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MG MGB Technical - Positive to negative, generator not working.

I have just converted my 67 B from positive ground to negative ground. I followed the instructions in the Moss catalogue and everything went smoothly, so I thought. The previous positive earth tachomoeter which I rewired in accord with Moss works fine, the fuel pump works fine, but the genearator (dynamo) does not charge. In fact I think its putting out a minus charge.

I followed the polarization procedure in Moss and sparked the field terminal (small one) on the generator several times with a 14 gauge wire from the new hot terminal on the fuse box. It sparked every time nicely.

Also when I connected another digital electrical meter to the original coil to check the tachometer reading, I get randow rev readings that are inconsistant with the in car tach. Do I need to replace the coil with a negative earth coil? BTW which way do the wires connect to a negative earth coil?

Many thanks Andy

1967 MGB. Almost negative earth.
Andy Preston

Andy - The coil does not need to be changed, only the connections to it. The primery connection marked - should be hooked to the points in a car set up for negative ground and the terminal marked + is hooked to the ignition switch. When you connect the diagnostic tach to the coil it should be connected to the terminal that goes to the points (distributor) and to ground. To check your generator for output, disconnect the two wires from it and isolate the two wires from ground and each other (in other words, don't let the ends touch each other or ground). jumper the two terminals to each other and connect a voltmeter from those jumpered terminals to ground. Start the engine and slowly increase the revs. The voltage from the generator should be positive and should increase beyond 12 volts quickly. keep the revs low enough that the voltage doesn't go over 20 volts or you will damage the generator. If you are getting a positive voltage that increases quickly past 12 volts, the generator is good. If the voltage will not increase past 12 volts, there is a problem with the generator. If the voltage increases quickly past a negative 12 volts, then you will need to re do the polarization. Good luck - Dave
David DuBois

For what it's worth, i polorized mine, (positive ground) at the control box -- voltage regulator. seem to recall that the two must be polorized as a unit.
glg

The voltage regulator does not need to be (and cannot be) polarized. Digital meters in my experience are polarity sensitive when reading things like rpm and dwell but not voltage and current. In a negative ground car the +ve or red terminal from the meter should go to the coil -ve terminal and the meter -ve or black terminal to ground. For a converted car the original coil would have had CB (Contact Breaker) and SW (ignition SWitch) terminals, which would now have a white wire on the CB and the distributor wire on the SW. Other than that, your polarization technique is correct and what David says about testing the generator should reveal any remaining issues.
Paul Hunt

Andy,
I've done this a several times over the years and on two occasions I have had the regulator fail immediately after the conversion. On both failures, the regulators were very old and tired. A new regulator fixed me right up.
David

Dave, Paul and David, thanks for your help. I also have a "Lucas fault diagnosis service manual" and checked the generator output both ways. I used the Lucas method for checking the output from the "Armature Circuit", I got no steady reading, just a series of flashing different numbers with a negative sign. I used Dave's method and also got hte same result. I checked my digital volt menter and that was reading correctly.

So, somehow when I changed from positve to negative earth the generator stopped generating current. I followed the Moss methodolgy to the letter. I disconnected the generator prior to changing the battery terminal connections. I flashed the field terminal twice to repolarize the generator. The generator doesn't smell burnt out. I removed it from the car and disassembled it and it looks nice and clean inside. the bushes are almost new. One thing that I found strange is that there is continuity between the output terminal and the field terminal. i.e. my ohm meter shows a continuous circuit. Is this correct or is there a short inside the generator.

I'd like to find the reason for the problem before I buy a new generator and possibly ruin that one as well.

Andy Preston

Digital voltmeters can sometimes be more bother than they are worth which is why I have both types. The output from a generator (when not connected to a battery) is varying DC and the sampling method of many digital meters means that you will not get a steady reading. By removing the wires from the F and D terminals and bridging the terminals as described before, with an analogue meter on the terminals start the engine and slowly increase the revs. As was mentioned before don't go above 20v, and you should see 20v before you get to 1000rpm. If you do then the generator is OK as long as the polarity is correct. If you only see about 0.5 to 1v the field winding is probably faulty. If only 4 to 5v the armature windings are probably faulty. Both the F and D terminals go through there respective windings to ground, which is why you can see continuity between the two, but the voltage test is the only sure way to check the output.
Paul Hunt

Andy - I concure with Paul regarding the digital meter - reserve it's use for more static situations. Use a good old fashioned analog meter, it will follow the voltage swing and enable you to better see what is happening.
Good luck - Dave
David DuBois

Paul and Dave, once again thanks for your responses. I used my analogue meter (dusted it off) and the only reading that i could get was when the field wire was connected and then I only got 2V from the Armature terminal. For some reason it looks like the generator is not working correctly. I took it apart again and have a continuity reading across every adjacent contact strip of the armature. I tried recalling my grammar (high)school physics and I think I remember that only the opposite strips on the armature should have continuity. But as Paul will tell you I went to school a long time ago. I think that a new generator is called for. I'm having it checked out tomorrow by a friend of mine who is an electrician just to be sure. I'll keep you posted of the results.

Thanks again guys, Andy
Andy Preston

You and me both, Andy :o)
Paul Hunt

Hi Guys latest update. Electrician friend cleaned the commutator and insulation between the commutator strips and now its working fine. Puts out 20 plus volts. However when I check the voltage at the battery there is no increase in voltage when the engine is running. I assume that there may be something wrong with the regualator. I tried to check this using both the Lucas and shop manual but when I remove the lead from the regualator the engine stops. Think I'll just buy a new one.

Thanks for all your help, Andy
Andy Preston

This thread was discussed between 27/05/2003 and 30/05/2003

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