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MG MGB Technical - Early Generator (Neg /Pos Ground)

I have an early MGB that was originally a positive ground system. The PO had converted it to a Negative Ground system. Now my generator has gone out. I am about to order a new genetrator, but VB or Moss doesn't offer a Negative Ground early generator. I recall hearing somewhere that there is some way to make it negative ground by simply grounding one lug of the generator or something like that. Does anyone know what I'm talking about, or how to accomplish making a stock generator work and charge correctly on a negative ground car?
Ricky

There is only one generator. You can simply make it positive or negative ground by taking a short length of wire and with one end connected to the hot (brown) lead from the solinoid, brush the other side of the wire to the small terminal on the generator. A small spark is produced. Make sure the leads to the generator are disconected while you do this. Also, besides converting the batteries, you'll need to reverse the wires on the coil. If you have a Moss catalog, all the directions are in the electrical section--same page as the coil, opposite the distributors. Its titled: "Positive to Negative Ground Conversions". hth
Paul Hanley

Ricky - Once you have installedyour new generator, but before attaching any wires to it, do the following:
Attach a jumper wire to the field terminal (smaller of the two terminals), strip the other end of the wire back about 1/2" and touch it quickly to the battery lead that attaches to the starter selonoid a couple of times. You should see a spark when you do this, just be sure that you only touch the terminal quickly, you don't want to leave it on long enough to cause smoke (and burned fingers). Remove the jumper wire and attach the standard wires to the field and output wires and you are good to go.

If you haven't already ordered a new generator, I would suggest that you look in the yellow pages and find a local auto electric shop and take the generator to them. Chances are, they can rebuild the generator for much less than you can get a new one from Moss or VB. The early generators take all of an hour to rebuild by someone who knows what they are doing and that includes the coffee brake.

If you can't find someone locally to rebuild your generator, contact me at the above e-mail address, I have at least one rebuilt generator in our garage that I will sell for less than either Moss or VB. Good luck - Dave
David DuBois

Paul, David
When you do what you describe ("Attach a jumper wire to the field terminal (smaller of the two terminals), strip the other end of the wire back about 1/2" and touch it quickly to the battery lead that attaches to the starter selonoid a couple of times"), what are you actually doing to the generator? Are you reversing the magnetic field and so the direction of current flow? Trying to remember basic Physics (field,motion,current, RH or LH??)

Richard
Richard Atkinson

Richard!

Your question seperates the truly knowdgeable from those of us that just know how to follow directions!

The proceedure above is refered to as "flashing" the generator and it changes it's polarity. That's what it takes to work and that's all I know 'bout that!

Always been curious myself. Good question.

Paul
Paul Hanley

Paul is correct, the procedure is known as flashing the generator (the only kind of flashing that you won't get arrested for). The theory behind this is that when the generator first starts turning, it depends on the small amount of residual magnatism in the field poles to start generating voltage. This small amount of voltage if then fed back to the field coils through the regulator to bring the voltage output of the generator to the proper level (about 14.6 volts). Once that level is reached, the regulator switches a resistor inthe feedback loop to limit the voltage output of the generator (other wise the generator voltage would continue to climb to the point that it destroys the generator). Flashing the generator establishes the correct polarity of the residual magnetic field in the fild poles to give the correct voltage from the generator. Cheers - Dave
David DuBois

Almost complete. The residual magnetism generates a small output voltage which is fed back via the regulator to the field winding, which increases the total field magnetism so increasing the output voltage, which increase the field voltage still further etc. and the output voltage rises very rapidly. As soon as this voltage reaches about 13v the cut-out relay operates which connects the output voltage to the cars circuitry and so starts charging the battery and powering all the loads. Immediately after cranking this is quite a heavy load which can take all the generator output.

As the battery recharges its voltage rises, and when the voltage gets to about 15v the voltage regulator relay operates which inserts the resistance Dave mentions into the field winding, reducing its current, which reduces the output voltage and the charging current. This drops back below 15v so the relay releases again, taking the resistor out of circuit, which increases the output voltage, which operates the voltage regulator relay again. This results in the voltage regulator continually 'buzzing' which can be felt if you touch the armature gently, and you can see a small spark at the contacts. Now comes the clever bit. The rising battery voltage i.e. its charge gradually allows the relay to be operated for longer and released for less i.e. changes its duty cycle, so gradually reducing the *average* charging current until a stable point is reached. Any increase in load after that, i.e. headlights, causes a small voltage drop which changes the duty cycle of the relay change to compensate. MGBs have 3 relays and the third is a current regulator. If the battery is very discharged or a heavy electrical load is otherwise placed on the system the high current could damage the generator, so when the current exceeds a certain point i.e. about 22 amps the current regulator relay operates, which also inserts a resistor in the field circuit (actually the same resistor) which also reduces the current, which causes the current relay to release, so increasing the current again, controlling the average output current through duty cycle in the same way as the voltage regulator. Of course, when the current regulator starts coming in to play the system voltage will reduce. If the load keeps going up the current relay maintains the generator current at a safe level, the system voltage keeps dropping, until eventually the battery is no longer being charged, and any increase in load after that is discharging the battery.

The alternator voltage regulator does much the same thing but without relays, using the continuously variable characteristics of electronics in place of the 'on or off' characteristics of relays.
Paul Hunt

Paul - Thanks for filling the details. I have always been a bit hazy about the actual operation of the regulator so always state it operation in general terms. Your explanation will go into my own archives. Cheers - Dave
David DuBois

This thread was discussed between 18/10/2004 and 19/10/2004

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