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MG MGA - Gas guage / Generator light

While on a recent rally my gas guage suddenly cranked over to EMPTY and stayed stuck there. On the dry home later at night my generator ignition light was shinning brightly. My headlights, dash, flashers etc all worked well on the 45 min. return trip.

Is there any possible relation between the two incidents or should I start checking out the charging system IE: generator , control box , battery Etc ?
Gordon Harrison

If the fuel gauge is working and pointing at the "F" mark, not on the peg, that implies that the signal wire between gauge and tank sending unit is shorted to chassis ground. Check for short at the "T" terminal post on back of gauge. If the fuel gauge does nothing when ignition is switched on, needle stay's sitting on the peg, this implies loss of input power to the gauge. If the fuse blew then heater blower, screen wipers, turn signals and brake lights would also stop working. If those other items work, check condition of the green wire between fuse and gauge. Look for short at the "B" terminal post on back of gauge. If the fuse blew the shorting fault could be in any of the associated green wire circuits.

Ignition light on is most likely a separate unrelated problem. Test function of the generator first. If bad, repair or replace. If good, then check function of the control box. If control box is bad, it is likely junk and needs to be replaced.

BIG caution here. Any time you fix or replace a generator or control box, check generator function first, but also check function of the control box to be sure it provides the right output voltage AND will not allow overcharging at high engine speed. It is downright disheartening when a faulty $40 control box takes out a $100 generator by overcharging, but even more of a let down if it also takes out the new replacement generator.
Barney Gaylord

Mr Gaylor,
I had the control box tested,and the larger coil inside has a fault which was causing a poor ground. It is not fixable so I will be replacing the unit. You would have like to see this guys shop, with all of the older equipment to test every type of electrical unit. Easy going guy who even invites you into the shop to see what he is doing to your units. Very old school professional. I hope he is training someone to take over for him whem he retires.

He did mention one thing that I did not understand....he said when I install the new CB it will polarize and everything will be fine. What does this mean and is there anything I have to do to install the CB properly.
Gordon Harrison

The control box is not polarized and will work with negative or positive ground. Just make sure to get the wires back on the same terminals. The generator is polarized and after installation has to be flashed to give it the correct polarity.
Jeff Schultz

It's not the control box that polarizes, it's the generator. If you didn't touch the generator it is probably still polarized. If you replace the generator or change polarity of the system (reverse the batteries), then you need to re-polarize the generator. Most of the time the generator will polarize itself the first time the cut-out relay kicks in to connect the generator to the battery. Just to be sure, its a good idea to polarize the generator first. Very simple, just touch a hot wire to the small "F" terminal on the generator and it's done. If everything is electrically connected you can accomplish the same thing by momentarily shorting together terminals A and F on the control box.
Barney Gaylord

Thank you
Gordon Harrison

A little side note.

The new control box says to clean the contacts before installation...so I lightly (with very fine enery paper) gave the contacts a light sanding. Is this correct?

I notice the new CB has much smaller coils inside and a much smaller resistor ???? underneath the unit. Is this simply your typical less costly method of reproducing these CB's. And finally is it possible or cost effective to have the CB rebuilt?
Gordon Harrison

This thread was discussed between 01/09/2009 and 08/09/2009

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