Charging issue

[Roler]

It's been a while, and so it was for the XJ
Had been sitting a few months, charged the battery, did some pre-start fuel conditioning and started up fine.

As the engine was idling, did some testing in the engine bay and noticed by battery voltage being below 12.4.
Putting increased load on it, made it even go to 12V. Jeep kept running pretty much the same during this.
Increasing the rpm's would bring it up to 13.8 Volts, what it has always done (never has charged over 13.9 in the past 12 years, even with the previous alt in there) . Letting it idle back down would drop the batt voltage again to below starting voltage.

It is a 2nd-hand alternator from the junkyard, about 2 years ago.

- Cleaned up all the grounds, including the 2 on the coilmount.
- Did a load test on the battery, reading remained within good range.

- Did voltage drop test: No voltage drop over the positive or negative side between alt and battery
(about 0.006V)

- B-terminal showed batt voltage with engine running idle and with increased rpm's.

- Gave the alt a few loving taps, in case it would " free up" something.



So in short: it doesn't charge the battery at idle, but seems to be able to put out enough when raising the rpm's


I think the voltage regulation ( from the ECU) could possibly at fault, but this would be less likely as I understand from reading up. It would mean that only at idle the ECU doesn't tell the alt to start charging, which I think would be unlikely (but its a Jeep)

Alternator the cause?

Anything else I could or should check?

Cheers

[LuluBelle]

time for a new one it should charge at at least 13.8 on idle

[layback40]

Could be crap on brushes. I once had a spider nest in an alt. It caused a problem like you see. Easy to remove the brush holder. Just have a look.

[Roler]

Can never hurt to take ii off and clean it up to see if the personal TLC is accepted as make up for letting it sit for months...

I have not had much luck to find some info about how to check the ECU'S 'signal' to the alt to tell it to start charging. Probably probing some connection at the ECU or at the alt (or both) I reckon to get some voltage reading. I just like to check to make sure (and to learn a bit more). Anyone some advice on that?

[Roler]

Just an update
After it finally crapped itself giving of a little puff of smoke, ordered a new alt from Rockauto and all is well now. Charging 13.9V at idle.

[eksjay]

Out of curiosity, what brand was the old one? What brand is the new one?

I replaced a properly reconditioned Denso (the original) for one my mechanic said was made in China. I still have the Denso unit just in case the Chinese one craps itself. The Chinese made one has been going for 3 years without a problem.

[Roler]

Old one was a replacement from the junkyard. OEM sticker on it. Lasted not long: sitting in an unused vehicle is probably the worst for an alt.

Now went with brand new alt instead of a reman. Was done with changing it.
TYC. Came with test results and all.