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View Full Version : Testing an alternator - Rocket Science!



632 Regal
01-20-2018, 08:48 PM
Went to the local auto parts stores to have an alternator checked... Simple procedure. Basic Bosch alternator, one 12 volt connector, a field and ground to the case. Electric motor a gauge and a belt and anyone can do this.

http://www.bimmernut.com/forum/attachment.php?attachmentid=8603&d=1516496440
8603
Advance auto. Fail before it started, could it be those tester machine extra 2 plug in wires that the alternator does not have a connection for? Other than that the 3 other wires were correct. Fail Fail and Fail, the machine never even started to spin the alternator. Try to sell me a new one even though...

Next we go to Autozone, they have to be smarter cause they advertise it all the time. Were kinda on the way but not so lets try anyway. Looked to be hooked up right for the 3 wires but the other 2 that had nowhere to go? Why do these guys insist they know everything when they don't? Machine kept asking if the white wire was connected. Broke the machine on test. Actually popped a breaker that they can't reset cause the box is locked. Belt looked jammed to the side where it was binding on the guard. Try to sell me a new one anyways.

On the way back there are a couple more parts stores, lets see if this thing works or what. I have the bearing kit, voltage regulator and a new slip ring ready to go but want to make sure it is worth rebuilding. Already took it apart and the only possible thing that 'could' be bad is the regulator.

O'Reily auto! For the WIN! Not really. The guy seems confident and seems calm with me standing there by him. Busy store, lots of disruptions. Checked stuff out, seems 2 extra wires have nowhere to go but might have been joined in the connector as one of those 'if not this than that' kind of connector. First run the belt was too tight or something, made a lot of noise. Hooked it up so the belt was loose, could hear things were not right. I asked him a few times if it is spinning and he affirmed yes. Anyways he pulled it out and said the pulley was warm, it shouldn't be so that's whats wrong. Machine said fail fail fail. Loose belt couldn't cause heat on the pulley?

Another Advance auto on the way back. Different machine in this store, very confident guy. Had some connector with 1 extra wire and couldn't figure where it went. I told him this alternator only needs 3 wires to test, battery, field and ground to the case. Goes back to the computer and pulls a 7 wire connector out, I was way too perplexed for this. I told him we only need 3 wires and he countered with, "not for this alternator" "I looked it up" I wanted to just grab my alternator and leave. Anyways I act like a consumer believing his superior intelligence knowing I could check this thing in 5 minutes with no instructions. I was walking around and came back, this guy actually took the back plastic cover off the alternator and put the alternator back into the machine THEN crammed the white white onto a field wire to a diode. My goodness! there has got to be awards for people like this. I walked away again cause I couldn't bear the stupidity. He eventually walked back out and told me "it's only 'getting' 8.1 volts and won't be able to 'crank' anything". Why didn't he turn the machine on?

WTF is that? So there is only one more parts store around, think it's a NAPA and will check when I get out that way.

What a scene. Why is a 2 wire with ground alternator so hard to test? 3 of the 4 places claimed to never see an alternator like it. My goodness these were more than common in the '70s although not Bosch AFAIK. These 'advanced' E34 alternators must be so complex that the major auto part testers cannot handle. Looked it up on realoem and this style alternator is on like 5 bazillion different vehicles. If it was simple to replace I would and test it live but spending 2-3 hours to "test" something seems like useless effort.

Was not going to even say anything here but after a simple check today I was astounded at the results of what would seem to be a simple test. My goodness I should have said it was from a 1970 chevy.

shogun
01-21-2018, 05:22 AM
if you want to do some bench testing DIY, here are the instructions for a 1990 Audi 100 with a Bosch alternator http://b2resource.com/PDFs/1990audi100alternatorrepair.pdf

632 Regal
01-21-2018, 07:46 PM
There is no capacitor on this alternator, not sure if it makes a difference. Also the one I want to test is not in a car, if it was I would know if it works. My current alternator is getting weak, or old and decrepit, probably worn brushes but time to refresh for another 200K miles. Don't want to press bearings on where I need a wheel puller to remove it, ruin the bearing race. I should just put the new stuff on my current alternator but if I slip I am out of a vehicle. And that's my only transportation at the moment.

shogun
01-21-2018, 11:25 PM
Maybe you can fabricate a cheapo bench tester? Problem will be to to simulate the exciter function. It is used to excite the field windings of the alternator, which in turn creates the amount of amps.
To turn the alternator should be possible with a power drill.

632 Regal
01-27-2018, 08:33 PM
Maybe you can fabricate a cheapo bench tester? Problem will be to to simulate the exciter function. It is used to excite the field windings of the alternator, which in turn creates the amount of amps.
To turn the alternator should be possible with a power drill.

I have a dead battery for the batt and field terminal, ground would go to the case, I can rig something up but have no way of checking the amp output. You would think out of 4 different stores I would get at least a single conclusive test. Next time I will tell them it is for a 1970 chevy, should make it far less complicated. Mention BMW and the world gets impossible to function in.