Car Alternator Diode Testing

[barrymurphy]

Hi,


When testing the diode on a car alternator all the tutorials state place a multimeter to diode setting, place the + cable to the casing and the - cable to the battery post ( B post) and you should get a reading of .5v to .8v.


Swap the cable positions i.e. the - cable to the casing and the + cable to the battery post ( B post) and the reading should be OL i.e. no current flow.


I have had three different alternators and when I do the test above I get the 0.5 to 0.8v for the first cable positions as above but I get approx 0.7v to 0.9v when I do the second test. The same on all three.


However if I split the alternators, so removing the rotor, and repeat the tests, then I get the results as expected with good diodes, 0.5v to 0.8v one way and OL the other.


I have done this test on an alternator that I knew had a bad diode(s) and the second reading i.e. the - cable to the casing and the + cable to the battery post ( B post) and the reading was 1.7v. A lot higher than my 0.7v to 0.9v.


So, to do the test properly should the rotor be removed? where is the 0.7v to 0.9v coming from when the rotor is in place?


No tutorials mention this phenomenon anywhere. I do not think I am doing anything wrong, the test is straightforward.


I was assuming diodes were bad in alternators because of the 0.7v to 0.9v readings but in truth they were fine.


Are all the videos wrong or what am I doing wrong? And if my readings are correct, then to assume a bad diode the readings should be a lot higher i.e. approx 1.7v rather than the 0.7v to 0.9v.


Confused.


(P.S. I never did the AC voltage test at the battery)


Thanks


Barry

 

[ericmark]

Depends on alternator, the old Paris Rhone alternator with just two diodes may be you can test without unsoldering, but in the main we have 9 diodes and they all connect together through the stator so step on has to be disconnect stator which often means unsoldering.

I remember the old Lucas 10 and 11 AC alternators massive slip rings, really low output not much better than dynamo the main advantage is they could rotate faster so lower engine cut in speed, but people would fit them with dynamo pulley so no gain.

 

[Peter.N.]

Diodes should be tested out of circuit and will read about 0.6v forward and virtually open circuit in reverse. I appreciate its difficult to do that but if you can disconnect everything else from the circuit you should get a fairly accurate reading.

Peter