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MG MGB Technical - Voltage Stabilizer?

Can anyone tell me whether a defective voltage stabilizer can cause a false hot indication on the temp gauge?

I have checked the radiator, thermostat, and sending unit (500 ohms cold, 100 ohms hot), yet the temp gauge quickly goes off scale hot. When I measure the temperature at the radiator cap, it shows 175 degrees.

Is it the voltage stabilizer? Other ideas?

Cheers,
Lew Palmer
Lew Palmer

Lew - Does the fuel gauge also give you a funny indication? I believe that both the fuel and temp gauge receive their power through the voltage stabalizer. If the fuel gauge is not acting up along with the temp gauge then I would suspect a short to ground on the wire grom the temp sender to the gauge. A quick check would be to disconnect the wire from the sending unit ans see if the gauge still reads off scale. The next check would be to read the voltage at the output of the stabilizer (or the power input terminal of the temp gauge. The voltage should be about 9 - 10 volts. If higher than that, then the stabilizer or the ground to it would be suspect.
Good luck - Dave
David DuBois

Dave,

This is a fresh restoration, so we have not had enough fuel in the tank to give much of a reading. Besides it was only last week that I got the fuel gauge to work at all. The complete insides of the fuel sender were missing. A new sender fixed that.

The temp gauge starts out OK. It climbs to normal, but then continues to rise all the way to hot and beyond. Yet the candy thermometer stuck in the radiator neck show 175 degrees with a 180 degree thermostat. 185 with a 194 degree thermostat.

If there were a short in the wiring, we should see an immediate rise in the gauge, but we don't. It rises quickly, but not immediately. I was going to check the voltage stabilizer output, but the manual warns about using anything but a hot-wire voltmeter. Maybe I'll check that next with a normal multimeter.

Also maybe the solution is the same as on my (now deceased) Miata air bag warning light - a piece of black tape over the gauge. <grin>.

Cheers,
Lew
Lew Palmer

Lew,

The black tape across the gauge may not be such a bad idea, one less thing to worry about. I have read the output of the voltage stabalizer with a standard (analog, average responding) multimeter and didn't have any problems. A digital multimeter would not know which reading to display and would keep changing readings (new technology isn't always the best, but those of us with MGs already know that). As I recall, there are two different sending units for the later MGBs, so it is possible that you got the wrong one (even if you ordered the right one). Cheers - Dave
David DuBois

I have had this problem a couple of times on different B's. The first thing you should do is stick a thermometer in your coolant and figure out if it is running hot or not. In my case, it was indeed a false signal - the coolant was normal. I messed with the voltage stabilizer, adding a redundant jumper ground wire to make sure that was not the problem. That wasn't it. I replaced the sender and that wasn't it. I cleaned every ground strap, cable and wire I could get my hands on and THAT was it. I would start with confirming the temperature of your coolant, and then go on to basic ground cleaning. On two different cars that seemed to be the ticket. I have noticed in general that the temperature difference between "N" and almost in the red is very small (maybe 20 degrees?). This makes for unecessarily alarming readings, IMHO. An actual temperature reading gauge (digital?) that could be installled temprarily could allow us to learn what the various needle positions really mean. Hmmm....next project?

Dean
Dean Lake

Lew,

My gauge gives odd readings and I have been wanting to test the sender and gauge seperately.

The resistance figures of 500 ohms cold and 100 ohms hot, are these the specs for the sender? Do you know what hot and cold are in terms of temperature? Do you know what the reistance should be at normal? I can cope with either C or F.

Thanks

David

David Witham

I just checked two senders in my cars with cold engine, around 70 F degrees. The 74 reads about 725 ohms, the 68 around 750 ohms. I have a new spare sender and it checks around 680 ohms cold. I heated it with a hi watt soldering gun and the resistance dropped to 150 ohms. I don't think I had it heated to normal operating temp so 100 ohms may be about right. FWIW, Clifton
Clifton Gordon

If you have a bad ground, the resistance can change as it heats up. Think expansion and contraction. You probably have a weak ground connection somewhere. Keep on hunting, cleaning and tightening.
R. L Carleen

A failed voltage stabiliser either causes no reading on both gauges or a slightly high and fluctuating reading on both gauges. However the voltage 'stabiliser' switches 12v on and off once a second or so, so this is what you see rather than a steady 9 or 10v. Going off the scale indicates that the wire to the sender may be grounding as has already been said, or the thermistor in the sender has failed, but both of these would cause the hot gauge reading as soon as the ignition is switched on with a cold engine. If it starts off on C then rises through N to H as the engine warms up then either the thermistor has failed in a very strange way, the sender is the wrong type, or the engine really is overheating.
Paul Hunt

Lew - Now may be the time to dump the sender in a pan of water on a camp stove next to the car (suspend the sender in the water with a piece of wire that is also attached to ground), along with a thermometer and boil the water. Watch where the gauge goes for a given temperature reading and what the gauge reading is when the water boils. If you want to really get carried away replace the water with whatever mixture of coolant you use. I did this when I made a converter to use a MGB sender in our Magnette and calibrated the gauge to read where I wanted it to at boiling. Good luck - Dave
David DuBois

Thanks to all who responded. I'm happy to report that although the gauge reads higher than I would like, after changing the gauge with a spare, the temp hasn't shown above 5/8 scale.

Time will tell for sure, but for now all appears fine.

Cheers,
Lew Palmer
Lew Palmer

If the temp gauge is like the fuel gauge you should find two slotted plates on the back (may be covered with cork plugs) that allow calibration of min and max positions. If you should want another usable spare.
Paul Hunt

This thread was discussed between 04/08/2003 and 07/08/2003

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