View Full Version : Fuel gauge always reading full

08-25-2004, 10:44 AM
I was so happy the other day when I got my fuel gauge and temp. gauge to start working. I was really worried about the temp. on the engine but have found that it never get above the 1\2. But now I have this problem with the fuel gauge and need some advice as to how to track it down.

The fuel gauge always reads full. I have put in a new sending unit in the fuel tank and have taken it out to move the float up and down to see if I could get the gauge to come down when the tank was empty.

What steps should I take to test this? I am not real good at the electrical stuff so any help would be great. This is a 1970 GT6. Thanks for all the help. Tom

Mickey Richaud
08-25-2004, 11:29 AM
Hi Tom -

I just went through this with mine - doing exactly the same thing. Bought a new sending unit (which I didnt' need!), and had everything apart several times. Assuming the gauge is OK, which mine was, the problem most likely is improper grounding. My TR3 gauge needed to make positive contact with the dash metal, and once I finally had that it worked perfectly. Also, double check all connections. Don't assume anything!

Good luck, and let us know what you find out.


08-25-2004, 01:08 PM
Hello Tom,
if the sender cable is to earth then the gauge will read 100%, this applies to fuel and temperature gauges. If you disconnect the sender cable at the fuel tank and, with the ignition on, the gauge still reads full, there is probably a short circuit on that cable to earth.
Good luck,


08-25-2004, 06:23 PM
Piman, How do you suggest tracking that down then if it has a short? Tom

08-25-2004, 07:10 PM
Disconnect the wire from both ends and check continuity to ground from either end. If you get any reading you have a short.

08-26-2004, 03:48 AM
Hello Tom,
if it has a short try and check the loom where it passes through any apertures or where it could get trapped. Sometimes it is easier to run a new cable, assuming you pinpoint the cable as being the culprit, following Philkile's advice.


Mickey Richaud
08-26-2004, 07:28 AM
You said you had the sending unit out and moved the arm up and down. What happened?

You might try running a wire directly from the "hot" post on the sending unit to the gauge while it's out and see if that changes your reading. Also, when you do this, move the arm up and down to see if the gauge moves as well. Be sure the "hot" post is completely insulated from the body of the gauge.

Again, mine was doing the same thing yours was. When I made sure the body of the gauge was grounded, it worked. When I removed the gauge from ground, the needle stuck on full.

By the way, on the back of my gauge, there are two small posts which hold the two coils inside in place. The posts are in slotted holes that allow the coils to move diagonally. I found that when you move those posts back and forth in the slots, the reading is changed. I'm experimenting with them to calibrate the gauge. Be extra careful, though, as the wire on the coils is very thin.


08-26-2004, 11:56 AM
Hello Mickey,
I don't understand how the gauge being 'grounded' affects its reading as, to the best of my knowledge the circuit is insulated from the gauge body. If you want to check the operation of the gauge by removing the sending unit then the body of that should be connected to earth otherwise there is no circuit.
By the way Tom , the fuel and temperature gauges are interchangeable so you could test the fuel sender circuit by connecting it to the temperature gauge. Also, another test for a short on the sender cable is to disconnect from the sender and measure the voltage on the end of the wire, it should be about 10 volts. (Ignition on of course)


08-26-2004, 01:35 PM
Curious Piman and Mickey,
My TR6 runs both the fuel gauge and temperature gauge through a voltage regulator located on the back of the speedometer (go figure THAT location). A triple ground wire with an eyelet afixes to the thumbscrew that holds the gauge in place. Very iffy location for a ground. Does the GT6 have same voltage regulator and, yes, they do go out.


Mickey Richaud
08-26-2004, 06:41 PM
Hi,Alec -

I don't understand it either. All I know is that when I hooked everything up and held the gauge in my hand, the needle stayed on "Full" and when I touched the body of the gauge to ground, it moved and registered appropriately! All else was connected correctly.


08-27-2004, 03:42 AM
Hello all,
yes all Smiths(?) instruments of that type are fed by the voltage regulator which gives about a 10 volt supply.


08-27-2004, 06:22 PM
OK, I am glad to get such a good conversation going. I have changed the voltage regulator as well because it was bad and then when I was testing I fried the new one as well by swapping cables around and killing the voltage regulator. So after another $10 and a weeks wait I got another regulator and this time I am not going to fry it.

I will do some testing with a voltage meter this weekend and let you guys know what I figure out. I also have the wiring diagram and a friend who may understand it better then I who can maybe help me.

As for the gauge being grounded I don't think it has to be because it is in a wood dash and the temp gauge isn't grounded unless the bulb and bulb wires do the grounding.

Oh I sure love the prince of darkness. That is Mr. Lucas and Mr Smiths.

As you can tell this is not one of my strong areas. I would rather take something apart that I understand and can't start on fire or burn out. Have a good weekend everyone.

Thanks for all the help.

08-27-2004, 08:23 PM
There is metal behind that fancy wood dashpanel. The guage is likely grounded to that steel dashboard. You can't see the steel dash from the drivers seat.

08-30-2004, 11:18 AM
With my carb issue this weekend I didn't get to look at the gauge. Hopefully later this week. Tom