2002 Cooper and this thing is KILLING me. Daughter purchased car (her dream car), and I've been tasked with working out all the kinks. So far, it hasn't been bad and the car is enjoyable to work on, but this latest issue has me stumped and pulling my hair out.

The alternator isn't charging whatsoever.
Removed alternator and had tested at Autozone--checked out 'good'.
Eventually found the positive cable from alternator to starter was very loose at the starter and tightened. Figuring this was the fix, I put it all back together--still not charging.
I checked the field/excitation voltage going to the alternator:
Key off: ~0V
Running: -1.4V
Key off, after running: 10V

I've read where these are PWM, but find that hard to believe and assume it's just a 12V excitation current, but am not sure. Should it be receiving 12V (depending on load, of course)? I've also read where there's a grouping of diodes in the dash and, since the 'batt' indicator is illuminated, could this be the cause? (I pulled the speedo/dash, but didn't notice any destroyed components or even any 'grouping' of diodes.)

Any help is appreciated. This is a *NEW* battery, so I can assume it's not the battery. If someone can verify the excitation/field voltage values for me, that might help me narrow it down to the alternator as I'm still not sure that's not the culprit.

 

Do you know in details how Autozone tested/checked the alternator as good?

I am pretty sure that the engine DME has a enable/disable signal going to the alternator, and that should be checked. As to reference to PWM, I think that is only relevant to the regulator which is built-in as a part of the brush assembly. All modern alternator are PWM controlled for load modulation/regulation.

 

The signal from the DME was what I was measuring. It was consistently at -1.4V while running. I'm hoping someone knows what this voltage should be (from the DME) before I take the front end off this thing again.

 

DMM is a very crude tool for diagnosing this problem. To dig in deeper and properly analyze the signal you will really need an oscilloscope. -1.4V has to be a byproduct of the DMM not knowing what to make out of the pulsing or fluctuation signal. Still you should not read negative unless your ground reference is wrong, or suffered from IR drop.

Still waiting for the detail of Autozone "test".

 

Also it begs the question if you pulled the front end to troubleshoot the alternator charging, what tests you performed to satisfy yourself that the alternator is good that you proceed to put the front end back? Did you simply take Autozone's "checked good" as gold?

 

A battery requires at least 13 volts to charge. Below is the link to a page that has the wiring schematics for a R50. What is the voltage on the wire going to the battery?

https://www.newtis.info/tisv2/a/en/r...24-alternator/

 

Hooking up a scope shows a negative signal of -1.4V, with the very occasional (random) positive and negative spike. No discernible waveform whatsoever. The negative voltage definitely seems strange to me, but again, I'm not sure WHAT it should be with this system. Given that, after shutting the engine off, this same signal jumps to 10V and stays there for a bit, I'm assuming I have the polarity of my leads correct.

As for the 'Autozone' test, it's tested for voltage and current output, though I'm not sure how much they load it down.
Battery is getting low and will charge it up over 13V, just in case.