Erratic cruise control?

[jimbof]

So today for a good 10 minutes or so the cruise control on my car was behaving very erratically - like someone using wellies on the accelerator pedal.  Can anyone explain to me what the feedback loop is for cruise control on these cars so I can try and work out what is wrong?

[jimbof]

Ideas anyone?  If I don't have cruise enabled the car drives perfect, but as soon as I enable it it will lurch around with the RPM surging up / down.  No codes whatsoever in VCDS.

[Gaz]

Mine tends to surge a bit at low speed cruise.  Particularly if I switch it on at a point where it might ordinarily be thinking about changing gear, such as lower power on a gentle slope at 30mph.

 

Afraid I haven't got a scooby doo really.

 

Gaz

[jimbof]

Thanks for the reply.  It isn't that - I've seen that in the past on occasion (particularly uphill on the verge of shiftpoint), but this a real case of "1st time learner driver clutch control" or similar, making the car hop along.

 

this morning the car isn't doing it.

 

I am guessing on these cars there is only one mechanism for controlling the throttle (is the pedal just a sensor on these cars and not actually connected up mechanically?) - so in that case I must be looking at some kind of sensor which is inputting to the cruise control which isn't in play when I'm driving without cruise - does anyone know what sensors are used to make the cruise control decisions?

[alliv]

Is the brakelight switch anything to do with it I wonder?

[Gaz]

Pretty certain it's fly-by-wire, in which case the brake switch would cancel cruise.  I'd be surprised if it'd cause fluctuation rather than just cancel it.

 

Gaz

[whitep]

I did hear that a failing maf sensor causes erratic cruise

[dieselV6]

I concur MAF a possibility, other possibilities include an intermittent signal from the speed sensor and/or accelerator pedal transducer, also possible is sticking variable vane (VNT) actuator on the turbocharger.

Brake, clutch switches are unlikely to cause it, as they disengage CC, same for faulty CC stalk wiring.

[jimbof]

When it was failing it was fluctuating the RPM up/down by 2-300.  It was a little like cruise coming off and then going to resume, quickly, several times a minute.  

 

I can't see why the accelerator pedal would be implicated - ?  It only happens in cruise, not when using the pedal.

I wondered about speed sensor - does anyone know if it uses the ABS wheel speed or the gearbox speed for cruise?

 

Wouldn't MAF also be noticeable with cruise off? And the VNT?  Or are they used differently with cruise on?

 

It really was only doing it with cruise on - as soon as I started driving the accelerator myself it would disappear.  It hasn't done it again yet though.

[dieselV6]

CC uses gearbox speed signal, abs signals are only used by brake related systems. When cc is on, you can still accelerate, it returns to set speed once accelerator released, so faulty, fluctuating throttle signal would also cause the symptoms.

Anyway, if it only does this with cruise on, without any trouble at all while in same gear and at same speed/engine load, perhaps it is a faulty stalk wiring or the switch after all.

[jimbof]

It hasn't done it for about 500 miles so just waiting for it to recur.

 

Obviously I'd had quite a lot of gearbox work done, so perhaps the loom or sensor have seen better days.

 

The surge was usually actually inversed - usually down from the set speed (lurching forward) and then then up - but the change in RPM was faster and more abrupt than if cruise were just activated / deactivated, or adjusting the speed via the steering wheel.  So I think it must be some sensor playing up.

 

Interested in the MAF theories - I've seen quite a few posts on the American Passat sites implicating MAF in jerky cruise control.  I just don't quite get why MAF would only affect cruise (though I guess probably a lack of understanding on my part).  I understand MAF would likely not throw a code whereas most other sensors would?  I have no codes on the car.

 

I guess I've got 111K on the current (probably original) MAF - maybe time to read up on the graphing of MAF performance in VCDS and see what it looks like.