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MG Midget and Sprite Technical - Fuel gauge defunct?

So today I fired up the new engine. Well not actually new, it's the original, expensively rebuilt. Fired instantly once I had corrected the dizzy drive (180º out as usual!). Sounds lovely. Next job was to find out why a fuse kept blowing. Eventually narrowed it down to the fuel gauge. Or I think so. The fuse stopped blowing when I disconnected the gauge, so I removed it altogether. Then I thought, could it be the sender unit? I have pored over the wiring diagram and can't work out what is more likely. Of course I have searched for articles and videos, but they all talk about testing and calibrating the gauge. There is nothing about my problem.

With the gauge removed and the wiring still in place, the fuse doesn't blow, so it's not the wiring. As these gauges are horribly expensive I need to be sure as to what is at fault.
Les Rose

Les,

Have you checked the resistance between the sender unit wire connection and earth to see if it's in a sensible range.
David Billington

Les
It's unlikely to be the sender because it falls to about zero ohms with empty or low tank. The gauge has 2 terminals marked T (tank I.e. sender) and B (battery). With gauge disconnected measure resistance between B and metal body of gauge - should be about 160 ohms. B to T should be about 60 ohms. If a fuse is blowing with gauge connected then it sounds like the B terminal is shorting to the gauge body (earth) which seems unlikely unless a stray earth wire was touching it or the metal of the B terminal connector itself was touching the gauge body.

I doubt it's the gauge internally. Check resistances and report back.
Bill Bretherton

It's not the gauge. I just set off to run in the engine, after 10 minutes at fast idle, and 100 yards down the road it stopped, with a fuse blown again. Walked home for a new fuse, put it in, blew instantly. The fuse that goes is the 30 amp one between the white cable from the ignition switch and the green cables for wipers, fuel pump, fuel gauge, indicators etc. I have disconnected all these one by one and only removing the fuel gauge stopped the fuse from blowing instantly. This I think was just a coincidence. Here is a list of everything connected to that fuse:

Wipers
Fuel pump
Fuel gauge
Indicators
Reversing lights (standard switch on 1098 gearbox)
Brake light switch
Heater blower

Why does it blow so unpredictably?
Les Rose

Les
The only items in constant use are the gauge (now discounted) and the pump. Pump maybe? You could try pressing brake pedal to see if hydraulic switch is shorting. If fuse blows without pressing brake pedal and with heater, indicators and wipers off, then it appears to be pump with intermittent short or one of heater/wipers/reversing/indicator switches intermittently shorting.
Bill Bretherton

Chafed cable perhaps - making intermittent contact somewhere on the fuel guage supply?
GuyW

I think I've narrowed it down to the reversing lights. I will have to check the wiring all the way to the back of the car.
Les Rose

Yes it was the reversing lights. One of the soldered terminals had been knocked and was shorting against the switch body. I was misled by the fuel gauge, which was beset by dodgy connections on both power and sender sides. It seems that as soon as I start investigating one problem, rummaging through the wiring causes lots of others. The gauge is now working, and I have the benefit of having read up about it and fully understand how it works, which I didn't before. Now to get the main beam warning light to work...
Les Rose

Well done Les. A short on a reversing lamp not that easy to spot either as it would appear to be fairly infrequent and therefore rather random!
GuyW

I am relieved it isn't the gauge. Have you seen the price of these things? Here is one from SC Parts.

https://www.scparts.co.uk/sc_en/fuel-gauge-1-187036.html

Almost £300! But for an AH 100 they are wanting £834.

https://www.scparts.co.uk/sc_en/fuel-gauge-1-9050.html

It's identical to the Frogeye one, except for the part number on the face. I get the impression that big Healey owners expect to pay silly money.
Les Rose

'Ow much? 'Kin hell. Glad I've got a spare then, should mine ever fail.
anamnesis

At those prices, they must come with a free "get you home" insurance policy for when you run out of fuel 'cos the gauge reads wrong!
GuyW

Anam

If you have a Frogeye gauge excellent!They were unique to the MK1 MK11. The later cars are different I understand and the Frog one won't work.
Bob Beaumont

I didn't know the frog fuel gauge was different, I thought all the fuel gauges were the same prior to the change to the voltage stabiliser type, whenever that came in. -- 1967/8 maybe?

My car gauge and my spare.





anamnesis

Anam

This is a Frog gauge it has the smiths number on it
FG2530/31. Like yours it is externally lit.


Bob Beaumont

Ah yes, very nice Bob. Pity they didn't keep that pattern, I like it. I'd not noticed that. Rare and hence expensive then.

D'ya reckon the FG is short for frog? 😉
anamnesis

HA HA!!
Bob Beaumont

Anam/Bob

I think the gauges without 1/4 and 3/4 on the face work in the same way and probably have the same internal resistances.

The gauges are simple in construction and are unlikely to fail unless you try to adjust them without being VERY careful. I posted about this recently as I'd had to rewind one of the internal coils in mine after trying to adjust the gauge.
Bill Bretherton

There's one on ebay from Italy Bill. Refurb'd.

At that price, much as the additional divisions do look nice, I'll pass. 😁.

https://tinyurl.com/mw7yyevk

Worth looking in boot sales etc. What other cars of the era had them?




anamnesis

Silly money! 50 euros for postage as well!
Bill Bretherton

Perhaps those in need could capture an image of the original dial, print and stick it over their existing dial.

Job done.
Philip Sellen

The sender unit is different too. They are matched.
GuyW

The gauge is similar to that fitted to an Austin Healey 100/4 (Late type) or 100/6 but as Guy says the sender is matched.
Bob Beaumont

This thread was discussed between 18/01/2023 and 25/01/2023

MG Midget and Sprite Technical index

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