Car Alternator Diode Testing Ask questions and find answers to many subjects relating to electrics and electrical work

[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]

Connecting an oscilloscope to the battery you can see the wave forms and the even pulse of charging showing if diodes and regulator are OK, however other than that the diodes need disconnecting from the stator, there are some makes where they are bolted on, but many need a soldering iron to unsolder first.

The alternator can had down to two diodes or could be twelve depending if single or three phase and if star point rectified, I know Bosch did start rectifying the star point, plus specials with 12 and 24 volt outputs, but only seen them on grove cranes.

In the main there is no current regulation on an alternator, but the CAV AC 203 did have current regulation designed for buses in the main and to keep battery charged with internal lights on, the 440 regulator was a special with M1 and M2 terminals that went across a steel resistor shunt which also worked the dash ammeter.

Since I have left the trade things have moved on, with the engine management telling alternator when to charge, and use of DC to DC inverters to combine outputs and special regulators to stage charge the batteries, or pulse charge the batteries mainly narrow boats.

In the main they use 9 diodes, 6 main diodes and three for the regulator supply and warning light. So 3 negative and 6 positive. Paris Rhone made a single phase alternator with a fuse one side of the two stator windings and a diode one each winding on the other side so each output was half wave rectified and each winding added together gave full wave, some had a third diode to work warning light, idea was to reduce cost, but in UK these alternators were expensive.

Paris Rhone and Delco both made brush less alternators, but not like the larger versions with rotating diodes, the rotor coil was static and just the fingers rotated.

And the 12/24 volt Delco had an auto transformer built in to boost output to 24 volt for the start battery, there was no regulation of the 24 volt output, just the 12 volt was used to regulate.

The Lucas alternators had a Zener diode well they said it was avalanche diode as the zener effect is only up to 3 volt, these were around 16 volt designed to stop damage should the output lead become detached, it was not very successful.

The early Lucas diode pack was damaged by vibration, most auto electricians replaced them with Durite ones which were far better, many of the French alternators would burn them selves out if you fitted larger batteries they relied on the battery being charged to a point where regulator cut in before they over heated.

Railway alternators were another story.

So yes 0.6 volt drop is common, many multi meters have a diode check option and it actually measures volt drop, but to test in the main the stator needs disconnecting.

[barrymurphy]

What a comprehensive reply. Thanks. The 0.6v drop is common - is this on what is deemed should be the OL side? i,.e. +cable to post - cable to case. or the other way?

A man who fixes alternators did say to me that you had to disconnect from stator to test properly. My stators are soldered to the rectifier. BUT my removing the rotor from inside the stator seems to have the same effect. Does that make sense?

thanks

Barry

[barrymurphy]

and I forgot to ask, how are those videos online bench testing the alternator diodes getting OL when they are still connected to the stator and I cannot with the three alternators I have. All read 0.07 to 0.9v compared to their OL.

[ericmark]

I made a sideways move in the 80's and 90's flitting between auto electrician and maintenance electrician and installation electrician, so it was 25 years ago or more when I repaired alternators for a living.

There may be a tool today which can detect a faulty diode without dismantling the alternator other than full blown oscilloscope, the Schottky diode, also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action, I know these are used in split charging devices so you don't have the 0.6 volt drop, there is no reason why they can't be used in an alternator, so it seems unlikely anyone can make a tool to test all alternators, there is bound to be an odd one out some where.

I know you can buy special regulators that allow the alternator to stage charge, things have moved on.

If you had said I have a Lucas 11 AC alternator and ABC happens, then since so old I can likely explain exactly what is going on, but in general there are so many different types and makes, battery sensed, machine sensed, but in regulators, external regulators, electronic regulators and pure mechanical regulators, Datsun I seem to remember had mechanical regulators, which was a surprise as expected Japanese cars to be all electronic. So can only answer in very general terms.