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MG Midget and Sprite Technical - Temp.gauge malfunction

Good evening Gentlemen
End of last year driving season I realised, that the radiator had a minor leak. I bought a new one, an aluminium one. At the same time I fitted an electric fan and fitted a 82-87 dergrees C switch in the outlet from the radiator.
I fired the midget up today for the first time this year. I warmed up the engine, but it took a decade to see the temp gauge move. Far too long. It did build up the temperature slowly, but when hitting (gayge reading) approx 57C the fan was switched on. Shortly after it switched off again.
I the had a short spin. The temp reading was around 48C.
I felt the same heat as I used too, lots of heat from the heater and when home again the reading was still approx 48C. But everything felt very okay.
Then I took an infrared thermometer and pointed it against the cylinder head just aside the the temp.gauge sensor. The readin on the infra red instrument showed 89C. That sounds correct and I therefore asume my temp gauge isn't working correct anymore.
Could it happen, that the capillary tube has got a leak?
Jan
Jan Kruber

I would have thought that if there was even a tiny pin hole in the capillary tube the gauge wouldn't work at all. Strange one that. I don't have an answer.
Mike Howlett

I had an aftermarket TIM capillary gauge that started to under-read, so it is possible.
Dave O'Neill 2

Possibly the needle has slipped on its spindle? Or, the capillary tube connects inside the guage to a curved (bimetal?) strip. As I recall there is some sort of means of adjusting this with small screws inside the guage and there are two pairs of small datum marks on the guage scale to aid setting the correct needle callibration
GuyW

The curved strip in the gauge is a Bourdon tube https://en.wikipedia.org/wiki/Pressure_measurement#Bourdon_gauge .
David Billington

Jan,
what gauge do you have that reads 57 and 48?

Infra red thermometers readings have been covered here before (far too technical for me, see Archives) but don't take them as gospel.

Have you still got the engine driven fan fitted?

Where are the two calibration dots on your gauge?

You could try disconnecting your electric fan to see if the gauge gets moving more.

It does sound like your gauge might be low but what did it register before?

I'll send you something on water stats in case you also changed that.

Nigel Atkins

Gentlemen
I have bypassed the truth a little miserably when stating that I haven't touched the instrument's. Common for all my instruments is, that this winter I have swapped the light bulps for LED's, supplied by Moss.
When reading your suggestions I suddenly recall this. Could it happen, that the LED's are penetrating to far in the instruments, preventing them from work as they should. I must find out today, I'll be back.
If this is the case, I don't hope internals are damaged.
I'll be back.
Jan Kruber

With regard to your fan switch, I fitted an adjustable Kenlowe controller last year, but I couldn't stop it from leaking.

I decided to fit a switch directly into the radiator top tank, but didn't know which temperature range to go for, so I tested the Kenlowe controller in a pan of water, together with a thermometer.

The Kenlowe was switching on at about 96º and off at 94º. I bought a switch from ebay for a Skoda Fabia, which switches at 98/93º. It seems to work fine, so far.
Dave O'Neill 2

Jan,
I control the correct reading by putting the temperature sender in boiling water (I am living 6 m below sea level here). The infra red temp meter should measure 100 degree C as well.

Flip
Flip Brühl

Jan those Moss LED dash bulbs look a bit over the top are they instead of a cabin light, you must be lit up as thought you're in the movies at night, and wow are those prices each or for a full set and how many in a full set?
Nigel Atkins

Gentlemen of the board, it was those damned LED's!!!
Today I had the speedometer out of the car and with shaking hands I turned over the drive with a screw driver - it worked, thank good. When looking in the bulp hole, I saw a turning device, the LED obviously prevented the device from turning. I reassembled the speedometer, now with a plain bulp. I swapped the temp/oil gauge LED for a bulp too and took the Midge for a test drive. Speedometer readings instantly, the temp rising far more rapidly than yesterday, ending at the excactly same area as last year, 190F, or 87-88C with 16-17C outdoor temperature.
So the instruments seem not to be damaged. Moss should attach a comment that the LED's are not suitable for the Smith's, right?
Anyway, when back in the workshop, I left the engine on a few minutes to see when the fan would switch on. The gauge showed 210F or 98C before the fan turned on. Is that acceptable, or should I find a switch with a little lower range?
Enjoy your saturday evening :-)
Jan
Jan Kruber

Nigel
I love LED's, but you are right, they are quite pricey. The benefit is formidable though, they don't fit at all, %#"/&=)¤f*** !!!
Jan Kruber

Jan,

Any possibility you could make a short spacer tube to prevent the led bulbs going in too far. Maybe a short length of the sort of thin walled tubing available in many hobby shops would do the job.
David Billington

This thread was discussed between 05/04/2019 and 06/04/2019

MG Midget and Sprite Technical index

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