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Bench testing the Generator

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vping

vping

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Next up on TB's list, Bench testing the Generator. What is the best way to do this.
 
You will need some way of spinning the generator at a variable rate. Short the two terminals together then attach a multimeter, set to the 50 volt scale, between the terminals and the generator case (If I recall correctly, the prewar MGs were all negative ground, so connect the positive side of the multimeter to the terminals and negative side to the case. If the generator is supposed to be a positive ground unit, just reverse the leads). While watching the multimeter, start spinning the generator. The voltage should start to build up rather quickly and you will need to stop increasing the RPMs as it reaches 20 volts (if you don't limit the RPMs, the voltage can climb very rapidly to levels that will burn the winding out in the generator). IF you see the voltage climb to 20 volts, the generator is good. If there is no voltage or it won't climb above a few volts or is it stops at 12 volts, the generator still has a problem in it. Good luck
 
External vltage regulators are another item entirely, but you may want to make sure your's is good too... So I'll throw the question...

How can you check an external voltage regultor in a generator equipped car?
 
Try this

https://72.14.203.104/search?q=cache:-6XM...t=clnk&cd=3

Generally it's your control box........
You NEED to have a spare.
$45.00 will save you needless hours of torture.
If you're one of those curiosity crusaders who want to divine the true cause of a Lucas fault.....God love you.

I'm just a parts replacer. /ubbthreads/images/graemlins/driving.gif
 
HI Vping,Just connect the generator to a battery as though it were a starter,if it shows high starting torque and spins you are good to go.You must support it firmly to do this test.---Fwiw--Keoke
 
Pre-war MGs were positive ground as were all MGs right up to MGB.

EDIT: Make that "some" pre-war MGs, not all!
 
Mine has been converted to negative ground. Right before the PO parked the GT, he purchased a new Regulator & Generator not knowing what was bad. He replaced the regulator and the car worked fine. He gave me the old regulator & the brand new generator is still in the box in my garage.
 
There are some on-car tests you can do to the generator. Testing generators on the bench, like they were a motor may work... and may not. Years ago I had a friend with a '64 MGB. When we "motored" his generator it worked but it wouldn't charge the batteries. We took the generator to a motor shop where they put it on a device called a "growler" that does some form of inductive testing of the field coils. It turns out the field coils on my friend's B failed that test.

I don't have the on-car tests with me but I can look them up for you if you're interested. Basically they involve disconnecting the wires at the generator, tying the spade lugs on the generator to each other with jumper wires, then measuring voltage between them and chassis ground at various RPM.
 
Steve - "Pre-war MGs were positive ground as were all MGs right up to MGB." - Not true, that is what I always thought, but pre war cars up to at least the TA were negative ground. Exceptions appear to have been the S,V,W salons. The rest from the M type through the P types are all negative ground according to the wiring diagrams in the Blowers manual.

Doug - You are describing the test that I outlined earlier to bench test the generator. It is much easier on the car in that you don't have to rig up a way to spin the generator and control the RPM.

Kenny - The way I alwas do it is to check the generator as I described and if it is good, hook it up to the regulator and see if it brings the voltage up to the voltage level as described in the shop manual. If it does, then the regulator is good. There are Lucas flyers available on how to do comprehensive tests on the regulators, but the bottom line is, does it charge the battery in conjunction with a working generator.
Cheers
 
Sorry Dave, I didn't mean to repeat your post. I remember that in addition to the 20V condition you mention above there were two others. If you only get around 1V it means "X" is wrong, if you get 6V it means "Y". I'll have to look up what those conditions were... if anyone is interested.
 
Dave, I did some investigating on this. It seems that all pre-war MMM cars were negative ground. All SVW and T-Type cars, including TA and TB were positive ground. The switch to positive ground became popular in the 1930's. Ya learn something new every day!
 
Steve - I have heard (don't know if it is true) that the switch to positive ground was for some preceived rust control due to cathodic action. If this was true, it was a diamal failure. I have also heard that it was to put the center electrode of the spark plugs at a negative potential to improve electron flow (as in a vacuum tube). If this was true it would have been a fuitle attempt as the same thing is accomplished by the polarity of the coil connections.

Doug - Repeating a test write upon only reinforces its validity. I have never heard of the interpretation of the readings and would be interested in seeing them. I do know that I overhauled the generatore in our TD one time and upon testing it, the voltage would go to 12 volts and no more and when it was hooked up it wouldn't eep the battery charged. I even had a guy in an auto electric shop try to tell me that that reading was ok. I finally found that when washing the generator back plate with solvent, I got some of the carbon material from the brushes into a crack of a stand off insulator and it was acting like a zener diode. It would work fine until it reached 12 volts, then break down and not allow the voltage to go any higher. I had a machinist make me a new insulator and every thing worked fine, in fact it is still working some 30 years later.
Cheers
 
Doug - Thank you for linking that page. One thing the author neglects to mention when testing the generator is that one needs to be careful to NOT allow the voltage to go above 20 - 30 volts. With an unregulated feedback from the armature to the field coils, reving the engine can very quickly increase the voltage to the point where it will burn out the generator. Also, a low field coil resistance reading does not necessarily mean that the generator is trash. This condition is usually caused by the wire between the two windings shorting to one of the long screws that goes all the way through the generator to hold the two end plates in place. Asimple repositioning of the wire and some shrink sleeving on the screws gets the generator back in condition.
Cheers
 
That's a good point about the field coil wiring. I'll make note of it for the future!
 
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