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I have just spent two hours on my TR3A fuel gauge.. The needle on my old unit would not move more that 3 mm and I have been using a dipstick for a few years. Today I took it out and replaced it with an overhauled unit. I cleaned up the connections and made a new ring connection for the green/black wire from the tank. The other terminal has a plain green cable and a black wire which is larger section. The small green one is live when the ignition is on. The black cable on the same terminal  is hard to trace. I have been looking at the fuse but there does not seem to be a large black wire.

So II cleaned up everything and fitted the unit with an earth from the bridge connector to the body brackets which hold the dashboard. Now the gauge goes straight to full and is very quick. I have two gallons in the tank. At one point there was a short that blew the fuse but I replaced it and I was hoping for a working fuel gauge before next week when I am driving the TR to Classic Le Mans.

Is it safer to disconnect the gauge to avoid any electrical  overheating while I am away from home?

Please advise, Thanks

Richard & B

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Richard, the gauge works by the sensor in the tank varying resistance.   The more in the tank the less the resistance.   So "straight to full" sounds like a short circuit in your wiring. It's a good way of checking gauge function, but any short circuit could be a fire risk.

I can't comment on TR 3/4 wiring colours, but very much worth a check!

John

Edited by john.r.davies
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That black wire sounds like a modification Richard. It isn't original so has probably been fitted to pick off 12v for something else (but why black?)

If the gauge goes straight to full it means there is no connection to the tank sender.  If you earth the terminal on the gauge that goes to the tank, the gauge should show empty.

If it does I would first suspect the new ring connection you have made on the green/black wire since that is the last thing changed.  If that looks OK then earth the green/black wire at the tank. Again the gauge should go to empty. 

If it does then either the sender is faulty or maybe the tank is not earthed.  If it doesn't the green/black wire is open circuit somewhere. 

 

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5 minutes ago, john.r.davies said:

 The more in the tank the less the resistance. 

No John - the fuel sender on the sidescreen cars works the other way round to the later cars.  The sender is highest resistance with the tank full. 

The gauge has two coils - one powered directly which pulls the needle towards full and another via the sender which pulls towards empty. By doing that, there is no need for a voltage stabiliser as variations in voltage are seen by both coils and so compensated. 

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Thank you, Rob! 

Then the correct mode of operation for the early gauge, that you describe so clearly, would indicate no working connection to Richard's sensor?    Or, another fault that causes no current in the signal coil so 'straight to full'?

John

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On 6/23/2022 at 3:27 PM, john.r.davies said:

  So "straight to full" sounds like a short circuit in your wiring. 

John

Not so with a sidescreen, reading full (or beyond) means open circuit to the tank sensor

Bob

 

Just now read the replies from Rob above, which say the same thing.

 

Edited by Lebro
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You seem to have a lot of TRouble with fuel gauges / senders Richard. I have repaired / calibrated yours twice now I think ?

Bob

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Thanks very much Bob. I have been out in the sun and I am trying to trace the wires.  I have one green wire to terminal T and one green and one black to terminal B. The black one is a feed for the heater control only. I will try and trace the other B cable this evening.  It is live and therefore must come from the fuse unit. The black cable is not live unless connected to the live green. I have a wiring diagram which does not show heater controls or overdrive switch and I need to know how the wiper switch is powered. I will move the back panel and check out the sender unit later this afternoon.. My service manual is for TR2 with a supplement for the Tr3

The gauge quickly goes to full and stays there. ???

Any help would be apreciated.

Richard & B

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6 minutes ago, Richardtr3a said:

The gauge quickly goes to full and stays there. ???

Any help would be apreciated.

The wire to the T terminal is open circuit Richard. Read my post above (third one in the thread).

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I dont have a diagram to hand, but from memory green should go to B & green black to T.

Bob

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Thanks Bob, I must have an ongoing wiring problem.  Maybe the diagram in my manual is not correct so I am looking for an update from a local member on the run out this Tuesday.

Richard

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Green is the colour of all fused wires coming from the ignition switch, so that must be connected to "B" (battery) on the gauge. The other wire, therefore connects to "T" (tank) on the gauge one end, & the tank sender the other. 

Bob

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The complication is the extra black wire. That must also be connected to 'B' to provide power to wherever it goes. The wire on 'T' ought to be green with a black stripe if the wiring diagram is to be believed. 

 

 

 

 

Edited by RobH
typo
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This morning I stripped out some trim and disconnected the Gwen wire. I connected it to earth and the gauge went to empty quickly. I tried a few more times and that included the earth connection on the sender. 
it seems that I have sender problem when it is fitted but ok on the bench. 
The black cable is a power supply for the overdrive and the spot lamps. 
The SDG is having a run out tomorrow and a friend is bringing a wiring diagram. I hope that it is better than mine

 

Thanks Richard & B

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Check that the sender mounting plate is earthed Richard - It could be as simple as that. 

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Thanks for the advice. The mounting plate is on a gasket using sealer but the fixing screws are fitted as normal. Tomorrow I will try a temporary extra earth and see if that improves  my fuel gauge.

Thanks Richard & B

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Hi Richard

Not sure how widescreen wiring is set up but I have fixed an earth wire from car body to one of the screws holding the sender unit to the tank on my 4A. I had a poor earth because of the gasket and fibre washers. I used a ring connector under one of the fibre washers.

Keith

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I tried an earth from the sender mounting plate but it made no difference. I took the sender out from the tank and tried moving it by hand but the gauge remained at full until  I  earthed the green  cable and the gauge immediately moves to empty. So I am going to France tomorrow with a dipstick.

It looks like a sender problem,. Where can I buy the best one ? 

Thanks Richard & B.

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To repeat, the green wire goes to B, the wire to the tank (T) is green/black. Earthing the green wire should blow a fuse.

Bob

Edited by Lebro
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Bob,

I should have written green/black earthed will make the gauge move to empty. Thanks for the advice.  Where can I buy a high quality sender next month, when I return from CLM.

Thanks Richard & B 

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Thanks for the link. I know Revington and am sure that their sender is the best quality. Before I buy one is there a way of testing my existing sender off the car. Leaving soon for France so I will get back to the problem next week.

Thanks Richard & B

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To test the sender off the car connect a resistance meter (DVM on resistance setting) across the body & the output connection. move the float up & down & you should see the reading change.  If the meter reads very high resistance in all positions then the coil is likely to be broken somewhere. The second time I looked at your gauge & sender I found the resistance wire inside the sender burnt out at one place, I presumed due to incorrect connections placing 12 volts onto it.

Maybe the same has happened again ? I am happy to look at the sender for you if you can get it to me.

Bob

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  • 4 weeks later...

Hi Bob,

I am trying again to get the gauge working. I may have to try Revingtons sender. What do you think ??

As you get older the garage is no use when wet or cold and also difficult when very hot. Tomorrow I am working on the Banjo washers and will run some checks on the wiring.

Thanks Richard & B

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