Fuel Gauge Reading Backwards

[doates]

Hi All,

My fuel and temp gauges were not working so I installed a new voltage stabilizer yesterday. Turned the ignitions switch to on and the fuel gauge immediately climbed to show Full. Original gauge and sender. I know the tank is very near empty.

I did not get a chance to run the car to see if the temp gauge was correct.

Any ideas?

Thanks in Advance

Dennis

 

[71MKIV]

E means enough

F means finished.



Did you get the wires onto the guages correctly?

 

[doates]

If it means taking the speedometer out again to change the wiring on the stabilizer - I will stick with:

E means Enough

Thanks....Dennis

 

[DrEntropy]

View it with a mirror?

 

[Grantura_MKI]

+1,Doc!! Sounds like a faulty petrol sender? Have you checked?

 

[TR3driver]

More likely, IMO, a short somewhere in the wire from the gauge to the sender. With the wire shorted to ground, the gauge will always read full.

Wiring the gauge, or the VS wrong could cause strange readings, but not that far off.

Easy test for short vs bad sender is to pull the wire off of the sender (assuming that isn't too hard on a 70 Spit). If the gauge falls to empty, the sender is the problem. If it stays at full, there is a short.

 

[linklaw]

I had a Jaguar Mk VII that I converted to negative ground. After the conversion, my fuel gauge read the opposite of the way it should. EMpty when the tank was full and vice versa. I switched the wires at the back of the gauge and started to read correctly. YMMV. John

 

[John_Mc]

I had a case of fuel gauge reading full all the time. The PO even included a fuel tank "dip stick" to measure directly into the tank. Turns out that the one of the wires to/from the sender had been switched with a similar colored wire. Easy correction once I had an accurate wiring diagram. I think it may have been the Green with Black stripe (GB) wire to the sender had been switched with the Black with Green stripe (BG) wire to the ignition key warning circuit or something like that. Might want to check that out.

 

[71MKIV]

Yea, gotta dollar that you squashed the sender wire against something putting the guages back in. The whole bundle to the rear of the car is right there next to the tach. pull the wire off the sender in the back and see what happens.

 

[doates]

Thanks for all the info.

Now I am really confused.

I pulled the green/black wire from the sender and the gauge fell to Empty and I thought "crap now I need to pull the speedometer again" but just for fun I switched the position of the wires on the sender and the gauge remained at Empty. So then I took the sender out and reconnected the wires (reversed) and when I lift the float the gauge reads full and when I lower it reads empty. Switch the wires and it is reverse.

I know polarity should not matter here but it sure seems to.

Thanks Again....Dennis

 

[71MKIV]

Had something all written out then got to thinkng,

Now I dunno??

gotta think about that one somemore.

 

[TR3driver]

Just a thought, is it possible your sender has provisions for a "low fuel" warning? My Stag does, and if I switched the two wires it would act just as you describe. The senders are usually grounded through the tank, unless someone has added a ground wire.

 

[mattdanchris]

I had the same problem with my 67 TR4A. My car did not run when I bought it (gas tank full of crud). I got a new sender off of Ebay and the gauge read backwards. All the wiring was checked and it was correct. I bought another one from Moss and it read correctly. I was told that some of the earlier TR4s used a different sender. Not sure if that is correct, but it worked for me.

 

[dklawson]

My comments below address typical Smiths fuel gauge systems and are not specific to any TR model. Randall and the others will be much more familiar with when gauge systems changed.

The earlier gauge system did not use the voltage stabilizer and the gauges are "fast acting". They work using electromagnets. The sending units for those gauges show increasing resistance from empty to full. (That is to say, fuel sending units for the magnetic gauges will read about zero (0) Ohms when Empty and somewhere between 70 and 90 Ohms when Full.

The later gauge system added the voltage stabilizer and the gauges work through resistance heating of a bimetallic element. The sending units for those gauges start with high resistance and moved to a lower value when full (nominally 270-240 Ohms = Empty, to about 30 Ohms = Full).

If you mix the sending unit and gauge types as Matt suggested, your gauge will read backwards.

 

[TR3driver]

If you mix the sending unit and gauge types as Matt suggested, your gauge will read backwards.

For the TR series, the change happened between TR3 and TR4. AFAIK all TR3/A/B had the balanced type gauge without a VS; and all TR4/A had the thermal type with VS. But both senders and gauges look similar (for early TR4, later gauges were a different style) and would physically interchange, so it's not impossible that someone sold/installed the wrong part (especially when eBay is involved).

But Rimmers has a voltage stabilizer listed for all Spitfires MkI through 1500, so I presume that all Spitfires came with thermal gauges. But that still doesn't mean someone hasn't installed the wrong parts at some point.