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MG Midget and Sprite Technical - Electrical temp gauge

Hi all,

Slowly getting round to some other bits and bobs while I wait for suspension bits, i noticed that after being laid up for the winter, the temp gauge has given up.

I have tried earthing the wire from the sensor and the gauge did nothing, i presume this means gauge? Or could there be another contributing factor?

Cheers all!
Karl Bielby

Could be a bad connection somewhere behind the dash. Does the fuel gauge work OK?
Dave O'Neill 2

You did have power, right? Sometimes it's the obvious we over look.

Check your connections from power source to the gauge and continuity from gauge to sending unit.

If everything is good, and the gauge doesn't do anything with the ignition on and nothing when you ground the wire, then in all probability the gauge is toast.

Clare
Clare Ravenwood

Power in terms on ignition was on.. i haven't checked the power at the back of the gauge though.

Cool, will check that when i get chance. I presume there will be an illumination live and a live for the gauge?
Karl Bielby

Yes, Karl, ignition on. Your fuse could have failed which is why you need to check power to the gauge itself.

If your gauge is illuminated, that will be or should be, on a different circuit. The illuminating light comes on when the main lights are turned on. Of course, some DPO could have wired the gauge into the headlight circuit but very unlikely.

You should have power to the sending unit through the wire as the resistance in the sending unit makes the gauge show the reading.

Good luck, let us know what your findings are.

Clare

Clare Ravenwood

Ah yeh, it was working as it should with regards to lights previously so the wiring should be fine in that respect. I did check all fuses and they looked fine but i didn't continuity check them. Odd thing is my front passenger side indicator has also stopped working, probably unrelated though?

Il hopefully get a good poke around over the weekend.

Cheers for the replies!
Karl Bielby

Karl,
check out the wiring diagram, there's one in the Driver's Handbook which will also tell you which fuse covers what.

If it's only one corner then it's unlikely that the indicator fault is related, if all could it be.

Without you putting up a vehicle profile or saying which year your car is can widen possible answers, not that any Spridget will necessarily fully comply to original factory specs and fittings, there's still quite a bit of variation in the 1500 model over the production years.

You could also check the voltage stabiliser is fitted correctly, earthed and working.
Nigel Atkins

Ok I'll have a look for a handbook online.

Apologies i thought i had filled all the vehicle profile info in when i joined up! Will look into it.

Ta
Karl Bielby

Assuming it's a 1500 see Ref: 0058 here -
http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

to confirm, -
http://brooklandsbooks.co.uk/index.php?route=product/product&path=12_138_631&product_id=557

If you have a 1975 model I have a s/h more original copy you can have for a small donation to charity.

ETA: site might have lost profile link on you might not have profile set to be viewed.
Nigel Atkins

Why yes, it was unselected to display!

Ta, Il have a look at those.
Karl Bielby

Karl,
a 1979 1500 will have a few extra wires for the items that were added to the later models but basically the same, certainly for electric sender, gauge and voltage stabiliser so check all these and the connections and earths from the fuse and fusebox forward.
Nigel Atkins

Hmm well iv just had a very quick play, there is continuity between the sender wire and earth. I pulled the gauge out and cleaned connections and it's come alive. However it only reaches 3/4 with the sender earthed...
Karl Bielby

You've lost me there, but continue with your checking and cleaning connections, do it in a logical run, say from the sender all the way back to the battery and check all earths along the way including the main battery and engine earths, not just to help resolve this problem but to discover, or help prevent, other or future problems.

All connections (and earths) want to be clean, secure and protected, don't just rely on a visual check.
Nigel Atkins

Yeah I've have a good check over. Found a couple of connections on a bad angle caused them to fray, so they will be replaced. Unfortunately just the heater blower switch though!

If i am correct, when you earth the temp sensor, the gauge should go to max deflection? Mine only reached 3/4 temp.

Where about a is the main loom earth? I have found the engine earth and the one in the bulkhead by the battery.
Karl Bielby

Karl,
I'm not good at lots of things Spridgets including 'lectricks but as you test the sensor by its resistance I'm not sure your right about that as surely it's always fully earthed by being screwed into the engine - but don't go by me.

Look after the heater blower switch if it's original as they're often broken and replaced and I think if you get them new were about £50 but again don't go on my memory with this.

And to complete the set, sorry I don't know where the main loom earth is (on 1979 1500) ...

but someone else will.
Nigel Atkins

Karl there is no main earth loom, the front and rear are earthed separately. Look for black wires screwed to the body in strategic places which go into the loom.
As Nigel said check all the earths, especially the engine block earth in the engine bay. This is the one that is most important for numerous reasons.
From memory the gauges are fed from a voltage regulator attached to the back of the speedo, which feeds the gauges with about 10 volts IIRC.
Check this first with a digital multimeter.
Someone may recall the range of the sender in OHM's but it may not be 0 (hot) to whatever (cold). IOW, earthing the gauge may not be instructive.
I fitted a digital gauge - no more issues
Cheers
Rod
R W Bowers

Again I might be wrong but I think the instrument voltage stabiliser may be screwed to a part of the body or a panel in the dash area. IIRC the screw is what earths the IVS so check that is clean and secure and IIRC some (all?) IVS have to be fitted the right way up, they had an arrow punched in them to show you.

You want to check all the connections on and to and from the IVS as well as it's earth so you're back to checking all etc., etc. ...

Keeping going to (both) ends as it'd be a worth while job if only for assurance and you may find or prevent future problems - and of course Sod's Law the one connection or earth you miss or ignore will be the one causing the problems.

Don't forget things like even cleaning the fuse ends and fuse box internal and external connectors - all checks with battery disconnected and better still being charged up to full out of the car.
Nigel Atkins

Ahh interesting, i have just got the speedometer back from overhaul.. wonder if the new voltage regulator is a bit different in some way?
Karl Bielby

Apologizes to Rod and yourself as obviously the IVS is on the back of the speedo on 1500, my poor mixed up memory.

If you have a brand new IVS then put the old one back on as new ones can be very poorly made (IIRC?).
Nigel Atkins

Your memory/knowledge out weighs mine Nigel!

Sounds like a good place to start, Il definitely give that a go next time I'm over.
Karl Bielby

>>Your memory/knowledge out weighs mine Nigel!<< - only on some midgets perhaps and only because I'm old so have experience, most of which I didn't understand or can remember, or have read it here (or in the 'good book').

If this was the case generally then I'd advise you not to leave home unescorted.

Often, not always, the simple basics get you there, you won't usually progress successfully if you've not covered the basics.

Much of servicing and maintenance on classics is basically just cleaning and lubricating (and protecting).

As I meant to put last time, keep going.
Nigel Atkins

This thread was discussed between 10/04/2017 and 19/04/2017

MG Midget and Sprite Technical index

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