Component testing

This Section deals with testing the individual components of an alternator. It is not the definitive list, but a guide. So don’t think that just because the component appears to test out without problems that it has no faults – unfortunately life isn't that simple.

I am going to assume the alternator has been stripped, cleaned and that you have read the other relevant sections of this web.


Check for circuit through windings with meter or test light. Typical resistance is about 3.5 ohms for 12 v and 8 ohms for 24 v. Test this  by connecting between the two ‘slip-rings’. (The part which the brushes rub against). Next check that the rotor windings don’t go to earth by putting one of the leads to the body. Visually the rotor windings should not be burnt and the slip-rings themselves relatively un-grooved. Although minor grooves are acceptable if they become to deep the slip-rings will  have to be replaced.  It will have been worthwhile putting your finger onto the slip-ring and spinning the unit prior to dismantling to check that it is concentric. Small variations are acceptable but if in doubt skim  in a lathe.


Test for circuit between each of the phases with a test light or meter and then check that it doesn’t go to earth. Ensure that the windings are un-burnt and covered in lacquer. N.B. many stator faults are caused by the windings shorting between each other and not through going to earth. This fault is almost impossible to detect, so the visual examination is all important.


This is the most difficult to explain, test procedure so I am going to describe how to test in only the simplest terms.  

The rectifier consists of 1 positive and 1 negative diode for each phase (wire) coming out of the stator. These are called power diodes and split the power into positive and negative voltage. To check a diode ensure that electricity can flow in one direction but not in the other by checking  with your meter or test light and then reversing it (One way you should get a reading, and the other way you should not)

In addition, many rectifiers also have excitation diodes these smaller (normally) diodes are tested in exactly the same way.

A three phase alternator will have three positive and three negative power diodes plus usually another three excitation diodes.

I appreciate that this is not a good description however, if you run into trouble e-mail  or phone us and we will help if possible.


Difficult to test when it is off the vehicle. These days they are frequently combined with a brush box and tend to be changed on any major rebuild. This is the one part of an alternator that will have intermittent faults. So depending on what the fault is it may be worth changing it especially if no other problems can be found.


Bearings – If noisy replace them.  N.B. Always check bearing housings for wear.


Check for cracks/stripped threads or broken studs.



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