Fuel level gauge reading not correct by 1/2?

[Tom '74 911]

Hello -
Let me start by saying that I just took apart and cleaned the fuel level sender. There was a minor amount of varnish on the wires that I lightly sanded off and the float slid up and down w/apparent ease - so that should be good to go.

The issue is that the gauge reads full when full, but 1/2 full when empty. When I had the sender apart, I plugged it back in and played with the float position to confirm this. The low fuel light does work - but when it comes on, the gauge reads 1/2 full.

In a nutshell, the travel of the needle reading seems to have been reduced by 1/2. I know it did work correctly at some point in the recent past. It's been fishy all summer, and I finally took apart the sender to check it's function.

Is there a resistor or something that may have failed in the gauge itself?

Thanks for the ideas,
Tom

[Bill in OKC]

I had a similar problem with my fuel gauge - but it read half full when full - there was a short inside the fuel gauge but only when it was screwed to the housing - so it looked good when I had it out. I posted a pic here a couple of years ago but can't find it now. Part of the gauge shorted against the inside of the housing when it was screwed in.

edit: found it Fuel gauge stays at half-full

[Tom '74 911]

Bill -
Thanks for the link. I read through your thread and another that was linked w/in it. I suppose my next course of action is to open up my gauge and do some investigating. I think I'll wait until winter sets in though - I don't want to miss any drivable days as the next one could be my last for a while.
Tom

[ossiblue]

Would it be possible to read the voltage of the wires at the gauge using a multimeter? That way you could remove the sender, turn it upside down, and read the voltage which (I assume) should be around 12v. Turning the sender rightside up should send the voltage down as resistance increases. If you get the complete range of voltage from the sender, then you know the problem is in the gauge.
Caution: I'm no electrical expert and have never done this. I may be wrong about the voltage/resistance (does increased resistance cause the gauge to rise or fall--I get so confused) but in theory, at least, it may help you diagnose where your problem lies.

[Tom '74 911]

L.J. -
That, or some variation, sounds like a good approach. Once the snow sets in for good, I'm planning on taking it all apart again to do more troubleshooting - from the sender to the dashboard gauge.

One more question - when I took apart the fuel level sender the other day, there were only 3 wires, not 4 - is that normal? There was no evidence of a 4th wire - ie:nothing looked broken or missing. I seem to recall reading through some other threads that mention 4 wires? Can anyone else verify the number?

Thanks,
Tom

[Tom '74 911]

Bump -
I'm back investigating this again. I pulled the sender again to verify my previous findings and they were confirmed:

1. When I slide the float all the way to the top, the gauge reads full.
2. When I slide the float to the bottom, the low fuel light comes on, but the gauge reads 1/2 full.

Anyone care to make an edumacated guess as to if it's a faulty sender, wiring or gauge? Or help me with some direction to figure it out?

Thanks,
Tom