Fuel gauge - strange readings, any ideas ??

[COLVERT]

Originally Posted by Sonic ZS

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.

This vital bit of information must surely be worth a click on my THANKS button.---

[Sonic ZS]

Originally Posted by COLVERT

This vital bit of information must surely be worth a click on my THANKS button.---

Sorry.....there you go, have two

[SD1too]

Originally Posted by Sonic ZS

Did Rover ever acknowledge or officially explain the on-board diagnostics procedures anywhere ?
Is it mentioned in the workshop manuals anywhere ?

Yes, in RAVE.

Simon

PS It's not "on board" diagnostics, that's the system that uses the socket under the steering wheel. It's the separate instrument pack diagnostics. Everyone gets it wrong!

[T-Cut]

Originally Posted by Sonic ZS

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

It was originally a BMW system that was adapted to the 75/ZT. I doubt MGR knew very much about it. In fact some of the functions described in the BMW routines don't work the same in our version. I don't think it was designed for owner use, but rather an in-house servicing aid in parallel with T4..

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

I produced the How To write-up the hard way. By testing each step against the original BMW docs and experimenting. So it's all trial and error and the reason why we'll only refine our understanding by doing the work.

Is it mentioned in the workshop manuals anywhere?

Nobodby's ever mentioned it. Maybe nobody's looked? Have you got a RAVE disc?


TC

[Number 6]

Originally Posted by 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

Is there any chance of a quick run down on how you do a Software rest in the diagnostic mode please.

Number 6

[Sonic ZS]

Originally Posted by SD1too

Yes, in RAVE.

Simon

PS It's not "on board" diagnostics, that's the system that uses the socket under the steering wheel. It's the separate instrument pack diagnostics. Everyone gets it wrong!

Thanks Simon, if you or anyone has Rave, could you have a look and see what it says about the 'reboot', number 21 please ?

PS - parked in work tonight on completely level ground, gauge showing 3/8ths of a tank when I switched off. Came out later, switched on the ignition and the needle immediately shot up to 3/4 - odd in itself. But then after a few seconds it slowly climbed up to full, stayed there for a few seconds, then gradually dropped back to 3/4 again ???

I'm beginning to think there's a ghost in the tank, playing with the sender arms....

[Number 6]

Originally Posted by Sonic ZS

Thanks Simon, if you or anyone has Rave, could you have a look and see what it says about the 'reboot', number 21 please ?

PS - parked in work tonight on completely level ground, gauge showing 3/8ths of a tank when I switched off. Came out later, switched on the ignition and the needle immediately shot up to 3/4 - odd in itself. But then after a few seconds it slowly climbed up to full, stayed there for a few seconds, then gradually dropped back to 3/4 again ???

I'm beginning to think there's a ghost in the tank, playing with the sender arms....

Well thats scuppered that then as I dont have Rave Will just have to zero the tripometer every time I fill it up and every 400 miles top it up again.

[SD1too]

Originally Posted by Number 6

Is there any chance of a quick run down on how you do a Software rest in the diagnostic mode please.

Activate the instrument pack diagnostics and choose test 21 John. T-Cut has created a "How to ..." as guidance. Gozo ken will be able to tell you what happens next as he's done it during the past few days. Alternatively, explore it yourself. You can escape at any time by turning the ignition key off (not recommended if a reset is under way though!).

Originally Posted by Sonic ZS

Thanks Simon ... could you have a look and see what it says about the 'reboot', number 21 please ?

Unfortunately Paul it doesn't give a detailed explanation, only a generic guide to activation and selection of "sub menus" (except the programmed check of the instrument pack displays; test 2.0).

I'd try test 21 if I were you.

Simon

[Sonic ZS]

Tried the software reset first thing today, but it hasn't solved the issue on my car unfortunately - an hour or so later the needle was going up & down all by itself again whilst parked with the ignition on.

So spent a few hours playing with some known value resistors (that all fall within the normal range of the senders in series), to see if I could get the gauge to show what I was expecting.

Having measured the resistance of an old spare set of pump & filter units on the bench, I confirmed the upper and lower values. Interestingly, the O/S and N/S circuit boards are actually slightly different in appearance and gave different upper values.

Drivers (pump) O/S: 5 to 165 ohms
Passenger (filter) N/S: 5 to 195 ohms

Both units have the lowest resistance (5 ohms) when the float is at the bottom i.e. low fuel level.

I also checked the resistances at varying points along the track, and again confirmed that the values are pretty much exactly linear (which makes life much easier.... )

Armed with this info, you can calculate that the gauge should show a full tank when it 'sees' approx 350 ohms, a completely empty tank at 10 ohms, half a tank at about 175 ohms and so on.

So equipped with the following resistors, I disconnected the plug on top of the pump, inserted each resistor into the wires that go to the dash, then switched on the ignition to see if I could fool the gauge into giving the expected reading. (Approx values due to 10% resistor tolerences):

41 ohms (act. 48) - 1/8 of a tank
82 ohms (act. 89) - 1/4 of a tank
150 ohms (act. 174) - 1/2 a tank
270 ohms (act. 297) - 3/4 of a tank
330 ohms (act. 362) - full tank

Each resistor produced exactly the expected reading on the fuel gauge, and having tested the varying values a total of maybe 20 times, in different orders, there was no sign of any unusual behaviour of the gauge needle.

So my thoughts are moving towards the sender units, or connection on top, but will investigate further next week once I've made up a temporary link lead to run the in-tank fuel pump with the senders disconnected - I'll insert a resistor and see how the gauge behaves over a few days and under varying parked and driving conditions.

Will report back with my findings in due course, but have already bought two new tank seals in anticipation of 'going internal'....

PS - they are the same seal as used on a Discovery 2

[Sonic ZS]

Quick update on this one - managed to spend a few more hours on it today, and made up the temporary link-lead to allow power to be supplied to the in-tank fuel pump, whilst leaving the sender units disconnected.

This meant that I could now run the engine, whilst substituting known value resistors in the place of the senders, by carefully inserting them into the end of loom connector.

Contrary to other posts in this thread regarding possible algorithms and the fuel gauge remaining fixed whilst filling up, I can now confirm the gauge does in fact move both up and down with the engine running or with just the ignition on, if the resistance is varied.

Once again, having obtained the anticipated results at the gauge with the car's tank senders removed from the circuit, it's looking very likely that the fault lies with one or both rheostats.

However, I'll test it for a few days with resistors in place, just to confirm the eratic needle behaviour has stopped, before having a look inside the tank.