75ZT Community

Is there a difference between the fuel gauge senders of early and late cars?

Running the on board diagnostic I get the following results:-

_6.0 __0000
_6.1 ______
2_6.2 __0000

This is on a late petrol car, I cannot see more than one sender.

This is the first time that I've dabbled in this area. Any comments please?

Does this help Bernard ?


6 Fuel level sensor outputs in litres (Fuel tank has left and right side sensors)t
6.0 Fuel levels both (eg: 109330 means left half sensor = 10.9 litres; right half sensor = 33.0 litres)
6.1 Total tank level (eg: 0439 is 10.9+33.0 = 43.9 litres)
6.2 Indicated value with diagnostics: The Test Number (6.2) is preceded by 1, 2 or 3 (eg. 1-6.2 or 2-6.2 or 3-6.2)
If first digit is 1 - both sensors are OK. This is followed by three left signal digits, then three right signal digits.
If first digit is 2 - one sensor has an error. Side in error will display - - 0 . Functional side will show its three digits
If first digit is 3 - no sensor signal, no display. Probably - - 0 - - 0 (Needs confirming -TC)

Yes thanks Rob, a little. But I still can't work out just what is wrong as the data doesn't seem to fit. The first digit '2' - one side has error seems to be the relevant fault but the three right digits are all '0'. That's the part I don't get.

What is the year of the car and engine variant ?

What does the fuel gauge read ?

Facelift MG 1.8 (!). Fuel gauge indicates zero with 40 litres in tank. Display works from diagnostic.

The two sensors are in series, so the pack can't differentiate between the two sides.

If you have zero display it could be either sensor, the wiring between the tank and instruments, or there's a link between the two sensors.

From the workshop manual:
'The sensors are connected in series, so if one of the sensors fail (open or short circuit)the fuel gauge will default to zero and the low fuel warning will turn on'.

So how does the diagnostic read the two sensors separately? Or is the diagnostic write-up incorrect?
Also where is the passenger side sensor located? I can't see any wiring crossing the tank. Sorry all the questions.

It can't read them separately, the wiring according to the workshop manual makes it impossible to do anything but read the total series resistance. It might be there was an intention to do it like that in the diagnostics, but more likely someone has been guessing based on something else. The first two fields look like total resistance, and tank contents in tenths of a litre.

The wire won't run across the tank on the outside. In fact the link is in the plug on the right hand side, the green brown in pins 1 & 3. The wire will then run over to the other side inside the tank.

To test, use a meter between pins 1&4 and 3&6, with the plug disconnected. You should get a similar resistance on both if the car is level.

I will say, though, I'm basing this on the workshop manual. The wiring diagrams for all the model years agree, as does the technical description, but it could be wrong.

EDIT:
found a picture on ebay and the link is in the tank.

Thanks Duncan. I'll try that tomorrow.

OK, across 3&6 about 60Ω and across 1&4 open circuit. So I'm assuming that this is the driver's side sender that has failed.
I spoofed the system with a resistor across 1&4 just to prove a point but not sure which is which until it's dragged out of the tank.

1&4 is indeed drivers side. Well right side so drivers in the UK.

Thanks Duncan. Smelly job for tomorrow.

Bernard wrote:So how does the diagnostic read the two sensors separately? Or is the diagnostic write-up incorrect?

I think I read recently on the OC that the diagnostic info was actually taken from from BMW info - so it would appear that the fuel level bit is different on the R40.