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MG MGB Technical - voltgage stablizer fried

two weeks ago I my fuel and temp gauges went dead and I saw some "smoke" coming from my dash. After doing some reading, I found the voltage stablizer- which was fried and I bought a new one. I replaced the new one and it immediately "fried" as well. Can someone help me to determine the fault before I try to install the third stabizer? Hopefully the third time will be a charm.
Thanks,
Bud
Charles Yanak

Charles,

It sounds like there is a dead short on the stabilizer output that feeds the gauges. I don't remember what color wire this would be (green/yellow?), but don't put another one until you remove and fix the short.

Regards,

Steve
Steve Brooks

Charles, The stablizer load wires going to the fuel and temperatue gauges is light green/green. The input is green. I would have thought the fuse for the green circuits would have blown before frying the stablizer. Have you checked to be sure you have the correct fuse in the green circuits? Don't connect the LG/G output until you find the problem as Steve suggested. Check the wiring from the LG/G terminal back to the gauges and senders for a short circuit using an ohm-meter or test lamp. You didn't say what year car you have. Does the wiper motor, heater blower and tachometer work?

Clifton
Clifton Gordon

I hate to say this now but if something fails like that one should always determine the cause before replacement, as you have found out. It's quite possible that with various marginal connections plus the resistance of the stabiliser heating coil there will be insufficient current to blow the fuse but enough to damage the stabiliser. I'd also say that any short to ground between a gauge and its sender - whilst it would cause the gauge to rapidly go to full-scale-deflection - wouldn't burn out the stabiliser for some time at least, and you would priobably notice the gauge being past its normal maximum first. Indeed, shorting the sender wire to ground *briefly* is one of the standard tests with a non-functioning gauge.

I'd connect a high-wattage bulb like a headlamp bulb in series with the green and the stabiliser. Then if the short is present the bulb will glow at maximum brightness as well as limiting the current, in a circuit without a short I'd expect only a dim glow from the bulb if at all, there should be little if any effect on the operation of the gauges. If the problem is becomes intermittent, which is quite possible if it is a wire or connector shorting out and you go pulling the wires about, you could leave this bulb in-situ while you are tracking it down.

For crude diagnosis you could leave the stabiliser out of circuit altogether - just join the green to the light-green/green - and if the short is present that *should* blow the fuse. Again if intermittent you could leave it like this, but be aware that the fuel and temp readings will be higher than normal, and fluctuate according to system voltage e.g. how much electrical load is on the alternator.
Paul Hunt 2

Clifton,
As a bit more background, I have a 1976 MGB and about five years ago the voltage stabilizer smoked as I pulled out of the driveway, within a minute of starting up. I purchased a replacement at Moss and installed it without any issues the first time. This time, it smoked seconds after starting up. This time, I replaced the fuse, which was not blown, but just to be safe, I purchased a new one since the original fuse looked to be about 30 years old.
I did not check the blower motor and wipers, but last time this happened, they worked fine. The Tach works fine. I have an ohm meter but the wiring diagram I have leaves a lot to be desired. Any further tips on how to diagnose the problem is appreciated.
Bud Yanak

Bud, Paul Hunt's suggestions for trouble shooting the problem are good. Although it sounds like the green circuits going into the stabilizer aren't shorted it's safer to use a technique that will not continue burning out stabilizers.

For a wiring diagram that is easy to follow go to Advance Auto Wire;
http://www.advanceautowire.com/
On the left click on stock schematics. Find MGB,MGC, etc. Find page 31 for late US market MGB. That may not be identical to your wiring but it will be close. US market 73 & 74 should also be close.

If you have an intermittent problem you will need to leave some protection in the circuit as Paul suggested. Have you actually checked the new stabilizers to see if they are working before installing? I have a used stabiliser and did some tests today using a 10 amp battery charger for a power supply. Hook the red battery charger clip to terminal B and the black clip to the mounting tab and measure the voltage input on terminal B, it should be around 12 volts. Terminal I should have a varying voltage averaging 9-10 volts. It's easier to observe the variations with an analog meter rather than a digital meter.

If you find a short indication I would remove the LGG wires from the gauges and at terminal I and check for a short in the LG/G wires with an ohm meter or test light. Do the same between the gauges and their respective sending units.

Another source of information can be found at;
http://www.mgcars.org.uk/electrical/stabilizer.html

Clifton

Clifton Gordon

Clifton,
Thanks for the information. The wiring diagrams will be a big help. I'll try to debug the problem this weekend. From the looks of the wiring diagrams, and the symptoms I am experiencing, it appears that the voltage stabilizer feeds my temp gauge and fuel gauge. I suspect I have a short to ground somewhere and will hopefully uncover it. I’m not clear on how the sending units work, so hopefully the short will be in the harness leading to the gauges and easily found and repaired.
Bud
Charles Yanak

The sending units are simply a variable resistance - high when the tank is low or the engine cold, low when the tank is full or engine hot. Shorting a sender to ground *briefly* just to check the gauge starts to move is a common diagnosis technique and will not harm either gauge or stabiliser. Leaving it shorted for a long time could well damage either as the current passed will be higher than when the tank is completely full or the engine hot enough to put the gauge into the H zone.
Paul Hunt 2

This thread was discussed between 25/10/2005 and 06/11/2005

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