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MG Midget and Sprite Technical - Temperature Gauge Not Working

1500 with a mechanical temperature gauge. I was out the other night on a quick test run after some small repairs and I noticed the temperature gauge packed up and started going down. Odd I thought but I was one minute from home so kept going.

Checked coolant level this morning - there was some. Drove it today (15 mins to work) and no temperature reading at all.

Checked it again just now and topped it up approx 100 ml. A bit in the filler, a bit in the expansion tank.

A very small amount of mayo on the filler cap, but car hasn't been driven a lot lately.

Drives and idles OK.

Is the temperature gauge broken (I thought this unlikely) or is something more sinister afoot? i.e. head gasket (leak from coolant to oil passageway?)

Thanks for your thoughts folks.

Malc.


Malcolm Le Chevalier

The mechanical gauge works on the principal of an expanding liquid in a thin metal tube ( think its an alcohol based substance)The hotter the water the more the liquid expands and moves the needle on the gauge.

Can you remove the sender from the block? Not sure where it is on a 1500. It will be a metal bulb.

Immerse the bulb in hot water, if the bulb and the pipe work are OK the gauge will register a temp. this means the problem is somewhere else. If nothing happens then the gauge is defunct and needs to be repaired. This is a job for a specialist. Most of the usual suppliers do an exchange service.
Bob Beaumont

I have certainly had a 1500 temperature guage fall when the water level in the cooling system was low and left the sensor bulb high and dry. And especially to worry you, I can also report that this was a pre-cursor to a blown head gasket on one occasion! :-(

I would doubly check your water levels in case there is some sort of air lock forming. Try squeezing the fat rubber coolant hoses a few times to pulse them and hopefully force any air bubles out.


Or maybe its just a failing sensor. :-)
GuyW

Water level should have been high enough, and certainly no problems with it in the past six months. But having to top up the coolant is a worry.

Will see how it goes on the way home and monitor the situation. I hope it's not HG. I can't face doing it again :-(

I can't remove the sender without draining the top half of the system. I might do that though to check.
Malcolm Le Chevalier

Malc,
did you fill the from the filler with the pressure cap fitted.

100ml isn't a lot, are you sure you're not trying to overfill it.

Test the bulb and refill coolant using the instructions in the good book by IIRC disconnecting the expansion tank hose and raising it to make it highest point to help bleed and fully fill system.
Nigel Atkins

No dramas on the way home. Nothing weird and all hoses are hot when parked up.

Current, but untested, conclusion is sudden gauge death.

It's an aftermarket gauge so only 30 to replace. I ended up with it after mistakenly snipping the "wire" coming out the back of the Smiths dual gauge one about 8 yeras ago :-D

Malc.
Malcolm Le Chevalier

As all know I'm no good at mechanics but I've never had a problem with air and never had to do anything other that just follow the refill instructions from the good book and I've had to refill the coolant system on my present Midget alone far too many times.

In fact only last night I had to do an equivalent drain down on mine that you'll do but that's another story for another time.
Nigel Atkins

Love the avatar Nigel!!
Jeremy MkIII

“Love the avatar Nigel!!”

I’ve just had to turn that option on, in order to see what you meant. Excellent.
Dave O'Neill 2

IIRC instructions exist in the archive for refilling them yourself with starter fluid which is basically ether and the chemical used originally.
David Billington

I didn't realise that it wasn't on view to all, even upped my references to it, as on this thread, I thought it was strange no one had mentioned it. I didn't know it would also show on all previous posts. As it takes up a lot of room I will hide it from view after tomorrow.

I didn't know it was an avatar either, I'll get the blue paint on me.

Back on subject, David, I remember seeing the temp gauge repair advice either here on the MGB threads.
Nigel Atkins

After many attempts to repair my gauge sender I found this to be the best way.

First, fix any breaks in the capillary tube. It usually fractures very near the bulb, so you can drill out the stub and solder the tube back in. Then find a small brass nut - about 6BA. Drill a hole in the bulb end and solder the nut carefully so that it surrounds the hole. Have ready a short screw that fits the nut, with a fibre washer, a small disposable syringe with a thin brass tube attached using heat shrink tubing, a can of starter spray (which is essentially ether), and a can of freezer spray (from electronics suppliers).

Mount the bulb so the nut end is uppermost. Freeze the bulb until it's thoroughly frosted. Cool the syringe and spray 1-2ml of ether into it. Inject this into the bulb, and insert the screw tightly with its washer. As belt and braces I covered the nut and screw with Araldite Metal. This is all far easier and more reliable than other methods I have tried.

You will then need to calibrate against a known thermometer. The linearity might not be quite the same as originally but you are really worried about accuracy at running temperature. Hope this helps - I found it on a Jaguar site but have lost the link now!

Les
L B Rose

Thanks all.

Still yet to test the old gauge and going on holiday later this week so not going to get chance.

Car is running well though so doubt anything is seriously broken.

I will report back when I get chance for some proper spanner time.

Malc.
Malcolm Le Chevalier

This thread was discussed between 07/09/2018 and 11/09/2018

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