403 Fuel Gauge Reads Full All The Time!

pontarriere

pontarriere

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Automotive electrics has always seemed a black art to me - this problem has me stumped. My 403 gauge was working ok up to a couple of months ago, but then it started reading full all the time. I have checked all the wiring, tried swapping over gauges and even bought a new fuel sender unit from Franzose, but the problem persists. Can someone explain to me what the gauge actually measures - is it variations in current, resistance, or something else? And before you ask - yes, the tank is only half full.
 
They are a variable resistor interposed before the cable is earthed. If it is out of the car, you can test it with an ohmmeter by moving the float between full and empty. The reading should vary uniformly between the limits. If the sender is good, the dashboard gauge isn't responding.
 
Gauge reads full= high resistance or open circuit between sender & gauge.

To test, remove sender wire from gauge & with a jumper wire ground the terminal on the gauge. Should read empty.
 
Would this be the same logic for a water temperature gauge that just goes straight to max as soon as the ignition is on - and stays there?

I could install an after-market one, but logically, if I knew how to do that, then I would know how to fix the exisiting one.
 
The common Bosch temperature sensor has a resistor with two terminals. Ohms fall as temperature rises. If the system short circuits, the temp is at max.
 
If the fuel gauge reads full all the time, and you've tried another gauge and the problem still exists, then there must be a short to earth? Fuel gauges (and water temp gauges) are always connected to positive at the dashboard end, with the fuel tank sender merely providing a greater or lesser resistance to earth. Although you have checked the wiring, maybe a short to earth has been missed. Retrace you steps. Firstly, disconnect the wire going to the fuel sender and see if this changes the fuel gauge. If it does, you've found the problem - i.e, the sender. If not, then very carefully go over the wiring harness as there would have to be a short to earth somewhere.
 
Easiest way is to take the lead off the temperature unit, and touch it to the engine block.
 
I disconnected wire 65 from the gauge to the sender at both ends and used the continuity tester on my multimeter between the wire and earth - it beeped, so obviously wire 65 is shorting to earth somewhere. Because it runs under the roof lining it's hard to access, so I'm just leaving it disconnected and managing without the fuel gauge for the time being!
 
Good to narrow it down. It's been an extremely long time since I've had much to do with a 403 but I'd have really good look at the wiring under the dashboard and also the boot area.
 
Peter C said:
Good to narrow it down. It's been an extremely long time since I've had much to do with a 403 but I'd have really good look at the wiring under the dashboard and also the boot area.
Click to expand...
I can remember my 403 wiring, it had very little wiring compared to later versions.
With digital signalling of end devices there are very few wires again.
Saves on car manufacturing cost and weight. :)
 
check wire from the junction near the left hand boot hinge , and check wire from there to tank , little protection for it there and a lot of stuff moving around the boot can snag it
 
alpine said:
check wire from the junction near the left hand boot hinge , and check wire from there to tank , little protection for it there and a lot of stuff moving around the boot can snag it
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My car has a single wire running from the gauge to the tank - no junction (new wiring loom).
 
The erroneous reading is still due to a circuit compromise no matter how many wires. If not the wire, then it's how the sender is earthed to the body.
 
Problem now fixed. I ran a new wire from the fuel gauge under the carpets through to the boot and the sender unit. Gauge now reads correctly - I also bought a new sender unit from Franzose as the old one was a bit ropey.
 
pontarriere said:
Problem now fixed. I ran a new wire from the fuel gauge under the carpets through to the boot and the sender unit. Gauge now reads correctly - I also bought a new sender unit from Franzose as the old one was a bit ropey.
Click to expand...
Good result!
 
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