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MG MGB Technical - Temperature gauge

I have a 67 BGT. Had an electrical problem after replacing gearbox which was traced to after market alarm. Alarm was removed and headlamp wire to switch was replaced as it had melted! All fine since. Yesterday moved car and when I tried to start, it appeared to have a flat battery and didn't want to start. I persisted until I spotted smoke coming from engine bay/ under dash. The capillary wire and choke cable were really hot! I left it for a while after poking around and it started OK and seemed fine. Took it for a run and all fine. I then noticed temperature gauge had swung right round beyond maximum and into oil pressure zone. It stays there and doesn't move. It's a capillary type, not electric. I then remembered that when I had previous problem, as I cranked the engine to start, the fuel gauge moved up as I turned starter! Any ideas where to look for the short?
S P Ayello Wright

others here will know more about this than me but the choke (and capillary?) getting hot might be where they are acting as earth leads and getting hot because of the large draw from turning the starter motor

so it might be an earth short where your present main earths are not good in condition or connection

it should not be ignored or car driven until it is fully sorted
Nigel Atkins

Sounds like you have lost the earth link from the chassis to the engine block, when cranking all the current required for the starter is being pulled through the gauge & choke cables. Check around the 'Big' earth connections, battery & chassis to block (under the carbs.
P M Gregory

I agree, you have obviously lost the engine earth strap. This should be wrapped around one of the engine mounts, or it could be from one of the bell housing bolts to the chassis. It doesn't matter where it is, but you MUST have a substantial braided cable connecting the engine to the chassis. The starter motor can pull over 200 amps and all that power needs an earth return. The capillary tube and choke cable are not enough!

If you have overheated the capillary for the temp gauge, I'm afraid the gauge is goosed and will need to be sent away for (expensive) repair.
Mike Howlett

Definitely the engine earth strap failed, usually this causes the accelerator, choke and heater cables to get hot. I've not heard of the temp capillary getting hot like this but there's no reason why it should not as any alternative earth path will be taken.

Chrome bumper cars originally had the engine earth strap round the left-hand engine mount. Rubber bumper cars had them round the gearbox mounts to the removable cross-member.

Might be worth carefully pulling the oil/temp gauge forwards or removing it, opening up the gauge head, and seeing if the needle has swung round so far it has jammed, but can be released. Normally when the tube has failed the needle won't move off C.

If it has failed the normal process is to replace, it's not even an exchange item these days.
Paul Hunt

Speedy Cables (http://www.speedycables.com/index.html) offer a repair service for the dual gauge if yours is damaged.
Mike Howlett

Thanks for your help. The existing engine strap seemed OK, but I've added another from dynamo mount bolt to engine mount, body side, but as fault was intermittent I'm not sure if it's a permanent fix.
Temp gauge is stuck right up past 220 deg F at 80lb/ sq inch oil pressure! Any chance of bringing it down electrically or manually or is it an ex-gauge?
S P Ayello Wright

My V8 temp needle was stuck on the plastic light diffuser, which had dislodged itself.
Allan Reeling

Like I said it's worth pulling the gauge and investigating it's internals, it's more likely the mechanism has jammed on something from having been pushed round too far as Allan had.
Paul Hunt

Paul. Thanks. I'll try it over the weekend.
S P Ayello Wright

This thread was discussed between 19/05/2014 and 04/07/2014

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