Corrado maf

[Saichy 10]

Hi all. The trip computer in my raddo 9a is all working apart from my mpg. It constantly reads 99.9 how can i fix this so it works as it should do?

 

Dan

[Anton Sobriquet 0]

I've had the same problem ever since I did an OBD2 conversion a few years ago using parts from a '96 Mk3. Like you, only the MPG readout is faulty and is stuck at 99.9.

 

For VR6 cars tfrom '93 on, the Bentley manual indicates that if the distance travelled and average speed readings are accurate, the fault lies either in the fuel consumption signal from the ECU or the wiring between the ECU and instrument cluster. If those are both good then you need a new circuit board in the instrument cluster.

 

Quite what "the fuel consumption signal" amounts to exactly I don't know, but I believe the actual MPG calculation is done in the cluster, so presumably it's a total consumption figure sent from the ECU which the cluster then uses with distance travelled from the speedo to derive an average consumption value. This is conjecture on my part, however, and I am probably wrong.

 

To check the fuel consumption signal from ECU the Bentley manual indicates the following:-

 

"... see Diagnostic Trouble Code (DTC), checking using VAG1551, Repair Groups 01 & 24, Fuel Injection & Ignition".

 

I only found out about the fault recently through a thread here. I was blissfully ignorant before, thinking it was due to the conversion but found I had been misinformed and that others who'd taken the same route all had accurate readouts. It recently occurred to me to test fit a different OBD2 ECU to see if it works then, but I'm going to Stealth in a couple of weeks and will get it all checked out then.

 

LOL I just re-read your post and realised you ahve a 9A not a VR... I'll dig out the manual and see what it says.

 

EDIT Bentley doesn't cover the 9A engine, I've just realised...

Edited January 3, 2013 by Anton Sobriquet

[Wullie 1]

Not sure if the 9a is the same setup but on the KR if the vacuum feed to the rear of the clock is blocked or disconnected it will cause the 99.9 reading.

[Saichy 10]

Lol its ok mate i will need that info for when i do my vr6 conversion in my mk2 golf. Thanks for the info tho mate will be very helpful.

 

So you think if i take the cluster out and check the pipe for the vacum feed?

[Anton Sobriquet 0]

Could also be a vacuum leak, although I expect you'd know already or could be the vehicle speed sensor. The 99.9mpg fault was also a common problem in Mk1s caused by a corroded connector in the wiring from that sensor, and in all models with a vacuum feed to the speedo coming adrift or getting blocked, as Wullie says.

[Saichy 10]

Thats something else i can check then which is good. Stupid question time where is the speed sensor? For a guess somewhere on the gearbox? Probably not and i sound crazy :)