seen it once on Delco SI, once on Delco CS, and I think twice on internally-regulated Fords...

If it isn't the alternator, you have to do some random voltage checks. Try some of the fuses to ground. Most every red wire is constant power, if they are low it's the fusible link, a splice or a connector. Most of the yellow wires (but not all) are switched current, typically after the ignition switch.
The little yellow wire at the back of your alternator has to have near battery voltage (with the key in run), if the voltage is very low or non existent it will shut down the charging.
it may well be the alternator, but can also be in the wiring.
There was a TSB way back when for the early Renix, about that yellow wire at the back of the alternator. Clean of the old dielectric grease and make sure the connector is tight.
I don't really understand how you can get a 10V drop and/or where. If the battery drops 10V it's deader than chit. If it drops ten volts in the cabin, you have a very bad conection somewhere or a dead circuit and/or you are reading standing current and/or the left over juice from a light bulb somewhere.