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Having issues with my Mk1 fuel gauge reading 1/4 tank all the time. Anyone know what the readings, ohms I believe, should be for sender unit reading empty and full? I have a spare but want to make sure it’s working before installing. Thanks.

aprox. 20 ohms ( full ) to ~ 200 ohms ( empty ).

Check the range with an ohm meter. Reading should increase and decrease steadily as you move the float arm up and down.

Thanks Pete I’ll check

So after taking it out again and checking I put it back in and at a full tank I’m getting half a tank showing on the gauge and approximately 40 ohms at the sender unit. I’m thinking a new sender maybe required, some on Ebay for about $100 cdn which is better than some nos at $500 AUD.........

Lee,

It sounds like the wire wound resistor is worn. ~$100 for a new one is a good deal. Keep the old unit as they can be rebuilt.

So here's a question for you Pete, it's been suggested that I may have a MK2 fuel tank and sender unit. Assuming that I have a MK2 sender and a MK1 gauge, are they different 'full' and 'empty' ohms readings? In which case a full tank may only give me a half full gauge reading? Pretty sad that these kind of questions keep me awake at night..... thanks.

Mk2 tank will fit in a MK1 but won't be flush with the trunk floor in other words it would bulge a bit.

The resistance range on all the senders are somewhat the same.
10 ohms -250 ohms to 20 ohms - 200 ohms. The sender and gauge should match otherwise the gauge will not read completely full or completely empty.

You are reading 40 ohms on a 1/2 full tank or sometimes a reading of 1/4 full would suggest that either the float has a small leak or the wire wound resistor is worn.

I know this suggestion won't be fun. Remove the sender from the tank ( shake the float to be sure there isn't any fuel inside ). Hook up the sender to the wire that normally attaches to the sender. Attach a ground wire to the sender body and to the car body. Turn the ignition on and move the float arm up and down, have someone watch the fuel gauge.

Thanks for the tip Pete, if I was really smart I would have done the last bit of testing when I had the sender out over the weekend..... I did have a small hole in the float which I soldered up and then I filled the tank again, I guess it should be nearly empty by the weekend to remove it again. Do you know if the senders are all interchangeable Pete? I don't want to order one for, potentially, a MK2 tank and find that it doesn't fit.

There are 3 types of sender units for the MK1 GT, deluxe, and SW. None interchangeable same for the MK2. Once removed send a photo and i might recognize it.

Will do thanks Pete.

Here’s a couple from yesterday, not too clear

That measurement is 56.3mm by the way.

I believe it's one of these: C5BH-9275C for the GT 10/64 on or 113E9275C
meant for the deluxe and Lotus Cortina and made by Smiths.

Photo of C5BH

Hmmm looks a little different, the ‘box’ with the resistor in is bolted directly to the back of the face plate on mine, didn’t know there was a number, I’ll check, I also have another spare which is also giving me some strange readings but visually is the same. Thanks Pete.

Ok I lied, it’s the first one you said there Pete , the C5BH. Took the thing apart agin, actually starting to like the taste of gas..... it looks in half decent shape but I get about 40 ohms full and about 120 empty. I’m guessing it’s shot.

Any tips here will be welcomed. So I have installed a new sender unit and with a full tank I get 20 ohms but still get a reading on the gauge as half a tank? A little confusing, I tried a gauge out of a MK 2 and get the same reading so I’m pretty sure the gauge is ok. I thought it might be a problem with the sender being grounded inadequately, I put silicone between the tank and bodywork when I installed it to prevent any leaks, so I added another ground to the bodywork and one of the tank bolts but get the same result. Any suggestions?