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MG Midget and Sprite Technical - Fuel Guage query

My fuel guage has two terminals on the back. Neither are marked. How do I establish which one takes the power and which one goes to the tank? Any help gratefully received.
Thanks,
Neil
Neil T

Neil I have two others here on my bench.

Neither of them has markings either.

I suspect that 'cos they take volts in at their end and the circuit closes at the sender unit (I think it's this way round) either terminal will do. Gauge just measures the passing of current, sender decides the amount allowed to pass.

Probably wrong but hey!

Bill1

Uhm, you could be right Bill. Why don't they put the part no. on these guages and mark the terminals! I need Dr J E Davies of Cambridge! Got a nice new shiny alloy tank with new sender and the gauge won't work! I know it takes 9.25 gallons though!
Neil T

They should both take power from the gauge... and as the float moves, the sender will have differing resistance and allow more or less current which will be reflected in the gauge.

Do you have an AVO type meter? Measure the resistance seen from each terminal to earth as you move the float, and that will provide the info you need.

A
Anthony Cutler

OK, so it doesn't matter which terminal has power going to it then?
I haven't got an AVO meter.
Tank is full of fuel so moving the float is a problem unlss I take some fuel out of the tank.
Neil T

well, when you raise the float, you need to see effect on resistance - it could go up, down, or stay the same...

A
Anthony Cutler

Neil,
as Bill has put I don't think it matters

my 1973 car, as you look at the back of the gauge, has the light green/green wire to the left terminal

and the green/black to the right (hand) terminal

and my the gauge works

on mine the green/black wire is the one from the sender which earths out on the tank/body(? as the tank is secured to the body)

I'd first check the (two?) bullet connectors connections (clean, secure and protected) from the tank end to gauge

you could also check the the connections on the voltage stabiliser and make sure the stabiliser is fitted level(?)

check the fuse and connections to fuse box and check other items also on that fuse
Nigel Atkins

Thankyou for that Nigel.
I also have a mk111 midget and so tried my guage on that car. The guage worked. The wiring was as you described on your car.
The car I am having trouble with is a modified Mk11 Midget (these didn't have a voltage stabiliser). I have fitted a new sender unit (for the 68-72 Midget BHA4711. I choose this specifically to fit the tank I had made) and I have fitted an electronic voltage stabiliser. Interestingly at tickover the voltage at the supply is 10v (which I think is low). The voltage from the stabiliser is 8v. The tank is earthed to the bodywork.
I'm confused. There is an answer out there someshere!
Neil

Neil T

uhmmm, "dinner is served" so I have to go

but voltage getting to the guage should be average(?) 10v as far as I know (which isnt that much)
Nigel Atkins

I assume when you tried the gauge on the MK3 it read accurately and that your modded Mk2 is negative earth and that the type of electronic stabiliser is wired correctly and correct type to your earthing

in the (not always to be trusted but seems ok for wiring diagrams) Haynes for MK2 it shows:

. sender earthed
. green/black wire from sender to gauge
. two bullet connectors from sender to gauge
. gauge earthed
. 1098cc fuel gauge has terminals marked 'B' and 'T' with green/black wire to terminal 'T' and green wire from fusebox to terminal 'B'
. green wire also has double bullet connector to heater motor switch

looks like on 1098cc (positive earthed) cars the wiring direction is imporant - at least when using original gauge and wiring set up
Nigel Atkins

If its the same as mine, to test the guage simply disconnect the lead to the sender (i.e. infinite resistance). The guage, at least on mine, shows full. Now connect the sender lead to earth, and it should show empty.
As I say, at least thats how it works on mine. The problem is calibrating - something I am about to do. Can point you to a useful article on that if it helps
Good luck
Graham M V

got to drive my wife to drink now

so for both of you something on this site might help, it'll be MGB related by principles will be the same if applied to same type of wiring and earthing systems

good luck - http://www.mgb-stuff.org.uk/hometext.htm
Nigel Atkins

Man from the Deep West says

"Why don't they put the part no. on these guages and mark the terminals!"?

I have half of your request on my two Neil

Paper stick-on part numbers (I suppose)

No terminal markings :(

Excuse blurry pic

I'm sober


Bill1

I have some of those stick on numbers on mine too Bill but they bear no resemblance to the Moss part nos. Also the numbers on mine are different to yours. It's all chaotic if you ask me. Still no terminal numbers. Anyway, must mention that I went to the pub in it tonight and on the way back the gauge started to work!!!! What's going on?
Neil T

Hope it was a decent pub Neil

As to the gauges, 85% black magic I reckons
Bill1

Best pub in Pembrokeshire Bill, the Cresselly Arms at Creswell Quay. This is the car last night.


Neil T

Neil,
well lets start with the important stuff - we've yet to make it over to Janet's (?) pub but have been very fortunate enough to have been to Bessie's at Portfaen three times in as many years and eh the Bass was good - by the time I get out that way I've had enough of water as the others like seas, lakes, ect and I don't

back to gauge - if it started to work then I'd put my money on connections or wires, until it works for a while you can't tell if it's accurate

I don't know if you've modified your wiring so I put stuff before for 1098 as standard

my fuel gauge has no markings or label on the back but around the side is a paper label with 'CAEBONT AUTOs? BIMETAL ...'

is it that your mixing a later gauge and stabiliser with earlier wiring?
Nigel Atkins

Next time we're over that way Neil I'll call you and we can pop down there, looks very good

I bet my dad knew it from his days flying from Pembroke Dock back in 44/45

As for the gauge working it is probably bad connections as Nigel says, after all these years there's plenty of scope for duff connectors to play us up
Bill1

Bad connections or intermittent sender.
Dave O'Neill2

Right:
Nigel, I've been to the Duffryn Arms many times. It is certainly something else, in fact the Gwaun Valley reminds me of the hidden valley in Lorna Doone!! It is priceless and many people miss it.
Cresswell Quay is a must. It is also a one off. Firewater is the drink of the day!
Now to the car, my car's wiring is heavily modified (ie K series) but the fuel tank wiring was standard. However having now had an alloy tank made I have had to add a diferent sender and guage together with an electronic voltage stabiliser. I tend to think it is a poor connection as it was working yesterday for a while and then went daft again. I'll investigate further and especially any bullet connections. Next time I take the tank out I will be cutting a hole in the boot floor and then making a cover plate. This will then give access to the sender which is on the top of the tank.

