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Chicagoland MG Club: Driveline
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  Chicagoland MG Club:Driveline

Early MGB Polartiry Change and Generator to Alternator Conversion
January 15, 2005 - Downers Grove, Illinois

John Schroeder Let me start by saying I am glad I started this with almost two full months before the Missouri endurance rally. I had done some research before hand and discovered my tach was not convertible to negative ground. I called John Twist during his tech help hour and discovered I have a very rare tach. How lucky can I be? I pulled articles from the MG Enthusiasts bulletin board so I would be prepared.

We started the session at 10:00 AM Saturday. Converting from positive to negative ground seems pretty straight forward when you read some of the articles available on the web and figure out what needs to be done. I also had the Moss motors instruction sheet, which follows a step by step procedure which starts with polarity change. Alternators are negative ground only. The first thing to do is disconnect the battery and remove the terminal as positive and negative posts are different size. The fuel pump had to be changed since mine is only a year old and the new pumps are polarity sensitive. This is a really easy change with a new diode (you just need a screwdriver). With some care and soldering gun you could also change the terminals on the diode leads. I didn t know that when I ordered parts but the new diode is only $8 from Moss and was worth it to me. I also have a spare diode that I can change terminals on and keep for future use.

As mentioned in notes with the photos on the web site, my tach wasn t convertible, but Cowboy brought two non functioning tachs that had the proper leads to convert. We checked them out before we started the rest of the project. When we found they didn t work we opened them up so we could see what should be changed. I also have to reverse the current flow through the white wire loop on the back of the tach case.

We removed the generator and changed the distributor points, condenser, and rotor as I had read they could fail shortly after conversion. Next step was to install the new alternator fan and pulley. The pulley and key weren't sized quite right, so we spent about an hour filing, polishing, and fitting to get it all together. It didn't take long to get the alternator and bracket installed on the engine, but I had to go back the next day to cut the coil mounting plate to give the engine some room for torque swing. The instructions for wiring the alternator to the voltage regulator were clear, but somewhere in that jungle of wires I have a problem. The alternator is putting out charging voltage but my ignition light is on until I turn the key to on then it goes out and stays out while the engine is running. I am going to be spending some time pouring over wiring diagrams to see if I can find the fault. I will also be shopping for a replacement tach at the Swap Meet.

ADDENDUM: P.S. I think I've got it. The Moss instruction said to connect the brown/green wire to one of the big terminals on the alternator. That bothered me as this (the brown/green wire) was originally the field wire, so I tried it on the field terminal of the alternator and voila! I tracked the wires through the wiring diagram and decided to try the change. The ignition light is now out when the key is off, on when the key is on, and goes out when the engine is running. The output is 13.92 volts and there is no smoke or funny electrical smells. Life is good! Now I just need a tach and a radio and I am ready for Missouri and another great driving season.

John Schroeder -

Photos and notes at:

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