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Petrol gauge wiring


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One of a number of electrical problems with my TR 3 was that the petrol gauge always read full. Over the last couple of months I have fitted a new wiring harness and was careful to ensure that the body of the gauge (via its mounting thread) was earthed as shown in the wiring diagram. However, it stll reads full when there is, at best, a quarter of a tankful. Does anyone have any helpful ideas?

Dave Whitefoot (again)

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Dave.

It will read full if there is no connection between the "T" terminal on the gauge, & the variable resistance in the tank sender. 

1st check is to ensure the gauge body is earthed, then earth the "T" terminal on gauge, & it should read empty. If it does not then the gauge is faulty.

If it does the next step is to re-connect the (Green Black) wire to "T" & earth (to car body) the other end of it at the tank sender. If gauge reads empty (which it should)

then the wiring is OK up to that point, & the sender is under suspicion. Check the earth connection between tank sender body &  car body. if ok then the sender needs to be looked at.

Let me know what you find. I can probably repair either end if you wish me to.

Bob.

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Bob is very clever with this problem. He fixed mine and it has worked ever since. It would have been better if I had wired it in correctly first time.  Bob sorted it out and  he understands the problems.

Good luck Richard & B.

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Bob,

 

I have followed your diagnostic instructions -

1. There was no earth connectivity (using my multimeter) between the dash panel and the battery so I have run a new earth wire from an earthing point under the bonnet to the gauge mountinh screw

2. I touched the T terminal (Green/black wire still connected) with the new eart wire and the gauge needle shot to empty. I guess this means that the gauge is OK.

3 I have checked earth connectivity between battery and sender. This is fine.

4. I have checked connectivity from tank sender to T terminal on the gauge and so I think that the sender must be faulty.

5. Is it possible to remove the sender unit from the tank without taking out the tank?

Thanks for your help

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Hi Dave. That all sounds good, & I agree with your diagnostics. Yes easy to remove the sender, just remove all the screws, & ease it away from the tank (try to preserve the gasket) you may have to twist the assembly as it comes out to get the float out.

If you have a resistance meter then hook it up between the earth, & the other terminal on the sender & move the float up & down you should see resistance changing from zero (or a few ohms) when float down, to a much higher value (can't remember what exactly, but aproaching 100 ohms) when float is fully up. The resistance should change smoothly, not jumping around.  If you don't get a good result, then it can be taken apart to clean the resistance winding etc, I'm happy to do this for you if you wish.

Good luck

Bob.

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