Alternator question...
 

87 Turbo, have 2 alternators (after the original one died):

O'Reilly's reman, diode tests outside the car at about 650mV (at least I think that's how you read it)

Bosch used unit, tests at roughly 1500mV outside the car

Have the Reman unit in the car with a new VR. Battery tests at just shy of 12.6V. Voltage at the battery at idle is just shy of 15 and at 2500 rpm is pushing 16. All the classic symptoms of a failing alternator, e.g. lights brighter under throttle, battery and warning lights on at idle but disappear at about 2500 rpm, but doesn't seem to be draining battery. Positive and negative cables are new, starter is new and the alternator to starter cable looks fine.

Am tempted to just swap in the used Bosch unit, but curious as to why the diode test shows at such a high figure? Would also appreciate any tips or hints about what else I should be looking at, before I dismantle everything yet again : )

I would be doubly sure about the battery first, was it load tested?

Idle voltage of 15 at idle would be charging the hell out of the battery. Something else is amiss.

Just checked mine, fully charged battery, idle, just after starting, 14.05V.

Thanks, guys. Tried a different VR in the reman unit and a new battery, same problem. Also cleaned all the various grounds in the engine bay. Pulled the reman unit and will try the used Bosch unit tomorrow. Am beginning to think that the reman unit just came bad from the parts store, but will know for sure tomorrow. One of our sons has an 86 944 NA and the voltage holds totally steady at just shy of 14V, even under heavy load.

On the plus side, I am getting really good at removing and replacing alternators, I guess. Will at least report back what I find.

Just to close this one out, the alternator from O'Reilly's was definitely bad from the store. Put a new VR and pulley on a used Bosch unit from the Eisenhower era and voltage is rock steady at 13.8v while at idle, and stays there under load and across the rpm range.

I usually have no problems with stuff from the various auto parts stores, but this lesson will for sure stick with me for a while.

I went through five alternators from the parts stores before getting a good one (Bosch, but I tried them all.) Assume they’re going to come in bad; order two and return one type thing.

Order from Zims in Bedford Texas...

I dont buy any parts from Oreilys, autozone, rock-auto or the like, especially not for german cars. 99% junk parts, especially axles. You have better chances with used parts

A lot of the remanufacturing is done in Red China.

Zims...