TR7 Fuel gauge

[Stuart Wilson]

Not sure if this has been asked before.

On my TR7 the fuel gauge reads full as soon as the ignition is turned on. Tried another tank unit and it's the same, tried running a seperate wire from sender straight to dash multi plug, it's the same. 

I'm pretty sure I've swapped the actual gauge at some point too.

Could the fault be in the actual dash/printed circuit?

Also now have same issue with temperature gauge, going straight to max as soon as ignition on. Yes it could well be the sender unit, but I have tried one from another engine I have lying about.

Any suggestions or has anyone got a good complete dash cluster at a normal non bank breaking price

[Beans]

Did you check the wiring (position, earth points, short circuits?

[Stuart Wilson]

1 hour ago, Beans said:

Did you check the wiring (position, earth points, short circuits?

Ran a temporary wire from sender to dash multi plug, with the original wire removed.

 

Should say I had same issue on the previous car. So I'm presuming issue is with the parts I've swapped over which is tank and instrument cluster. 

 

Next I'll look at wiring up totally seperate from the original loom.

 

The temperature I'll replace sender see what happens. My thinking on this one is if it was the wire going to earth it would do that regardless if it was connected.

An after market temp gauge is much more straightforward to fit than a fuel gauge. But it may come to that

[Howard]

When the tank is full the resistance from the level transmitter is 10 ohms and empty approx 300 ohms, so if the gauge jumps straight to full there is a short circuit somewhere in the wiring. A short circuit on the flexible PCB behind the instrument can be repaired so long as you are very careful ( I had to repair mine so it is possible 🙂 .)

You may find the attached drawings assist you in tracing the circuits.

Cheers

Howard

1981 TR7 DHC Wiring Diagram Rev 7 03_2023.pdf

[Stuart Wilson]

As I've run a wire direct from sender unit to the cluster that has eliminated a problem in the main loom. 

So I'm thinking it is the Flexi circuit board at fault.

Next I'll look at removing the gauge and wiring it up independently.

 

Edited March 26, 2023 by Stuart Wilson

[Stuart Wilson]

Benching testing another gauge I have and an old sender which I believe may be faulty.

Ideally I'd like to take sender out of the tank, but not sure on fuel level. The sender in tank was new in last car, but didn't work then either. Starting to think it could be a faulty sender unit

[Howard]

Stuart.

Start by borrowing a multimeter and check the resistance on your spare sender. As you move the arm it should change from 10 to 300ish ohms. If it changes and is within these limits you can use it to test the circuits on the instrument cluster.

Connect the spare sender into the wires that are plugged into the existing sender. If the gauge still jumps to full irrespective of the position of the sender arm the problem is not with the sender. As I said before there has to be either a short circuit on the wiring (green/black wire) to earth, or a short on the printed circuit board or a faulty gauge.

Check the wiring from the flat connector that plugs into the back of the instrument cluster and again the resistance should vary as you move the sender arm. If it doesn't and it shows a very low resistance the problem is in the wiring loom.

If that's ok, see if you can remove the gauge itself. I have a recollection that this may not be as simple as it sounds, but if you can, carefully trace the connections by following the circuit board tracks from the guage back to the connector. Repeat the above test.

If it passes the problem is with the gauge

[RobPearce]

I don't think you've said whether you tested with the sender completely disconnected? In that condition, the gauge should read empty (should not move from its ignition off position). If it reads full without a sender then it's not the sender that's faulty!

There is also, of course, the classic mistake of swapping the wires at the sender end. One is a ground, the other is the gauge feed, but the ground terminal on the sender is connected to the sender body and thus to the tank and ground, so swapping the wires gives a dead short and permanent "full" reading.

[Stuart Wilson]

Be back at it tomorrow I think, I've emptied the fuel out...... probably over 30 litres.

I've tried that many different things I'm probably going to start again. 

Not sure if I've tried ignition switched on with wires off sender. That may be first task tomorrow.

 

Need to double check the wire positions on the sender. Three wires, black - earth, gr/b - gauge, gr/o - low level light relay.

 centre spade connector - earth, T - gauge, W - low level light. ???

 

 

 

 

[Beans]

This is how to connect the tank sender:

 

[Stuart Wilson]

Gone back to basics/the start.

Had everything connected up, fault on gauge. Disconnect wiring from sender gauge doesn't move. 

So looks like faulty sender, best get new one ordered.

Amazing sometimes try and overthink stuff when just need to start again and keep it simple.

 

Thank for everyone's input, I'll keep updating how it goes.

[Stuart Wilson]

Right update on the fuel gauge saga......

So with sender disconnected gauge doesn't move, so to me this pointed to sender unit possibly being faulty.

Bought new sender unit, plugged the wires on, just held the sender while someone watched the gauge. Exactly the same as before.

Sat in the car, took pin out of the multi plug and fitted in a wire took that straight to sender, then put earth straight from sender to battery and guess what it's exactly the same.

I have noticed with the sender in the empty position the needle is on the lower side of the full mark, in full position it's on the full side of the mark. So the gauge range is the thickness of the full mark.

I'm now thinking it will be staying broken, managed two RBRR with it broken plus trip round Ireland and various other events. I haven't got the time to spend hours going through wiring trying to find a fault, loads of other stuff I need to do.

If someone has a dash cluster that they know 100% works it would be great to try, but other than that it's staying how it is.

Edited March 30, 2023 by Stuart Wilson

[Chadow441]

Bit out of date now, I know, but check the gauge!  Pop it out of the dash pod and measure the resistance across the 3 studs on the rear.  See picture below.

[dave sweetman]

I have a similar problem! With another sender unit (new) and jump leads to the car harness I have found that with the sender unit held "upside down" the guage will move with the float arm and gives approximately the correct reading!So....the guage works ,the sender works out of the tank if held upside down,the guage in the instrument panel can not be incorrectly fitted,the sender unit can not be incorrectly fitted and the wiring seems to be ok.I have checked and cleaned the earth points......I am now out of ideas!!!!