T4 2.5 TDI accelerator intermittently not responding - Fixed!... I think.

[Pundit]

After just picking up my 'new' T4 2.5 TDI the dreaded non responsive accelerator pedal problem that I thought had been fixed by the dealer re-emerged.

Symptoms were total loss of accelerator response with the engine stuck at around 1200 RPM and a flashing glow plug warning light on the dash. Turning the engine off, then on again, would cure the problem albeit temporarily.

As I don't yet have a Vag-Com scanner I had to resort to 'old school' diagnosis.
I eliminated the Throttle Positioner Sensor after checking it with a multi-meter and a clean just for good measure.
Scouring several VW forums seem to suggest that either the brake light switch or main wiring loom could be the culprits. As the brake light switch was the easier of the two to check I started with that.

Pressing the brake pedal is designed to override the accelerator (in case the throttle jams open) as the status of the brake light switch is monitored by the ECU.

Upon dismantling the brake light switch I discovered a rather black looking contact terminal. After a good clean and spray I reassembled and re-installed the switch and have not had a problem since.

Although it's a bit early to be 100% certain it's fixed I'm suspecting it is.

So for those who may develop a similar problem with their T4 2.5 TDI don't automatically assume the accelerator pedal (TPS) is at fault.

Here's an excellent link to the fix...

! How to - brake light switch / flashing glow plug light - VW T4 Forum - VW T5 Forum

[Sunny43.5]

Good diagnosis there nice to see old school techniques still work .

[Pundit]

Good diagnosis there nice to see old school techniques still work .

Well I guess if it was truly 'old school' I shouldn't have even used the internet.

[Seano]

Cleaning and or replacing the brake light switch fixes so many faults (which is why I have a spare in the door pocket)...but this one is a new one to me!!

[Pundit]

Cleaning and or replacing the brake light switch fixes so many faults (which is why I have a spare in the door pocket)...but this one is a new one to me!!

I thought it was odd until I saw this and then did some more research...

T4 TDI Brake light switch

"Genuine VW brake light switch for 2.5 TDI T4's
Common problem sees you with reduced power (limp home, or limp mode) due to brake lights not working."

It appears it can affect the operation of the accelerator.
There's also a switch on the clutch pedal that is supposed cancel the cruise control if fitted.
The brake light switch will also cancel the cruise control when fitted.

[Pundit]

Well it turns out it wasn't fixed after all. The brake light switch really had me fooled as each time I cleaned the switch contacts the problem disappeared. It turns out it was indeed the wiring loom that emerges through the firewall near the brake booster, runs across the engine and under the expansion bottle and then onto the ECU. Will post some pics in a new thread shortly.

[jets]

I will be interested to see your pics. Thanks for sharing. Why not post again in this thread, keeps it all together.

[Sparkie69]

Well it turns out it wasn't fixed after all. The brake light switch really had me fooled as each time I cleaned the switch contacts the problem disappeared. It turns out it was indeed the wiring loom that emerges through the firewall near the brake booster, runs across the engine and under the expansion bottle and then onto the ECU. Will post some pics in a new thread shortly.

Hi there!

On the RHD T4's the engine harness runs over the tappet cover and takes a lot of vibrations.
What turns out to be a major problem-not only on T4's but also on numerous newer VW's is the use of
corrugated sleeving.The wiring vibrates in the sleeving (since taping the wiring harness is much too expensive
and too much of an effort for the factories !!) and the insulation chaffes away,leaving the copper partly exposed.

Wash your engine once or drive in rain and you are in for trouble...You'll get the dreaded Throttle sensor code and in time
the wiring will corrode off..

All I can recommend from experience is to open up the harness,fix all open wiring and wrap the wiring harness in 'Denso Tape' -then refit into the corrugated sleeving. The new harness price is way past rediculous over here....
(The sticky brown tape used by plumbers,etc. - prevents corrosion, I use it a lot,since we are at the coast..)

This is not a nice operation to do(since removing the harness is the best option),but will save a lot of future trouble and money..

A car wash with a pressure cleaner is definately a no-no

Hope this will help somebody!

[jets]

Thanks for sharing your experiences Sparkie.