Alternator Electrical Query

[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

 

[telectrix]

your unexpected reading would be a parallel path, perhaps through the alternator winding or some other partand removing the rotor eliminates the parallel path.

 

[barrymurphy]

your unexpected reading would be a parallel path, perhaps through the alternator winding or some other partand removing the rotor eliminates the parallel path.

thanks. When you look at this video, as with many others,

you can see that the OL reading is received on the new device. I get that when alternator split but not when intact. Why is that?

 

[telectrix]

it's because when connected there is either reading another diode in parallelbut opposite polarity, or something else. i always check diodes with at least 1 end disconnected.

 

[barrymurphy]

it's because when connected there is either reading another diode in parallelbut opposite polarity, or something else. i always check diodes with at least 1 end disconnected.

thanks.

so are all these guidance videos basically incorrect? even though they get the desired OL.

I was putting it down to a charge still being held somewhere, like a capacitor, but I believe when removing the rotor the capacitor is still on the bridge rectifier side and so splitting would make no difference to it

".To conduct the test, you must first make sure the circuit is unplugged and that all capacitors in the circuit have been discharged. As long as you do this, you do not need to remove the diode from the circuit."

 

[telectrix]

can't comment further as myknowledge of alternators is limited to the fact that they spin round and if they are working, the van starts OK.

 

[barrymurphy]

can't comment further as myknowledge of alternators is limited to the fact that they spin round and if they are working, the van starts OK.

many thanks

 

[brianmoooore]

Assuming a conventional car alternator, with the B+ lead disconnected you should get a reading a two diodes in series (about 1.2 volts) between the B+ terminal and the casing with the leads one way around, and open circuit the other way around. 0.6 volts will indicate that one of the six power diodes is short circuit, but not which one.
Similarly, testing between the D+ output and the casing will test the three regulator diodes, but not indicate which one is faulty if any is, although if you get 0.6 volts in both tests, it probably indicates that one of the three power diodes between the windings and the casing is short circuit.
The regulator pack could affect these readings if left in place, depending on its design.

 

[barrymurphy]

Assuming a conventional car alternator, with the B+ lead disconnected you should get a reading a two diodes in series (about 1.2 volts) between the B+ terminal and the casing with the leads one way around, and open circuit the other way around. 0.6 volts will indicate that one of the six power diodes is short circuit, but not which one.
Similarly, testing between the D+ output and the casing will test the three regulator diodes, but not indicate which one is faulty if any is, although if you get 0.6 volts in both tests, it probably indicates that one of the three power diodes between the windings and the casing is short circuit.
The regulator pack could affect these readings if left in place, depending on its design.

Thanks. But when I remove the rotor from within the stator and retest, I do get the OL as expected and the diodes seem fine on that basis. Why is that?

 

[brianmoooore]

Because there's a parallel path through the field coils in the rotor.
I would expect that removing the regulator pack would also remove this parallel path, but that may not be the case for this particular alternator.
The absolute best you can do without disconnecting at least one end of individual diodes, is to test the nine diodes in batches of three, since each group of three are physically connected together at one end, and connected together through the stator windings at the other end.

 

[barrymurphy]

Because there's a parallel path through the field coils in the rotor.
I would expect that removing the regulator pack would also remove this parallel path, but that may not be the case for this particular alternator.
The absolute best you can do without disconnecting at least one end of individual diodes, is to test the nine diodes in batches of three, since each group of three are physically connected together at one end, and connected together through the stator windings at the other end.

thank you