Fuel gauge - strange readings, any ideas ??

[marinabrian]

I noticed this the first time I used a pay at the pump forecourt.

Although the engine was not running I left the ignition in position 1 when filling up, as I was listening to an interesting piece on the radio.

When I restarted, the fuel gauge had not moved, despite having brimmed the tank.

It returned to normal on continuation of the next journey.

And my gauge operates normally under other circumstances.

Brian

[Sonic ZS]

Right, I've now had chance to test the resistance of both senders via the O/S connector in the top of the unit.

One is currently reading about 100 ohms, the other around 116 (so both within the expected range, from memory, of 5 to 165 ohms).

Then got a friend to rock the car and both readings fluctuated, again as expected from the sideways movement of fuel in the tank.

As the car has roughly 4 or 5 gallons in at the moment, just wondering if there could be a dead spot lower down on one of the sender tracks ?

After all, the fuel in a tank will always 'slosh' more when almost empty (as more free space to move about in), than when it's full. Hence the floats will move move at the lower ends of the track and wear on the contacts, in theory, would be greater ? The car is approaching 150,000 miles, so not an unreasonable possible cause of the fault maybe....???

Might need to take the senders out tomorrow, or at least wait for the fuel level to drop a bit again & see if things become more irregular lower down the scale, so-to-speak. What does everyone think ?

[Avulon]

Originally Posted by Sonic ZS

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

As already said, it's not normal to add fuel when the car is running. In fact the car doesn't expect this, the fuel level calculations may prevent any large increase in fuel with engine running from showing (as a final guard against any bad math in the algorithm that calculates the fuel needle position) as it wouldn't make sense in a normal engine running situation.

[Sonic ZS]

At lunchtime today, just before checking the sender resistances, the fuel gauge was showing a tiny fraction over half full.

Jumped in the car tonight to come home, switched the ignition on, and the needle went up to about 7/8ths of a tank.

Oddly, it fluctuated a little, both up and down before settling. It's all very odd....

Do the senders have a visible metal track and slider arrangement, or are the arms attached to a potentiometer ?

[T-Cut]

You could try the Software Reset option in the Diagnostic Mode menu. That's been reported to fix the wonky fuel gauge syndrome.

Maybe there is a software bug lurking in the system? It's more plausible than anything else I've read. Ask gozo ken about his experience.

TC

[COLVERT]

The senders are wired in series so either one can affect the other when they are on the move.

[Sonic ZS]

Originally Posted by COLVERT

The senders are wired in series so either one can affect the other when they are on the move.

Are you sure about this ? (polite question ) Only when I was looking today, they appeared to be in parallel (four wires total, two per sender on the socket).....although that would explain whey the diagnostics on the Rover doesn't appear to differentiate between the left and right sides of the tank ?

As the N/S sender is wired internally within the tank, surely if they were in series, there'd only be two wires going into the O/S unit, but as mentioned above there are four.

[SD1too]

Originally Posted by Sonic ZS

... when I was looking today, they appeared to be in parallel (four wires total, two per sender on the socket)...

You're right Paul, they do appear to be in parallel but they're not. Pins 1 and 3 are bridged via the GN wire. So yes, the IPK cannot differentiate between the two sides of the tank.

Simon

[T-Cut]

Originally Posted by Sonic ZS

Do the senders have a visible metal track and slider arrangement, or are the arms attached to a potentiometer?

Yes, both sides of the tank have a common or garden rheostat wired in series.



The empty/full resistances are as follows.



The rheostat system appears to give various problems other than physical wear and spurious values. There seems to be some new evidence of a software issue as well. I'd be interested to hear of any results you may get after trying out the Software Reset.

TC

[Sonic ZS]

Did Rover ever acknowledge or officially explain the on-board diagnostics procedures anywhere ?

If not, I'm just wondering how anyone ever found this functionality ?

Is it mentioned in the workshop manuals anywhere ?