AFN Pedal position sensor problem

[spanishfly]

Hi
I'm learning a bit more everyday thanks to this forum!
Today I'm trying to get my throttle to respond in first.
Van has an AFN from 1998 Passat estate that wasn't running when I bought the van.
It has the potentiometer mounted under the floor pan and I have cleaned the earth.
Van starts on the button first kick every time revs are probably a little high at tickover but very happy sounding.
As you press the accelerator nothing happens to start with even pressing it quite along way down. If you press it a few times it catches then the revs behave normally, this only happens when pulling away, when driving seems fine.
Potentiometer is 028907475AJ 0 281 002 286 green and gold label with short wire and cable. It feels like the tracks are dirty like an old radio?
I think I've read in here that this potentiometer can be bypassed? Anyone know if that's right and how I go about it or should I carry on hunting for a replacement?

Also my temp gauge never goes above first white bar far left, has a blue 2 pin head/guage sensor fitted anyone know what colour should be there please? Black? White? Grey?

[randolph57]

I can see the virtue of keeping the potentiometer 'nice and tidy' under the floor pan but it makes it a bit of a bugger for diagnosing faults like this. To me it sounds like a fault with the wiring from pedal position to 'passat' part of the wiring harness or you have slack in the system before potentiometer moves. Do you have a OBDII port ? if so get access to a Ultragauge/scan gauge/vagcom as it is quite easy to see the throttle position moving from 0-100%(or other way depending on set up) . This is where i would start first-The barrel potentiometers are pretty robust and I don,t believe this to be at fault.
I think the late 90's tdi's all came with a similar potentiometer (audi a4/passat/A6 etc) and i gathered a couple of these as spares on my scrappy trips
Some people use the later potentiometer in the pedal assembly and i believe attention to wiring connections/numbering is more critical with changing model year if this type is used.
I use the pot situated in engine bay using the old cable and makes it easier being at the 'buisness' end when chasing faults'
From memory the temperature sender should be a 4 pin unit (blue on mine) as the ECU needs to see engine temperature as well as a take off to the Instrument cluster gauge-so one or other is not connected on yours!. Again you really need a OBDII port and a ultra/scan gauge so you can read engine temp/throttle position and about 30 other engine parameters from the information the ECU is reading.

[syncroandy]

High tickover and no throttle response is known as 'limp mode', and happens when the ECU sees a serious engine management fault of some sort. It possibly is a miswiring of the accelerator sender, but it might be some other wiring problem. Its probably worth checking the wiring on the sender is correct, the should be like this:

(The plug on the sender is called 'T6a', the ECU plug is 'T80', the pin numbers are marked on the plug)

T6a/1 > yellow > T80/24
T6a/2 > red > T80/11
T6a/3 > brown/red > T80/23
T6a/4 > grey/yellow > T80/12
T6a/5 > (not conn, auto only)
T6a/6 > brown > earth

As randolph says, VCDS would be very useful as this should reveal the exact cause of the problem. The senders are quite robust and unlikely to have failed.

Is your glow-plug lamp flashing ? The ECU uses this to indicate faults. If not, its probably not wired up properly. Check your T3 glow-plug wire (white/red) is connected to the ECU on T80/41 (brown/green).

[spanishfly]

Thanks both of you. Really helpful and apologies I didn't see these replies.
User error! Well that and spending hours under the van struggling with linkage then gearbox and now I've messed something up and it won't run so pedal issue slipped off my radar until I can figure out what's going on.
I do really appreciate forum members giving up their time to try and help.

[spanishfly]

To answer more fully, the yellow glow plug lamp doesn't work on the dash clocks which isn't helpful though when I tried with a known good one there were some codes flashing.
I do have a port to connect diagnostics though was told it couldn't connect. This is probably because the owner isn't that familiar and me less so. Obviously need to go through the basics get the van running and then find a way for the magic box and ECU to talk to each other.
It never ends..!

[spanishfly]

Finally got round to trying to solve the lack of throttle response and discovered OBD doesn't connect so a bit stuck.
Data cable has 3 wires, a brown earth which is good a green and a red and black. Does anyone with an AFN know the pinouts from the ECU please?
Many thanks.

[syncroandy]

Hi, on the OBD port, red/black is battery positive. The OBD port green/black connection should go to the ECU pin 45 (of 80), which is also green/black. This assuming the immo has been removed.

Finally got round to trying to solve the lack of throttle response and discovered OBD doesn't connect so a bit stuck.
Data cable has 3 wires, a brown earth which is good a green and a red and black. Does anyone with an AFN know the pinouts from the ECU please?
Many thanks.

[spanishfly]

Thanks for the help Andy. The ECU green/black was broken. Now repaired so will see if this works.
Simon

[spanishfly]

Yeeeeasss! Comes up with various codes :

1/.01266 and 00626 Glow plug light open circuit - light doesn't work, when I rescued the van the giant 50A fuse in the engine bay was blown..? Since replaced which solved other problems but doesn't light up.
Have seen yellow glow plug light with other clocks so am I correct to suspect the Led? Are they hard to change?

2/.00563 AFM fault.

Am I right in thinking there isn't an easy OEM tacho solution for an AFN engined van? Not as simple as running a wire from the 'W" terminal if there is one? to the dash plug on the back of the clocks....

Thanks for the advice.

[spanishfly]

If anyone else has the same limp mode when pulling away in an AFN converted van I changed the brake switch and now all good!