Bill, next time you are in Pembrokeshire you will call and I will lead you astray down the Quay! Did your dad fly in those wonderful Sunderlands? They are amazing. There's a whole one you know on the bed of the Cleddau just by Pembroke Dock.

David, I will definitely check any connections. The sender was new but in fairness it has been placed in a new alloy tank and so I will have to check it has the right clearances available inside to operate. I wonder if the baffles in the tank are affecting it?

A better view of the Cresselly Arms

Neil


Neil T

Neil,
the pub is on my list for a visit, perhaps this year

you wont need the hole in the boot once it's sorted - unless you have the same sort of luck as I do on these things
Nigel Atkins

Dont worry Neil, a call is on the cards soon

Yes dad was air gunner - armourer in Coastal, one of his jobs (when not making tea of course) was sliding out depth charges on the rails under the Sunderland's wings to drop on U boats.


Bill1

Neil,
have you seen the other thread on here - fuel gauge - might be some info of use to you there in the later posts
Nigel Atkins

Yes, watching it develop. Thanks Nigel.
Neil T

Just a thought since you mention a new alloy tank, could it be that a new gasket or similar either between the sender and your tank or tank and car could be stopping the 'earth' return?

You could try temporarily connecting a return earth wire from the sender body to a good clean chassis earth and see if your gauge starts to work properly. That said, not easy to do without dropping the tank as you said you did not cut yourself a boot access hole.

Cheers
Steve L.
s lowiss

Hiya Steve, thanks for that. I did run an earth from the sender unit to one of the captive tank bolts anticipating that the alloy would be a problem. Incidentally the 68-72 Midget BHA4711 sender unit I used had three spades on the top. I connected the black and green to the middle spade and one of the outside spades to the earth.
What would the third spade be used for?
Neil
Neil T

Neil,
did you test sender and wiring before installation?

fom wiring diagram it shows:
. sender (negative) earthed
. green/black wire from sender to gauge (unmarked terminal)
. green wire from other unmarked terminal on gauge to 'I' terminal on bi-metal instrument voltage stabiliser
. green wire connected to 'B' terminal on bi-metal instrument voltage stabiliser going to terminal '6' of the (2 fuses) fusebox

or

. twin green wires connected to 'B' terminal on bi-metal instrument voltage stabiliser going to terminal '4' of the (2 fuses) fusebox (and other green wire going to heater fan switch)

if the sender is a non-original it might have(?) a second "live" connector that isn't used on your car(?)

Nigel Atkins

As a kid during the war, I had a great close-up view of a Sunderland which made an emergency landing in Mount's Bay.
It was allegedly hit at night by anti-aircraft fire from the Trinity House vessel "Satellite". The crew came ashore in a rubber dinghy as the plane was only a few hundred yards off the beach and flew off again the following day.
Nothing new about "Friendly Fire"

Back to fuel gauges-

Dave

D MATTHEWS

The only thought I have is that this sender unit is wired such that the centre connection (live from gauge) is connected to the moving wiper contact - which moves as the tank float level changes, and the other 2 spades are a choice of earth connections on the ends of the fixed resistor.

Then depending upon which 'end' you connect to earth to decides whether the resistance increases or decreases when the tank level goes down. ie it could be a 'universal' sender for differently operating fuel gauges.

Note this is only conjecture as i have not stripped my sender but do know the 'variable resistor' principle of how they work.
Steve L.
s lowiss

Nigel is correct about the wiring.

Technically it is a simple circuit in which the battery/alternator/dynamo supplies voltage around 13v or so depending upon engine speed etc, the voltage stabiliser then regulates this to an average of 10v, this then goes to the fuel gauge (an ammeter) which measures current flow depending upon the resistance provided by the sender as the current completes its circuit to earth.

Fuel gauges still work without the voltage stabiliser but not consistently/accurately.

The voltage stabiliser operation though is different depending on whether you have an original or a new electronic version. The original type is actually a bimetalic switch which turns the current on and off such that the switched on-time for the car voltage gives the same net effect as a 10v supply-continuous current. It is because the fuel gauge response rate is very slow that the switching on/off effect is not noticable.

Sorry if a bit geeky but this bbs is great for this kind of thing.

Cheers
Steve L.
s lowiss

I think it's already been suggested to check the supply wires for resistance but you don't need a meter to test if the tank is full (or empty) you can just rig up a test (12v) bulb

you can use the test bulb to check supply is getting through and in place of components - bypass your voltage stabilser by just connecting the two wires that got to it - so that the bulb remains bright

but test the bulb is still working before and after each individual check, you always keep check the test equipment, otherwise you could be chasing a fault that's in the tester and not the car, i.e. the tester bulbs has blown or has bad connections
Nigel Atkins

Crickey lads. Great info to digest there. Can't do it now as I have to head off to work. But guess what I'll be doing over the next few days!
I have the electronic voltage stabiliser by the way.
Of course I'm introducing the dual guage system of a 68-72 car to a 1964 model. I ve probably cocked up somewhere! Lots of advice here though. Thanks. I'll report back soon.
Neil T

This thread was discussed between 05/05/2012 and 09/05/2012

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