98 lumina alternator plroblem

[tennykimble]

i replace my -- working-- alternator (due to failed bearings) and the new one will not charge! i took then new one back to napa they said its charging good. i put a new battery (old one way 7 years old!) in the car and that didnt seem to have any affect. after quite a bit of reading in the repair manuals it comes down to i need to hook the car to a tech2 and "command " the generator "on" and that will give the 1 small wire ( only 1 wire in rectangle plug ) on the back of the alternator 12v, and if that happens the pcm is good. and if it dosent the pcm is bad. i checked the voltage at the 1 small wire (in the plug) and it has 5 volts with the key in the on position. i did put 12v to that one small terminal and it did charge, BUT it loaded down the engine pretty hard. so my question is how much voltage SHOULD the small wire get ? (under normal operation conditions?)

i find it hard to believe the pcm just went bad between alternator changes

[jeffcoslacker]

I can't help you specifically with the nominal voltage at that wire, but have you checked the voltage at the large stud/wire on the back of the alternator while it's running?

Should have 13.8-14.5v, if you aren't seeing same at the battery while it's running, you burned a fusible link on the charging wire.

But if you see battery voltage or less at the charge stud, you are on the right track thinking the reference voltage to the PCM is corrupt or something...

[tennykimble]

yes i checked the fuseable link - its good. the voltage is the same at the alt & batt when the car is running. about 11.5 volts at ( any rpm ) also i dis-connected the batt and did a conituity test between the two ends of the battery cable from the alt to the batt came up good -- with almost no resistance.

i recently talked to an automotive electrical guy he said it typical that gm uses 5volts to control the alternator, so that leads me back to the alternator being at fault. i went back to napa (again) and they are gonna order another alternator and see if theirs something differant between the two. they said they would also order a 99 lumia alt because it is a similar mounting pattern. luckily i got a guy that understands the problem!

[jeffcoslacker]

If it comes down to it, you may have to check the continuity of that reference wire from the alt to the PCM, if you can determine which pin it is at that end....there has been a few reported here that were bad and causing a problem like this.

[tennykimble]

thought of that too. i skinned the wire about 2 inches out of the pcm. 5 volts everywhere i checked. almost no resistance between the two ends. it does have connitunity, but no resistance -- something like .0001 resistance.

i still think its the incorrect alternartor. im thinking maybe the car was made in later 98 and possibly has a 99 alternator on it???

good thought though! --- thanks for the suggestions! im running out of ideas! ha ha

[tennykimble]

well, i was right... it was the wrong alternator, we ordered a 1999 lumina alternator and IT WORKED!! 14.3 volts! ( on my 1998 lumina. ) who knows why?!?!?

[ricebike]

wow, thanks for that update

guess we should check the date code of when the vehicle was made...

i came late in the thread, but as a parts person in the past, I did came across some of these exact problems with the manufacturer using parts from the next year