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MG MGB Technical - Voltage Stabilizer?

I have a 78B with a moss supercharger. I have a mild electrical issue which is probably either the voltage stabilizer or a bad alternator. I've noticed that my fuel gauge varies slightly based on engine speed. At idle, it will drop about 1/16th of a tank. After a few seconds of driving (maybe 5 or so) the gauge will go back up to where it was before stopping. I think my temp gauge varies the same way. my first thought was bad voltage stabilizer. however, when I start the car, my ignition light comes on and stays on until I run the rpms over 2k. Additionally, my turn signals blink very slowly at idle, but blink normally at speed. So, I'm afraid my problem is the alternator not putting out 12 volts at idle, which is more expensive than the regulator. Any other things to check for?
J. Lawrence

The variation in the gauges sure sounds like a voltage stabilizer to me. Not to worry about the blinkers not going as fast at idle. You are probably correct that then gen isn't giving a full 12+ volts at idle...so what? Mine has been doing that for 20 years or longer. It's a '72 B and as far as I know, it's the original alternator, but it has had a rectifier replaced only 18 yrs ago.Put a voltmeter on mine years ago and it will drop down to 10.5 or 11 volts at idle but picks right up off idle so I don't worry.
G. Wayne Hardy

Hi J.

Does the ignition light come back on when the revs drop to tickover ?. If it does you do have a problem.

Does the engine turn over briskly on the starter ?.
If it is sluggish take a look at the engine earth strap.

The instrument voltage regulator is a fairly crude device, I don't think you have a problem there.

Don

Don

"Does the engine turn over briskly on the starter ?.
If it is sluggish take a look at the engine earth strap."
Many of the old lags have been caught out and have fitted a second earth strap where we can see it - in the engine bay.

FWIW

RMW

The voltage stabiliser is in a metal housing and I think it needs to earth through that housing. Mine was all rusty causing it to make intermittant contact. A good clean and some copper grease fixed it right up.

Simon
Simon Jansen

Thanks for the replies. The engine turns over briskly at start up, so, it probably isn't a ground strap.

Don, once the ignition light goes off, it stays off until the next time I start up.

Looks like the voltage stabilizer is probably the culprit - I guess it could be much worse. This one is about 4 years old, which is about 3 years older than the one it replaced.

thanks for the help.
J. Lawrence

The voltage stabiliser wouldn't have anything to do with the ignition light. My money's on the alternator.
Rob Edwards

As Rob said, I would check the alternaor first. After the alternator is working correctly I think you will find the stabilizer is OK.

Clifton
Clifton Gordon

You can replace the voltage stabilizer with a solid state unit. I've seen them on e-bay. It will last a lot longer.

Or you can make one yourself using a LM317T voltage regulator available at Radio Shack.

http://home.comcast.net/~whaussmann/InstrStabilizer/Instr_Stabilizer.htm
werner haussmann

Thank you, Werner!
Joe Ullman

The ignition warning light should go out at about 900 or 1000 rpm. If the light is disconnected the engine may have to be revved to about 3k before it starts charging, then it will charge normally down to the usual 600 rpm or so. If your light is working and the engine has to be revved to about 2k then problems with the alternator are definitely indicated.

Until the alt starts charging and the warning light goes out the system voltage will be about 12.5v, after it goes out even at idle you should have about 14v (unless you switch everything on), slightly higher when revved to about 2k or so. This is why the turn signals flash slower with the light on than off, for example, and also vary slightly with revs and electrical load even when the alt is charging. However bad connections through the brown, white and green circuits will make this effect worse.

But because the temp and fuel gauges are supplied from the voltage stabiliser they should *not* be affected by voltage changes above 10v or so. 'Voltage stabiliser' is rather an amusing term, since it works by switching the system voltage on and off about once per second. It is the changing ratio of on time to off time which results in a constant *average* voltage to the gauges, which being thermal types are very slow to react to the switching on and off, and hence give a relatively constant reading for a wide variation in system voltage. As far as the varying gauges go it may just be that the can of the stabiliser is not making good contact with a ground, which it needs to do. This is entirely a separate issue to the warning light not going off until the engine is revved to 2k.

Paul Hunt 2

http://cgi.ebay.com/SOLID-STATE-VOLTAGE-STABILIZER-MG-midget-MGB-MGA_W0QQitemZ4645950925QQihZ002QQcategoryZ27380QQssPageNameZWDVWQQrdZ1QQcmdZViewItem#ebayphotohosting


This one is on ebay at the moment. I bought a solid state one and it helped
Frank Baker

Or just purchase an authentic Smiths voltage stabilizer, original in every way.

http://www.gaugeguys.com/Smiths/Accessories/accessories.htm



*Note, stay away fromthe reproductions that Moss offer. They are defective.
Boris

If it is your alternator, a bosch type replacement is available from Kragen auto parts (or whatever they call themselves in Texas just look up your local chain auto parts stores) for about $50-.
Just look up 1980 Ford Fiesta. There are two types of alternators available. You want the Bosch one.

http://www.partsamerica.com/ProductList.aspx?PartType=11&PTSet=A&SearchFor=Alternator

Worldwide Imp ALTERNATOR
Reman; with Bosch Alternator; 55 Amps
part #13107
Kimbelry

This thread was discussed between 22/05/2006 and 02/06/2006

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