MFA MPG - strange readings

[Trist28]

Having taken by S2 Avant out from hibernation - took for MOT and have a strange issue with the MPG reading (real time) on the MFA. It has done this previously and it always seems to be after it has stood for a long time.

Readings are very low, and decrease when I'm off the power and increase when on it so in reverse so to speak. But the range is quite small too so 7mpg off boost, on boost goes up by 5. Its not guzzling fuel and will probably settle down after 100 miles or so. Just wondering if anyone else has had similar problems. Cheers

[prj]

There are two things which the speedo needs to calculate MPG.

The vehicle speed signal from the output shaft of the gearbox (VSS), which is delivered in pulses, and a consumption signal from the ECU which is delivered as a duty cycle.

Most likely you have a bad contact on the VSS wire or some cold solder joints in the clocks.

[Error404]

Slightly off topic but Dmitri do you know how many pulses the ECU expects to see per revolution of the wheels? Is it 4 or 8?

[prj]

I don't know, never looked into it. Easy to find out though.

[irishs2]

Could it be 8 judging from the points on this ring?

[Error404]

Good pic, thanks. Saves me opening up my spare box

Seems to correlate with the info dug up in this thread

http://www.s2forum.com/forum/showthread.php?t=63731&highlight=VSS+speed+sensor

[UrS4boy]

Slightly off topic but Dmitri do you know how many pulses the ECU expects to see per revolution of the wheels? Is it 4 or 8?

Not Dmitri (obviously) but I think I have the answer (8) based on this diagram (as documented in this G22 VSS post):

http://forums.quattroworld.com/s4s6/msgs/168463.phtml

[bluequattro]

My s4 is reading same after changing clocks
Tick over mpg goes up to 18.5
Light load driving I've seen 12 mpg
Floor it and goes up to 20 mpg!
Doing same miles per fill up though
Brake light check works about 6 out of 10 when first start
Must be same problem

[Trist28]

Thanks PRJ and UrS4Boy for some really useful info. However I popped out and its working so I'm going to live with it!

I've spent a good few hours sorting the number plate lights out - wiring issue somewhere in between rear quarter and fusebox - re-wired now

[Trist28]

bluequattro - There is something quite cool about being able to nail it and MPG goes up

[bluequattro]

Certainly is, shame it's not reality though and it really does use fuel, unlike my tdi which sips like a Victorian lady
50 mpg vs 20 means I favour the smelly stuff at the moment