Service life of diodes is indefinite....

[Jack H.]

Meaning they'll last and last. What CAN happen is a given diode can suffer junction puncture and act 'flakey' at elevated temperature and high current flow. These are tricky to diagnose, because you have to recreate the actual stress of working conditions and not just run a 'go/no-no' test with the low voltage DC generated by typical multi-meters. Even more 'hideous' with a diode that's suffered junction puncture is it'll tend to 'heal' itself when left to sit for a period of time without bias voltage across it.

The only way to really tell the health of a diode is to run it on a curve tracer across its rated dynamic operating range OR put the fully assembled alternator on an exercise machine, get it hot and cycle it through its RPM range while monitoring the output waveform(s) on an oscilloscope for uniformity.

Most auto parts stores will sell you alternator rebuild components including individual diodes, brushes, and bearings. You can also solve the 'dated diode' issue by purchasing a 'core' alternator from a scrap yard that was built in the same timeframe as yours and robbing it for its diode(s). Regardless of the alternator's rated current output, all diodes are the same and therefore interchangable (it's the rotor and field windings that dictate the rated current output for a given alternator)....

[Terry M.]

[John G.]

Re: Lab scope to test alternator

I would bet an auto electric business in town should also have those parts. I would have sent the alternator out to be rebuilt but I have it in mind to do-it-myself and learn something along the way.

Gene - I tested the diodes as per the motor's manual test procedure using an ohm meter. One 'high' reading - switch leads - then one 'low' reading denoted a good diode according to the manual. There was one diode that read 'low' with test leads alternated between terminal end and diode case. That was the reason I judged that one diode to be bad. The inked numbers on the backs most of the diodes is barely discernable now, with the exception of one diode. BTW, I never knew these diodes had date codes .. ! ..

The test procedure outlined in the motors manual seems lame compared to testing 'hot and running'. Jack and Terry make excellent points.

Thanks for all replies ..
John

[Eugene B.]

Re: Lab scope to test alternator

John,
Before I disassembled my alternator, I took it to the electric shop and had it tested. I wanted a reference point so that I would know exactly what I was starting with.

If you are doing the rebuild yourself, be careful when you remove the nuts from the plate that holds three of the diodes. Mine was so tight that I broke one of the diode wires. Hence, the need to reweld it. Getting the pulley off can also be an issue.

All things considered, I'm not sure that I would do this rebuild again. It may be a job that is better left to the pros. Especially if you have the car judged.

Good luck,
Gene

[John G.]

Re: Lab scope to test alternator

Gene ..
Everything came apart pretty well on this alternator. Nuts, including pulley nut, etc., weren't so tight as to cause a problem. Everything is apart - just need to replace some pertinent internals and get it back together. After reading up a bit on alternators and their operation I have a better understanding of how these devices work. I was interested in seeing what the internals looked like too so apart it came . .! . .

I know what you mean by those pesky nut/terminal combinations whereby something gets broken, usually a terminal end, as a result of breaking loose a too tight nut. It didn't happen this time but I've had it happen in the past .. ! .. Seems 'universal' ..
Thanks .
John