Fuel gauge not working

[gregtheo]

Hi everyone, my fuel gauge hasn't worked since I got the car but I really need to sort it as soon as possible as its quite difficult guessing how much is in the tank!
Its a 78 fix head if that makes any difference.

The fuel light is working fine but the gauge doesn't even move, just sits on the bottom.
The wiring looks okay and its all plugged in on the sender unit.

Anyone got any ideas?

Thanks
Greg

[dursleyman]

If the wiring is sound and all connected OK it could still have a break or bad connection so you need to check continuity end to end. Then it could be the float in the tank could have a leak and sunk or come off the arm.
At the other end it could be the connector onto the printed circuit or the gauge itself.
You just need to work through it. The dash/gauge is fiddly but not too difficult to get at. The gauge just plugs in from the front. The tank sender unit is not too difficult either but it depends on whether the locking ring is corroded.
Done it all myself just recently so know how you feel.

Russ

1980 TR7 Sprint DHC, 1981 TR7 Sprint DHC
Dursley
UK

http://tr7russ.blogspot.co.uk/

[img]http://i1040.photobucket.com/albums/b410/dursleyman/TR7%20web%20picture/IMG_0479-Optimized.jpg[img]

[saabfast]

If you try the search function (top right) there are a number of threads on testing the various components of the fuel gauge system. It is not the most reliable part of the car - not sure what is really!

Alan
Saab 9000 Stg 1 (now passed to son for his family car)
Saab 9000 2.3 FPT Auto (now gone that others might live)
Saab 9000 2.3 LPT Auto (sold on, wish I had it back)
Saab 9-5 2.3 Vector Auto Estate
'81 TR7 DHC

[tr7jim]

Greg

The fuel gauge can be check in the instrument pod quickly by swopping the two main cables over on the fuel gauge. e.g. not the earth one.

The gauge works on low resistance so when full its full deflection on the gauge and when the tank is low is high resistance so low.

This could be case at all times if the float as let in fuel so always low on the fuel gauge. e.g. not raising up when the tank is filled.

Jim

[gregtheo]

Thankyou for the replies, I will check the wiring for continuity and swap the wires to check the gauge is working correctly.

What I find odd though is that the fuel light is working fine!

May just bite the bullet and get a new tank and sender as the tank is starting to look a little worse for wear. It is 36 years old after all!

Thanks
Greg

[TR Tony]

The wires from the sender come into the car at the rear of the transmission tunnel, and join the loom further up. The wires do not connect directly to the gauge, everything goes through the two flat multipin plugs that connect to the printed circuit at the back of the instrument pod.

At the sender your connections should be Green/black wire to terminal T; Green/orange wire to terminal W, Black wire to earth terminal. Green/black is the wire for the gauge - power goes from the pod to the sender, when the float arm is at it's high point the potentiometer gives full current to ground, as the level drops the resistance changes & alters the gauge reading.

If you have a short to earth in the green/black wire the gauge will show full all the time, if it shows empty all the time there may be a break in the wire but not shorting to earth.

Tony

<font size="1">1981 TR7 FHC Cavalry Blue
1980 TR7V8 DHC Jaguar Regency Red - sadly sold!
1977 TR8 FHC EFI Factory development car Inca Yellow</font id="size1">

[RJS]

I am a little confused. If the low fuel light is still working, then doesn't that tell us that the sending unit is good, and the wiring is good till it gets to the instrument pod?

I am thinking that it is either the printed circuit on the back of the pod, or the gauge itself.

Rob

[trekcarbonboy]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by RJS</i>

I am a little confused. If the low fuel light is still working, then doesn't that tell us that the sending unit is good, and the wiring is good till it gets to the instrument pod?

I am thinking that it is either the printed circuit on the back of the pod, or the gauge itself.

Rob

<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">
That totally makes sense to me Rob. However I had the same problem that was remedied by installing a new sender.

Craig '75 2.0 FHC

[HDRider]

They have two seperate circuits for the gauge and the fuel light. It is quite normal for the gauge portion of the sender to be defective and still have the light function as designed.

Edward Hamer
Petaluma CA

[RJS]

Thanks for clearing that up.

By the way, I replaced the sending unit in my car about month ago. Both gauge and light are working perfectly now.

Rob

[scarface031]

Disconnect one of the wires and short it to ground, it should give you a high reading on you're gauge. If it does, you have a bad sending unit. If it doesn't, you have bad wiring or gauge.

-Justin P