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MG Midget and Sprite Technical - Fuel Gauge reading in Reverse!

Well, another "issue" has arisen following the rewire of my car....

Before the car was rewired, the fuel gauge was not working. The previous owner and the company who did the rewire both tested the loom and circuitry and decided it was the sender itself that was faulty. I sourced a NOS smiths sender for a MK3 sprite in the states. I've fitted it today, the dial now reads full when the float is extended downwards and only half full when the float is at the top of travel. So i have 2 issues really.

1: its reading back to front
2: It appears to be under reading by half a tank.

I've swapped the wires on the rear of the gauge and the same problem shows. At the sender end there is only one red wire connecting to the spade, and the black earth is earthed at one of the 6 mounting screws.../

Any ideas?

Chris
C Fleet

Hi Chris

I have had exactly this problem since I acquired my midget. I believe that the sender and gauge need to be matched. The gauge measures the resistance at the sender unit. Reversing the wires will make no difference. Some gauges read a high resistance as full and others a low/zero resistance as full (early versus late but I'm not sure which way round it is). If you have an early sender and later gauge it will not read correctly and vice versa. I tried substituting different gauges without solving my problem and decided to live with it. However, if you are in the process of wiring up your car it is probably worth sorting this out.

Chris
Chris H (1970 Midget 1275)

Chris,

Whereabouts in 'Grampian' are you?

Would a guage and/or sender from a '76 be of any use? Both off my car at the moment if you want something to experiment with. (I don't know what age your car is and if there were any differences).

Cheers,
Malcolm
Malcolm Le Chevalier

Hi Malcolm,
I'm just outside Oldmeldrum.... 'Lily' is a 66 Sprite Mk3.... I have a feeling the dials won't match. Certainly the senders are different I believe...
Incidentally, are you in any clubs up here? since leaving the Morris minor scene, I've found myself out by myself...
Chris
C Fleet

Bend the float wire round so it operates the other way?
Guy

Unfortunately there is a stop in the way, preventing the bend round.. Only obvious difference between the senders is that the old one had a single paddle that moved along the coil. The "new" one has 2 paddles. One either side. I'm guessing this may change the resistance slightly if not the polarity... Hmmm,
C Fleet

Do you still have the old sender?

It seems you can repair it.

http://www.mgexperience.net/phorum/read.php?3,1928302,1928441

"....it is possible to fix a sending unit....i just did it
patience and diligence prevail
i disassembled the switch,gently sanded the connections,re dimpled the "scratcher" with a scratch awl
reassembled the switch part,and no it works "

"....it took me 3 hours to do it. that was dropping tank and all. there is a silver tab on the inside of the sending unit....it has a dimple in it........i tapped that dimple a couple times with a scratch awl, put it back together, then took it back apart to readjust it a few times.....finally got it to run smooth.

what actually happened was that dimple actually got a flat spot wore on it......once i deepend the dimple...it worked like a charm......there was some very very gentle sanding as well."

Lawrence Slater

Here's another thought.

Sussex Classic MG Car Parts, list --

Fuel Tank Sender Unit - Upto 1967 - Positive Earth - Midget. 21A168

And

Fuel Tank Sender Unit - 1967 to 1972 - Negative Earth - Midget. BHA4711.

Are you certain you have the correct sender for your year of car?






Lawrence Slater

I believe I have the correct sender, however I think you may have stumbled on the prob. The car has been converted to neg earth. Could explain the reversing I guess... But still no idea how to fix this.....
C Fleet

I don't think polarity matters - it is the resistance of the sender unit.
Chris H (1970 Midget 1275)

Well either you have to reverse the action of the sender, or get a later sender. You can get them easily enough.

What tank size do you have?

That will also be a factor in how accurately the sender reports full/empty.

Original was 5 UK gallons. But I think you can go up to 7 Galls. What's yours?
Lawrence Slater

If its resistance that matters it should be possible to re connect the fixed terminal to the opposite end of the resistance coil. The wiper contact will then increase resistance as it moves over the copper windings, where previously it was decreasing. or vica-versa

I still don't understand why bending the float wire wouldn't work. Even if there is a built in stop. I will try and explain. Think of a clock face. Suppose the float arm is horizontal at 3pm when the tank is full, and drops clockwise to 5.30pm at empty. Bend the float arm by 180 degrees so that it now is at 9pm when full. Now as the petrol level drops the float will drop to 6.30 position, but critically is now going anticlockwise as the level drops so the moving wiper contact will move in the opposite direction over the resistance coil from before.
Guy

Hi guy,
Yeah I get that, however it's not possible to bend the wire beyond the 530 position because of the stop.... Not without creating a z shape and shortening the wire and therefore the travel of the float.... Ideally I need to move the float to the opposite side of the sender... But that's a huge faff.

To be honest I need to refurb the fuel gauge, cracked glass, face tc, so I may try and source a correct gauge and calibrate that. I believe they have adjustment slots on the rear.

One saving grace in all this is that I don't have to drop the tank, as when restored and access panel was built into the boot floor :)
C Fleet

"access panel was built into the boot floor"

now that's a good idea!
Guy

"At the sender end there is only one red wire connecting to the spade"
I've never met a fuel gauge sender wire that were red! Should be Green/Black, as it should be at the back of the gauge.
Red wires are side/tail lamps.

FRM
FR Millmore

Guy, a chap called James Ballard, was talking about an access plate on www.british-cars.net, back in April 2010.

"A suggestion I picked up in the past which worked well for me is a nifty mod to the trunk floor.

Cut a circular hole in the floor above where the sender goes on the tank. Fashion a circle of ally plate larger than the hole you've cut. The plate can be screwed down with a couple of self tapping screws and a make shift seal (I used a strip of draught excluder rubber) around the edge of the plate.

If at anytime you want to adjust the sender or replace it in the future you just need to remove the screws and remove the plate.

A very handy mod indeed!"

James Ballard


I wonder why it wasn't a factory addition. Maybe it was deemed to dangerous to have people opening their fuel tanks, full of fuel. But it sounds like a good mod to me, as long as the boot floor strength doesn't suffer.
Lawrence Slater

FRM,
Yep I gathered that with the wiring. I think it was a 'mod' made by the previous owner when he replaced the loom for the second time not realising he kept burning them out because of a stupid wiring mistake with some over powered halogen spots. He bought the wrong age loom and modified it with his own cables. The front loom is all correct now, however whilst the rear loom is sound I've chosen not to replace it yet. Even though that particular wire is the wrong colour....
C Fleet

This thread was discussed on 11/01/2012

MG Midget and Sprite Technical index

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