autocheck / trip computer issue

[LHD_Kid]

Hoping someone might have an idea...

Cars been sat a few weeks. All starts works runs perfectly EXCEPT for the autocheck computer thing.

Again, seems to work except the miles per gallon is stuck at 200 and doesn'tt change.

What could be causing this?

Thanks!

[Cottis]

My coupe is not a daily but it does get used normally once a weekish. Recently though, it was unused for nearly 2 months (long story about clutch and gearbox) and when it returned to the road, I noticed the computer had a mind of its own on some of the info it was giving. The main problem I had was the range it was saying I had left. This just did not tally with the fuel in the tank (or the fuel gauge) The average and actual MPG seemed to be working (unfortunately ha ha) but the range thing really bugged me. I went for a hard 40mile thrash which normally uses about 80miles worth of range. It didnt move at all and claimed I had 100miles left despite this clearly not being the case.

Long and short of it is that suddenly a fortnight or so later after more frequent use, something clicked in somewhere and it suddenly started working. You know the little E sign and it telling you to find a petrol station. I think most S2 owners are familiar with this.

Maybe yours will sort itself out and they just don't like being stood about. Did your battery go flat? Mine was in a workshop having work carried out and they said the battery went flat, so guessing this might have something to do with it. It stood last winter for at least 2months but I always kept the battery fresh and gave it the odd 20min run in the garage to keep it moving as such. Never had the same problem. Bizarre.

[LHD_Kid]

Hmm, that's really interesting... the battery did go flat, I'll try running it a bit more over the next couple of weeks and see if the fairies will fix mine like they did yours... !

[newsh]

I think if it's stuck at 200 mpg it means it's lost the speed signal, probably a little damp in the cluster and a pin has corroded.

[LHD_Kid]

I think if it's stuck at 200 mpg it means it's lost the speed signal, probably a little damp in the cluster and a pin has corroded.

Actually John, that is a really good point.

No real evidence of damp or if any actual corrosion... However, I dismantled the cluster a little, cleaned the contacts ( Caig Deoxit too....) and hey presto... back working 100 % !!

Thanks chaps!

[s2driveruk]

hmm, mine is doing this at the moment - will see if i can clean up the contacts too!

[LHD_Kid]

I spoke too soon. Back at 200 MPG now really wish it did!

Does anybody have the means to tell me which part(s) feed the trip computer with the speed signal?

Speedo works fine so cluster receives the info ok...

Any ideas?

[LHD_Kid]

Any ideas from you smart guys?

How does the trip computer receive the speed signal inside the cluster?

[newsh]

Have a look for the How To install the outside temp display, it tells you which pin the speed signal comes out from to the rest of the car (to radio loom, diff lock switch etc and on ABY the ECU) with everthing switched off and using the continuity setting on your meter, make sure that wire is not shorted to ground. It might be trapped in the centre console somewhere if you've had the radio out

Im pretty sure the speed signal is made by the speedo circuit in the cluster and then travels to the Trip computer through the internal pins where the computer board plugs into the main cluster board. This will likely be where the contact is bad.

[LHD_Kid]

I've not had any luck tracing the problem. Was hoping someone might have more thoughts on where the poor connection or broken wire could be.
Again. Speedo works but fuel computer not. Also, distance till the fuel runs out shows 2000 miles permanently...

Edit: forgot to add, it is the same with 2 sets of instruments so the fault is really unlikely to be in the instrument cluster...

[A1QSHIP]

Think about it. Under normal circumstances when my Coupe is on a hard deceleration the MPG gauge goes to 200. May be that the idle switch on the throttle body is playing up.
HTH, Chris

[LHD_Kid]

Think about it. Under normal circumstances when my Coupe is on a hard deceleration the MPG gauge goes to 200. May be that the idle switch on the throttle body is playing up.
HTH, Chris

Hmm... plausible.... but wouldn't this potentially have other side effects? Apart from the trip computer everything is as it should be - idle is fine, throttle seems to work ok. cruise control works, etc etc etc...

I'll check it out anyway, clean and de-oxit whatever I can find....

[UrS4boy]

Any ideas from you smart guys? How does the trip computer receive the speed signal inside the cluster?

Better question is how does the trip computer calculate miles per gallon, or Litres per 100 km? It needs two signals: 1. Vehicle Speed (speed x time = distance) and 2. Fuel consumption.

1. G22 Vehicle Speed Sensor:

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

The speed signal comes from the G22 Vehicle Speed Sensor, entering a B3/B4 cluster on T26/20. If you have a G21 speedometer reading, then the issue isn't the signal reaching the cluster. Could be an internal cluster issue.



2. Fuel Consumption

Fuel consumption for the trip computer is based on the fuel injector duty cycle, i.e. based on factory calibrations and the presumption that the pressure across the fuel injector is always the same, regardless of vacuum or boost, the length of time that an injector is open can be manipulated into fuel consumption. Note that this calculation goes to sh*t when you change the injectors to say, RS2 injectors on an S2, or you change the fuel pressure regulator from say 3.8 Bar to 4.0 bar or 4.0 Bar to whatever.

Not being an S2 expert, I can't tell you exactly where you can find that wire. (See my next post). However based on my C4 UrS knowledge I can tell you that the signal initially comes from the ECU at pin T55/31 (this will be true for 3B, AAN, ABY and ADU). For the C4s, that wire is a blue/black wire in the red T6c connector in the C4's passenger side connector station.

5/6 Blue/Black (0.5) ECU pin 31 (T55/31- Fuel consumption signal for trip computer - BL/BK Enters the J128 Board Computer at Pin T10a/5 - Note: only used in N.America on 92 spec UrS4s)

Here is the C4 T6c:



It is also known that the T6c connector can get corroded, resulting in the loss of signals, including the G22 VSS signal (also in the T6c connector). Solution is to find the connector and clean the corroded pins.



OFF to find the S2 equivalent connector.

[UrS4boy]

S2 Specific Fuel consumption signal info

From S2 central's ABY/ADU ECU pin-out info:

Pin T55/31 = Fuel consumption signal, OUTPUT to J128/189 trip computer, wire colour= br/sw. (Note: According to S2Central.net, this wire could also be bl (blue) or bl/sw (blue/black = like the C4 UrSs) )

A quick search of wiring info at S2Central.net showed up this pdf:

http://www.s2central.net/S2_Schematics/ABY_Wiring.pdf

A Ctrl F search on that with "fuel" found the required diagram with the T5 connector containing that T55/31 signal on page 47:

Screen shot:



Which is presumably on the right side of this photo, the green connector at the bottom:

[LHD_Kid]

That's going to be a great deal of help. Thanks very much Dave - also helps to have a bit of an understanding of how it works, not just where the wires go..little something to do at the weekend