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September 29th, 2008

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Q & A: Alternator Conversion

Tuesday July 03rd, 2007
Q & A, MGA, MGB, MGC, MG T-Type, Midget
John:
I have an August, 1966 MGB (GHN3L 98066) on which I have just completed a major driveline rebuild. In the process, I installed an alternator conversion purchased from one of the major suppliers, with a "2-wire" alternator which retains the original RB340 regulator.

The car was already converted to negative ground. The alternator conversion calls for the brown/yellow "D" lead on the regulator to be spliced into one of the "B" leads (either to the ignition switch or the fuse box) and the "F" lead to be relocated to the "D" terminal. The alternator installation and the wiring conversion have been done according to instructions, checked and re-checked with no problems apparent.

The situation is this:
1) When the key is turned on, the warning light glows (no problem).
2) When the motor is started, the warning light goes out (no problem).
3) When the motor is shut off (and the key is off), the warning light glows (PROBLEM).

I pulled the regulator cover and noted that the cut-out is remaining closed when the key is turned off, so I disconnected the battery to open the cut-out. When running, the cut-out is closing at approximately 12.8-13 volts (no load), which as I recall is in the acceptable range.

I have checked and re-checked the conversion wiring, cleaned up all of the old regulator connections, and directly grounded the regulator. As the regulator I was using had been in service with the old generator for many years, I suspected a cut-out problem and substituted a new regulator I've been keeping as a spare, but the same problem occurred. I can't help but think that I'm missing something really obvious, but the brain is fading rapidly. Any suggestions??

Glenn

Glenn,

To keep the system working correctly, you really have to gut the old RB340 to keep the current from getting to earth. I just did one -- but you can do it too. Cut out the swamping resistors; cut out the F to D resi... read more