Polarity Change and Generator to Alternator Conversion
On a early model MGB
January 15, 2005 - Downers Grove, IL


Click for larger pictures, average 34KB.
Photos from Jim Michel. -- Notes from John Schroeder.

Session got started on time. It was a really cold, sunny day and the heat in the garage was welcome. Cowboy brought a couple of early electronic tachs to compare with mine. Neither unit worked, but we had a chance to compare a tach that could have its polarity reversed (Dave's) and one that could not (mine). After we checked out the tachs, we clamped the fuel line and removed the fuel pump.

 
The two pictures show the convertible tach. The power and earth connections must be changed inside the tach case. There is a voltage-in spade terminal and usually a ground lug inside the case. The polarity is changed by moving the wire from the ground lug to the spade terminal and moving the resistor from the spade terminal to the ground lug. My tach is grounded directly to the PC board and has no ground wire to swap polarity. Since we didn't get the tach prepared we didn t do the other operation which is to reverse the current flow through the white wire loop on the back of the tach case.


These pictures show the workings of two SU fuel pumps. The one on the left is an old non-polarized pump with a capacitor. The one on the right is about a year old and has a diode instead of the capacitor. The diode has to be replaced with the proper polarity or the terminals need to be reversed. Since I hadn't opened one of these fuel pumps before, I opted to replace the diode. Once we opened the pump and saw what was different, I could have just reversed the terminal ends on the diode, using a heat sink to protect the diode itself from the soldering heat. Then we wrapped the fuel pump with electrical tape to seal the cap and reinstalled it.


While it is arguably not necessary to change the points and condenser, there have been reports of the points failing shortly after a polarity change. My car was almost due so we swapped new points, condenser, and rotor in. We also reversed the wires to the coil at this time. The last picture is of Dave and Maynard (legs anyway) reversing the battery cables and reconnecting the clamp-on wire terminals. It wasn't quite time to connect them to the battery, but we thought we were getting close.


Next we disconnected and removed the old generator. It was time to assemble the new alternator, bracket, and hook everything up. We had a number of problems with the Moss kit. The alternator, key, and pulley did not want to go together. It took about an hour of filing, polishing, and fitting to get the pulley on the shaft. Next we put the alternator and bracket in place, replaced the fan belt, and snugged everything up. Now it was time to reconnect the wires according to the Moss instructions (enlarging the photo shows a close up of the instruction sheet).

Unfortunately, this is where we ended as the ignition light is not cooperating and I am still trying to find out what is wrong. I disassembled the coil and mounting the next day and cut away some of the bracket to clear the alternator when the engine torques over. Hopefully, I will be able to track down the problem and get everything working properly soon.

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.