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MG MGB Technical - Incorrect water gauge reading - revisited

Sometime last fall I posted a thread about my water temperature gauge reading H incorrectly on my '68 B.

A lot has happened since then (heart surgery) and I finally <today> got my car back on the road. Among other things I put in a new wiring harness, a different temp. gauge, a different sending unit.

The gauge still reads H. I tried a different (used but good I think) voltage stabilizer. I tested the thermometer while I was working on the car. I tested the water temp (in the past) with a candy thermometer and it's not oveheating.

The gas gauge reads correctly more or less. It shows about 1/4 tank gas. But the temp gauge slowly moves up and after 2-3 minutes of running, is sitting on H.

If the voltage stabilizer were bad, wouldn't the gas gauge read FULL?

Any ideas would be greatly appreciated!!

Dave
David Steverson

David "I put in a new wiring harness, a different temp. gauge, a different sending unit." You have two unknowns in the car, either or both could be causing the high reading. If you have the original gauge and sender, I would suggest you put those back in and see how they read. If you have a good reading with them, swap in the new ones one at a time, checking them out after each swap. I don't think that the stabilizer is bad or you would have goofy fuel level reading also. You could have a wire shorted to ground between the sender and the guage (remove the wire from the sender and see if the gauge climbs - if it does, remove the wire from the gauge and try again - it it still climbs, the gauge is shorted. If it doesn't climb with the wire removed from the sender, then the sender is bad). Cheers - Dave
David DuBois

Dave,
The car had this problem when I bought it. The sender was old, so I put one in from Advance auto parts. No joy. The current one came from a B suplier (B Hive).

I did ground the sensor wire (with iginition on) and the gauge did not move. Is that as it should be?

On a post I once found in the archives (and cannot find now), it was mentioned that you might go through several replacement sensors before getting one that works correctly. Any thoughts on that?

Thank you for your reply.
David

PS Warm weather seems to be here and my B wants to be driven!
David Steverson

There are two (at least) gauges and senders and if you get the wrong combination over-reading is one of the problems you get. However that is *over* reading, not pegging on the H, which implies grounding of the sense wire as Dave says. But then again, if it *is* grounding, it will shoot up to H as soon as you turn on the ignition, even with a stone-cold engine. In that case do the steps Dave advises, and you will diagose it to sender, wire or gauge. But you also indicate that the gauge does *not* move with the ignition on and a cold engine - either initially *or* when grounding the sender. It shouldn't do with a cold engine, but it *should* do with the sender wire grounded. If grounding the connection at the sender didn't cause the gauge to move, then your ground must have been bad either where you were picking it up or where you were connecting it to the sender. Or you were connecting it to the wrong sender (you want the one by the thermostst with the green/blue wire), or the wire is bad. But if you turn on the ignition and the gauge continues to read C, and only gradually rises to H as the engine warms, then it isn't shorted anyway, and you are back to the 'basic' over-reading scenario. In that case with the ignition on, engine running and up to temperature, disconnect the wire from the sender and make sure the gauge drops all the way back *past* the C to its 'ignition off' position pretty smartly. If it doesn't go all the way back then you could have a partial short, which will cause over-reading.
Paul Hunt 2

Paul,
I was wrong abou the gauge not moving when I grounded the wire, it does. I tried again last night, this time making sure I had the wire well grounded.

I was wondering if I could try this? >>
get a new voltage stabilizer and hook that up to the sensor and a gauge out in the engine compartment. That way i could switch things around until i obtained a working combination.

Thanks! David

David Steverson

If the fuel gauge is OK then the stabiliser is OK. And a stabiliser fault doesn't cause the temp gauge to over-read as high as H (when it should be on N) anyway. Do the other checks and let us know the result, like making sure the gauge only rises gradually from C to H and only as the engine warms, and drops smartly all the back below C again when the sender wire is disconnected, even though the ignition is on and engine running.
Paul Hunt 2

This thread was discussed between 02/03/2008 and 03/03/2008

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