Fuel Gauge Sender Unit

[Fourducs]

Maybe this question should be posted under the Fuel System. I dunno.

I just received a new aluminum fuel tank from SJ Sports Cars. The tank came with a fuel gauge sender unit installed. The unit in the new tank has two posts for electrical connections. The sender unit in my car has three posts and electrical connections. Can anyone offer insight other than recontacting SJ Sports Cars and inquiring about 3 post sender units. BTW, I was very pleased with SJ Sports Cars' timely filling of this order.

My car is a 1969 Elan +2, LHD, non federal, #2305.

[HCA]

The third terminal is for a warning light.

You will have to contact SJ to replace it with the correct unit, or do without the warning light and just relie on the gauge.

[Fourducs]

Thanks HCA. That makes sense.

[Lotusian]

SJ’s website shows the sender with two insulated terminals (1 for gauge and 1 for warning light) plus one uninsulated earth tag (ground).
Perhaps the earth tag has broken off or got squashed?

https://www.sjsportscars.com/files/X050 ... jpg?mod=86

[Bigbaldybloke]

Fairly simple to test if you have a multimeter, check between the terminals and the switch body, the low fuel indication is a switch, with no fuel the contact should be made and should be effectively a short circuit and the gauge is a variable resistor which will change gradually as the float moves up and down. Both circuits go to earth so if the tank is still out of the car, just measure each terminal to the tank and rotate the tank to move the float around in the tank and see what changes. If the tank is in the car it probably easiest to just take the sender out and test it on the bench, probably need a new gasket though. It there is only a level sender fitted and no switch you won’t get a reading from the terminals to the tank, you’ll only get a variable resistance reading between the two terminals.
Mind you a phone call to the vendor might be simpler and quicker!

[Fourducs]

I am missing the earth tab on my new sender unit. I see no sign of it. I may fashion one up and check the two insulated terminals to see if one lights up the low fuel light. The other one should change the indicated fuel level when I move the tank around as suggested by Bigbaldybloke. Thanks for the input guys.

[Fourducs]

I did talk to the vendor, SJ Sportscars. For the curious here is the answer:
The sending unit supplied with SJS's aluminum replacement tanks only has two electrical connections. One for earth and one for the fuel gauge. There is no switch for the low fuel light included in the sending unit that comes with the new tank. One cannot switch the old sending unit over, or install the replacement sending unit that SJS sells because they mount differently to the tank. The sending unit with the new tank is attached by 6 screws instead of a locking ring. I can drive without the low fuel light, although I would prefer to have everything working.

Other than that the tank appears to be good quality, the same size & shape, and fits in nicely. The threads for the drain plug and fuel line are correct. It does lack the two depressions on the bottom of the original tank that were designed to provide clearance for two bolts that protrude through the body under the fuel tank. I don't think that is going to be a problem.

[stugilmour]

Interesting. That might explain why Andy at Axminster mentions his tanks use the stock sender.

If you want to add the low fuel light function to your sender, the Spiyda Gauge Wizard would work.

https://spiyda.com/fuel-gauge-wizard-mk3.html

I just installed one in my Plus 2, hoping to get more consistent gauge readings. I am still in the process of adjusting things so hadn’t posted yet.

The Spiyda unit provides a separate switch-to-ground output wire for a low fuel warning light. I am not using this feature as I have a low fuel switch in my sender already.

The box allows you to match the sender and gauge settings for five steps from empty to full so the gauge readings hopefully become more consistent. At least with mine (Fuel tank and sender from Axminster, claimed to use the stock sender), the first half of the tank goes down on the gauge reading way faster than the lower half.

I mounted the box next to the tank sender. One needs to add a green (fused - hot in run and start) wire to the trunk to power the box though, or you could perhaps tap in to a green or green/black power wire that may have been added to the trunk area for a non-stock electric fuel pump. If you are rewiring the car I would recommend adding a fused green leg to the rear loom anyway.

HTH