Did my alternator just die?

[Bammo68]

Just took the 99 TJ out for a drive to see if I had resolved an unrelated issue with poor idle / hesitation (cleaned out the throttle body, IAC and replaced TPS). All seemed OK but half way up the street the "check gauges" light came on and the battery voltage indicator was showing >10V.

Shut it off and measured voltage on the battery - 12.7V on an 18 month old battery with no obvious issues. Started car, measured voltage at batt - 12.1V whilst running - Uh Oh. Measured voltage on the terminal at the back of the alternator itself - 12.1V. Gauge in car now showing 12V not under 10 but still way down on the 13+ V it normally shows. Weird it dipped so low before going back up to 12.1. Maybe it was shorting the battery to ground for a moment?

I think the battery itself is OK. This was an instantaneous change as I was driving which suggests the alternator just stopped working. If its only 12.1V at the back of the alternator body then does that mean its def the alternator and not the regulator in the PCM? that 12.1V its seeing is just the battery voltage I think.

First photo is the unit in the car - its a DENSO 56005 685AC which I *think* is a 117A unit? Before I go chasing a new one is there anything obvious I am missing here? Car is still starting and running well but obviously not charging the battery so I don't think its anything else and the earth leads all seem pretty secured. I can't help but feel I am missing something obvious here, but that's maybe because I have been battling TPS and IAC gremlins for the last week.

 

[Rickyd]

Some auto parts stores can test alternators if you remove it and bring it in. If it tests good you can focus on wiring or pcm

 

[afusco1222]

Try turning on your headlights, heater, radio, etc and check your battery voltage with a load on it. If it dips below the 12.1, I would replace it. Your tests are already indicating that your alternator is starting to go bad.

 

[Link]

New alternator time! Or rebuild what you have.

 

[Bammo68]

I think the first thing I'm going to do in the morning is check that the ground wire going to the plug at the back of the alternator is good. I'm starting to wonder if some of the recent O2 sensor issues I've been having might relate to a bad ground and Eric O recently did a video on a TJ where the O2 sensor ground was commoned to the alternator control line ground and both were causing faults. If it isn't that then it must be a failing alternator.

 

[TheBoogieman]

When I thought mine was going bad, I drove it to an auto parts store to test the alternator and battery. Jeep cut off and wouldn't run. I replaced the alternator in the parking lot.

 

[Badgerbiter]

https://www.extremeterrain.com/paperf-alternator-160a-0006-2422.html
I was looking at this beauty yesterday. It gets very good reviews.

 

[DuckNut]

Testing a battery without a load is just making yourself feel useful.

 

[Deputy Dodge]

https://www.extremeterrain.com/paperf-alternator-160a-0006-2422.html
I was looking at this beauty yesterday. It gets very good reviews.

That alternator won't fit on a 99. But K-suspension has one that will.

 

[MikeE024]

This was a fantastic tool for me until a borrower didn't return it.

It tests the starter, alternator, and battery. Good for smaller batteries as well (tests down to 40CCA...most I've seen test down to 100CCA).

 

[TheBoogieman]

Stick with Denso.

 

[Bammo68]

Did some more testing today - the control lead to the generator has a good ground so that's not the problem. At this point I reckon I just need to swap it out so will do that once my RockAuto order arrives!

 

[Bammo68]

Ok update time! I removed the battery and mounting plate and checked the battery temperature sensor. It seemed OK and measured 9.1K Ohms which is in-spec. I took the time to reassemble it, took the time to make sure all the connectors on the battery and alternator were clean and fully seated and tightened up the battery terminals themselves.

and

13.9V charging. Everything working perfectly. Unsure what the lesson here is other than always take the time to make sure there isn't a loose connection somewhere. Just because the car was starting OK doesn't mean that all the battery connections are tight for instance. I dunno, I just know that I didnt need to order a new alternator so now I'm happy!

 

[TheBoogieman]

Ok update time! I removed the battery and mounting plate and checked the battery temperature sensor. It seemed OK and measured 9.1K Ohms which is in-spec. I took the time to reassemble it, took the time to make sure all the connectors on the battery and alternator were clean and fully seated and tightened up the battery terminals themselves.

and

13.9V charging. Everything working perfectly. Unsure what the lesson here is other than always take the time to make sure there isn't a loose connection somewhere. Just because the car was starting OK doesn't mean that all the battery connections are tight for instance. I dunno, I just know that I didnt need to order a new alternator so now I'm happy!

Good for you.........................for now.

 

[Bammo68]

In this process I think I have discovered something odd about the way the "Check Gauges" light works. If what I am saying is crazy then can someone please correct me, but the behaviour of both the oil pressure gauge and now the voltmeter is similar.

Note this is on a '99 TJ that has a "proper" oil pressure gauge - I know newer models have a binary type sensor that's either OK or below.

Anyway - the observation is repeatable on both the gauges. When you start and run the car for the first minute or two the gauge shows the actual measured result - in my case, 12V on the battery and about 10 psi on the old motor that had oil pressure issues (at least when the oil got hot, on first startup it was more like 13psi). After a short time, the "check gauges" light comes on and the gauges drops to zero. I confirmed on the battery that it was still at 12V but indicating zero. I think the same thing was happening on the oil pressure - it was indicating zero when in reality it was something like 8-10psi. I'm speculating it does this to draw attention to the issue for the driver. Restarting the car immediately shows the actual measured value again and if it remains below some predetermined threshold it zeroes again some minute or two later.

Can anyone confirm that this is what is actually happening?

 

[Bammo68]

Good for you.........................for now.

You unutterable bastard. You just had to go an say that didn't you @TheBoogieman ? Here I was, happily thinking I had fixed the issue myself by careful effort, testing and good engineering practices and you come along and say "....for now". Let me tell you why you are so completely wrong and misguided. But I will have to do that tomorrow because today I need to replace my alternator after it failed again.

Bastard.

 

[TheBoogieman]

I know newer models have a binary type sensor that's either OK or below.

I thought they were Non-binary before 1/20/2025.

 

[TJ Jim]

I thought they were Non-binary before 1/20/2025.

And you wonder why nobody likes you?

 

[5632]

The simplest way to check the alternator is to disconnect the battery while running and if it stops running the alternator is bad.

 

[macleanflood]

Might want to view and monitor live ODB data. I'd be especially interested if it was an open/closed loop thing and that would be my suspicion.

-Mac