Fuel gauge - strange readings, any ideas ??
 

I've never been entirely confident of the fuel gauge in my '04 diesel, and last evening confirmed something is definitely wrong.

The car was covered in ice when I finished work yesterday, so I left the engine running to assist warm up & demist a little whilst I got fuel (I know I shouldn't, but this was for experimental reasons...!!). The car was on level ground all the time and the gauge was showing about 1/8th of a tank, with the low fuel light having just tripped on - it wasn't illuminated when I parked up yesterday morning.

So with the engine running, I watched the gauge as I put about £30 in, approx 5 gallons or so. But no movement at all in the gauge.

Then drove about 25 miles (without turning the ignition off) and the needle gradually dropped from the 1/8th position towards the empty mark. Obviously I went up and down various hills, which may have caused this ?

Then on level ground, switched the engine off, restarted, and the gauge immediately shot up to just over half - pretty much the position I'd expect with £30 of fuel.

So it appears both the senders and gauge are working, but why is the system not recognising the increase in fuel, yet still seemed to drop a little whilst driving ?

Does the fuel system have an ECU or similar, to control the readout, which might have a glitch ?

I find it hard to believe the wiring could be at fault, as the gauge does appear to read correctly once the ignition feed is reset with the key.

Any help on how the system works, or what might be faulty, would be greatfully received :xmas-smiley-008:

Run the onboard diagnostic - 19-7 -select 6 - will tell you if your fuel sensors are faulty - see link below for error codes.

https://www.the75andztclub.co.uk/for...hread.php?t=44

Hi Paul,

I don't think there's anything wrong with your car. The instrument pack has circuitry which processes the signal from the level senders and takes into account road speed; effectively it introduces an element of current fuel consumption into the gauge indication. My guess is that this is re-calibrated when the ignition is turned on. Since you left the engine running when refilling, the instrument pack didn't know that extra fuel had been added.

Thanks for carrying out that useful experiment!

Simon

Thanks for the info Simon, but unfortunately I've been monitoring the gauge for 3 or 4 months now and it was only last night that I confirmed in my own mind that there is definitely an issue.

I know this because on 2 previous occasions, I've refuelled albeit with the ignition on (not engine running, as last night) - and both times the gauge has moved, although not to the extent that I was expecting for the quantity of fuel added.

The last time, the gauge started rising as I pulled the pump trigger, but stopped on a quarter tank whilst I continued filling. Again, the gauge shot up once the ignition was turned off then on again.

Having also had numerous other 75/ZT variants over the last 20 years, this car's gauge just isn't indicating the fuel level as I've come to expect from the model.

My gauge is all over the place, one day you switch on and it shows full, next day a quarter full and on consecutive days any thing between 1/4 and full. So I have to keep watch on the mileage to be sure I dont run to low as I cant rely on the gauge.

Hi Paul,

No car is designed to be refuelled with the engine running or indeed the ignition on (plus it's contrary to forecourt safety regulations). Does your gauge always show the expected reading after filling with diesel with the ignition off?
John, I agree with Colvert. Begin by checking the resistance readings from the two tank senders.

Simon

That's pretty much what mine's doing, I can park on level ground with a quarter tank, come back later and it'll have gone up to a half ?? Or leave it showing over a quarter, start up later and it'll say almost empty and the light will be on ??

Would like to get to the bottom of it, as everything else now works really well on the car - but an inaccurate fuel gauge just reminds me it's getting old....:icon_rolleyes:

Hi Simon,

I appreciate the car should be switched off - as I'm sure you realise this is not a regular occurance and was done purely to test the movement of the gauge. However, I doubt any manufacturer designs a fuel gauge system so as to prevent the gauge from reading, just because the ignition is on ? After all, we know the system takes the 2 sender feeds and constantly averages them, and these will fluctuate both down and up, as the vehicle corners.

Additionally, as I mentioned before, the gauge has moved upon filling, although not as high as expected.

Thanks Colvert, I will try to check the sender resistance readings later today :xmas-smiley-008:

The system we have in the 75/ZT is more complex than the basic series circuit of rheostat, gauge and bimetallic voltage stabiliser in use for decades until IT came along. :( I've observed my gauge reading rising slightly (turning off the low fuel warning light) as I begin a journey after a cold start. This suggests that the IPK processing includes calculated instantaneous fuel consumption (using the road speed signal) when driving the gauge. It's effectively showing us the range; data which is also made available to the trip computer.

I agree with you Paul that it wasn't a deliberate decision by the instrument designers to require the ignition to be off when refuelling, more a consequence of the increasing complexity of electronic systems to provide us with additional facilities.

Incidentally, you and Number 6 and gozo ken have all reported abnormally large variations in gauge readings and you all have diesels. Is that a coincidence?

Simon :}