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can you check a dynamo outside the car ?


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hello all , 


first of all , best wishes to all of you !!


my daily driver (110 , 1987 disco 200tdi) has some trouble charging , the dynamo sometimes doesn't charge at all , if the Rpm gets above 1500-2000 it will kick in and charge normal.

i changed the dynamo for one i had laying around (was a 35amp v8 one , so i had to clock the front housing to fit). so everything is working like it should .


so the question is , can you check a dynamo outside the vehicle with a multimeter , i opened it up , and checked the brushes (i had the idea these were worn, but the where still good (only very dirty)) , checked the 3 diode's , also seem to work as they should . is there anything else i can measure before sending it off to the repair shop ?



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You can check it out of the car, but it's easier in the car. You need to spin it fast enough and provide 12V to the exciter pin. @elbekko had a go with a drill to test the alternator on a TR6 that was behaving strangely. We didn't get up to 12V but enough to prove it was generating charge. The problem turned out to be the amp meter in the dash. The local rebuilding place has a bench setup, basically an electric motor that drives a belt you can put on the alternator and measure the output. But it's easier to test on the engine by disconnecting the wires to the alternator and measuring. Could be a bad earth, not passing enough current to start from idle so you need revs to self-excite and get the voltage up.

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There should be 6 diodes not 3. You can test it off the vehicle but you will need something like a 1Kw rating electric motor to spin it with sufficient torque and speed to produce an output. Forget using a drill to do this. Plus you will need arrange pulleys and belts.

Best way to check is with it still in the vehicle and by measuring battery voltage with engine running. Even with engine at idle you should get at least 14v at the battery. This should not drop when you switch on the headlights and blower fan. If the voltage drops more than 0.5v, then the alternator is faulty.

Edited by simonb
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