Where to measure alternator output

[Frontside]

I am experiencing the exact same issue and I have been told that the alternator is good. New Alternator was 3 hours old when it would not keep the car running with any aux. on. Got a new battery, thought all was good then turn on lights and a few other aux 54 miles to work and started loosing power. If you have had any luck let me know.

 

[91 PBP 5079]

The best way to test an alt is on the truck. They cant fully load test an alt on the cheap litlle bench testers they have at the parts stores.
You can tell if an alt is good or bad with just a volt meter. But it is nice to have an dc amprobe to check and see what kind of amperage it is putting out. Turn everything on and see what kind of amperage it is puttin out. If the load exceedes the capacity of the alt. It can cause the voltage to go low If the voltage output is low and amps are high. This is Generaly caused buy a shorted battery (battery will be hot and stink) or some other high load. I usally check the charging voltage just off idle. Alt output can very especily with rebiults. You may be able to Check your tachometer signal terminal with the volt meter set to ac. You also may be able to check it with a volt meter with a frequencey setting . I am pretty sure the tach is mesureing frequency not AC voltage. But I could be wrong. Alt can still be Ok but weak or lazy. I know my spelling sucks but I get paid to fix thing not spell.

 

[peter sherman]

Buy a shitty rebuild ya good move!
Buy from Will or have it rebuilt from a GOOOD rebuilder that knows MM alts for baby Jesus sake.

 

[They call me Pete]

I did one better and bought a brand new 120 amp MM. Returned the NAPA one and got my money back. Case closed!!!