Question about alternators fooling tests.

Ok,

Jeeps been fine for a while now, I know how could that be. Anyways yesterday driving to work CEL was on, said ok I'll get it scanned after work. Stop for a bagel, get in car starts fine but wants to die.

Unless I'm giving it gas and keeping the revs up the car tries to die. As soon as the heat comes on or lights the volts on the in car meter drop. So I was pretty sure it was a good battery as the car started every time. Well I limped it to Advanced Auto today where the Battery, Starter, and Alternator all passed the test.

But then I came across a thread where, it was said that 91+ alternators will pass the test when they are actually bad.

Anyways 2000 Jeep Cherokee 120k

Terminals look clean also.

Any ideas?

Your statement that "terminals look clean" leads me to believe that you have not actually disassemble them, cleaned them and securely reattached them. You very well may not really have good connections just because they "look" good.

I'll go try that, but they were cleaned not to long ago.

Ok, just checked my grounds and terminals and they all looked fine.

Whats got me so confused is:

1) Check Engine Light went off

2) It will run fine for about the first ten minutes of driving - The way to Advanced it would die in the lot
I just went out and started it up and it ran fine for the five minutes I sat in it.

Check your voltage at the battery with a multimeter. It should be about 12.5 with motor off, 14volts with motor running. Most likely cause of problem running after warmed up is the CKP (crank position sensor). May as well unplug and replug a few times or even use contact cleaner on it to make sure you have a good connection there.

Next time it won't run, check for spark.

Man I need to send you a big paypal payment. You've saved my butt more than a few times. I don't know why I didn't put two and two together with that. I'm going to go run it a few times and see if I can clean it off.

Best place for replacement anyone?

Did nothing and it decided it just wanted to drive around for 20 minutes with no problems. This thing is driving me crazy.

Pull DTCs anyhow - even if the CEL/MIL goes out, the ECU will still store the code as a "History" measure. That way, you can see what was wrong.

The CEL/MIL goes out when the ECU detects that the fault does not recur after a certain number of run cycles - how many depends on the nature of the fault.

As far as the alternator goes, find a local shop that does nothing but alternators and starters, and dismount the thing and take it in. They usually have a bench that will wring the thing out but good, and can tell you exactly what, if anything, is actually wrong. I don't trust the parts house benches any farther than I can toss them (I know - I used to run one.)

Thanks, I'm thinking the alternator isn't the problem but I'm going to run through a bunch of tests tomorrow. Hopefully I can still see what the code was to help diagnose this a little bit.

Pull the codes as the computer will tell you what it thought the problem was. Ditto on getting a real test done on the alternator. They can test bad, but still have a bad diode that causing the battery to drain while sitting. Your symptoms certainly sound like a weak alternator.

Cleaning up the battery connections is a good idea, but that usually shows up as poor cranking.

Went and worked on his Jeep this morning to try and get it figured out.

Idle was low...around 630-650 according to the scanner I borrowed from my dad. Other than low idle, there were no other symptoms. Jeep stayed running fine on its own, both cold and warm. Pulled the IAC off to clean it, much to our surprise it was pretty clean but decided to give it a shot anyways. Put it back on and fired up the Jeep, immediate CEL with still poor idle. The code I pulled was a P0503 if I remember the number right...it was "Idle Control System Malfunction." Verified the connector was tight, reset the light and fired the Jeep up again. No more CEL but still poor idle. Looked at the TPS reading on my scanner with the engine running and it was around 16% at idle, which seemed low compared to other 4.0's that I've used this scanner on. For kicks, I threw it on my 99 and my TPS which is less than 6 months old came up 18-20%.

Long story short, after a couple of rounded off TPS torx bolts, a trip to Pull a Part in the pouring rain for a 96+ Throttle Body, we got a new TPS on and it seems to have helped. Idle is just a tad on the low side still, but it's steady now where as before it was slightly erratic.