Club 80-90 Forums

Hi All

My AFN TDI has developed an annoying intermittent fault on the throttle.

The power will suddenly die without warning. Take the foot of the pedal and apply throttle and it picks up again and may go any amount of time from seconds to minutes before it cuts again.

When it happens the reading on Scanguage will remain showing the throttle position at % even though there is no power/acceleration.

I have changed the TPS but it has not gone away so I am really asking if there are any other sensors or components that might be the source of this. It could be a wiring issue of course but I wondered if the MAF or boost sensors or the like could cause the power to immediately cut if they went faulty?

Its happening every time I drive it; some days more frequently than others. There are no errors being logged.

While it does not stop me driving the van it makes for interesting pull aways and i am sure it is not doing the driveline, mounts any good.

Any pointers appreciated.

AFAIK MAF and boost errors take the ECU to limp mode that needs restart to clear.
Put on VAG-COM and log/graph measuring blocks of pedal sensor.
If the TPS value keep being mismatched watching vag-com then I assume it is either wiring or bad ECU.
If the throttle position value corresponds the pedal all the time, then it could be a failing injection pump with sticking governor.
 

ZsZ wrote: ↑26 Apr 2021, 14:42 AFAIK MAF and boost errors take the ECU to limp mode that needs restart to clear.
Put on VAG-COM and log/graph measuring blocks of pedal sensor.
If the TPS value keep being mismatched watching vag-com then I assume it is either wiring or bad ECU.
If the throttle position value corresponds the pedal all the time, then it could be a failing injection pump with sticking governor.

Thanks ... VAG-COM was my next call but scan guage shows the TPS going from 0 to 99% without a problem .... when it cuts its usually somewhere around 60% but you have to take the foot off the pedal and reapply for it to pick up again. If you just put your foot down it shows 99% but there is no power.

As stated it sounds like a wiring problem but what type of throttle pot do you have? is it the barrel type or have you gone for the 'all in the pedal' variety? If the latter i would be looking closely at the extended wiring (assuming ecu in the rear).

Have you tried Dr Tim for any help?(ultimate engineering)

randolph57 wrote: ↑03 May 2021, 12:33 As stated it sounds like a wiring problem but what type of throttle pot do you have? is it the barrel type or have you gone for the 'all in the pedal' variety? If the latter i would be looking closely at the extended wiring (assuming ecu in the rear).

Have you tried Dr Tim for any help?(ultimate engineering)

Hi .. i have the barrel type it the engine bay. I have got as far as changing the barrel  and putting up this post so far ...the barrel change was to no avail. Im concentrating on the characteristics when driving at the moment. The cut in power can happen in any gear on acceleration and is quite intermittent. Sometimes its all the time, sometimes not at all.

I dont know enough about diesel pumps to hazard a guess at whether a mechanical failure in the pump could cause this as the poster above suggested. But the position value is maintained on the readout when the power fails; depressing the pedal causes the power to pick up again as does releasing and reapply the foot.

Strange one!

In the pump there is a stepper motor like device turning the shaft of the quantity adjuster together with a potentiometer like position sensor attached to that shaft. If the position sensor wear with time then it can confuse the ECU. The stepper can fail or the shaft can be worn and not operating smoothly. The quantity adjuster collar in the pump also can wear and stuck in positions.
So it is either can be a mechanical or an electrical problem in the pump if it is not a pure electric failure in the wiring or the ECU.

But the first suspect is the bad TPS signal that confuses the ECU.

Hi all. I have the exact issue on my 1z, kind of lived with it for a few years . I haven’t investigated it tbh but I was told it could be the vacuum pipe to the ecu?? I will look into this hopefully in the next few months. Seems to occur when the turbo is spooling up, foot of the gas then straight back on usually sorts it but I feel the engine is definitely not performing as it used too. No faults from diagnostics but I haven’t run any dynamic tests? Good to know what it could be. S.

Sorry for the slow reply.. not been on here for a while.

The symptoms described - sudden, brief, intermittent power-drops, often when under load - sound exactly like what I've experienced on several occasions due to a dodgy contact at the brown plug to the N80 needle-lift sender.

This component is critical in setting injection quantity and timing so if the signal from it is degraded it will impact on the power output.  I used some contact cleaner spray and narrow bit of card pushed into the receptacle to 'wipe' the contacts clean, and a cotton-bud on the pins.
 

Update on this .....

Its been a long and frustrating route to the source of the problem, but you cant beat first principle engineering and thanks to a couple of hours with Syncroandy of Syncrosports, VAGCOM a multimeter and a sequential approach he eventually found that the issue was with the LED bulbs I had fitted to the brake lights last year.

Put the old bulb back in and the problem was resolved.

My understanding of the high level science was that the ECU monitors the brake light circuit and relies on resistance/voltage measurements to discern the state of the brakes via the brake peddle switch. An LED bulb messes up the base readings and messes up the 'logic' resulting in the ECU cutting the fuel in my case quite intermittently.

Who knew ! 

If you update your bulbs you need to alter the wiring with a resistor to correct the circuits values.

Thanks Andy your a star! My garage have spent three months trying to get to the bottom of this by simply changing bits!

 

Thanks Mark, you are very welcome.

TBH it only took a couple of minutes in VCDS to see the likely cause - brake switch inputs incorrect - then another hour plus delving around in the rats-nest (Mark's words) of that TDI conversion, before finding the actual issue.

The ECU brake switch inputs are very important on any VAG drive-by-wire engine, because the brake pedal serves as a fall-back method to cut fueling to the engine, in the unlikely event of the accelerator pedal sender failing in a way that says 'I'm at 100%'.

There is typically a four-pin brake switch, one side closing when the pedal is pressed, the other side opening. The ECU has an input from each side of the switch, known as 'F' (the brake lamp circuit itself), and 'F47'. Again, this is a safety feature, if the ECU sees invalid inputs from these signals, it will go into 'limp mode', reducing power for safety.

The 'F' (brake-lamp) signal is normally 'low' (tied to earth, via the low resistance of the brake lamp filament), and goes 'high' when the brake lamps are switched on. There is a 'pull-up' resistor inside the ECU, so the default signal status with nothing connected is 'brakes on', this is a saftey feature. For the input to go 'low', the pull-up resistor must be overcome by a much lower resistance to earth. As designed, the brake lamp bulbs provide this resistance, remove or change those bulbs for LED's, and you will cause a problem.