Drive by wire.

Can one of our teccies explain how this is achieved in practice? My last car had a throttle cable - although newer versions of the same model went over to drive by wire. So it has been around for a long time -

15 years or more.

My 'new' car has it - again with Bosch petrol injection - and it seems to have a fault. At low speeds, when pressing the throttle pedal, it can do nothing - but the engine doesn't stall. Just like you'd taken your foot off the pedal. It's at its worst after leaving the car for an hour or so when it was hot before stopping. And seems to be mainly around the closed throttle position. Go for a fast takeoff, and it's OK. But not something you can do in heavy traffic.

It would seem to me like a worn track on a pot.

Hence the question. It is a pot they use as a pedal position sensor? Or some form of solid state rotary position device? How about the engine end? Is it a stepper motor, or a PCM controlled solenoid? Or something else? Not even sure if this car has a throttle body. Or several.

The other things that is confusing me is why it would be worse after the engine 'heat soaks'. As it is a mid engined car - so the actual pedal some way from the engine.

When I first experienced it thought it was a fuel vapour lock. But the perfect idle says otherwise.

The car is going back to the dealer for a warranty fix. But being a curious soul, like to have an idea what the fault might be.

Reply to
Dave Plowman (News)
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I dismantled mine on my old Fiesta last year, as it was running a bit erratically, and I suspected the pedal assembly (as it turns out, it was something else). But yes, there's a carbon track pot in there. At least that's what it is on mine.

Ask your question in uk.rec.cars.maintenance - you'll get a good answer in there. And maybe try getting an OBD2 fault code reader. Cars can be pretty good at diagnosing themselves these days.

Reply to
Dan S. MacAbre

Oh, I see you already know about u.r.c.m .

Reply to
Dan S. MacAbre

Depends on the car, the old ones were, the newer ones use a sort of rotary encoder. There are a few Youtube videos about it.

This one (diagnosis and teardown) is somewhat entertaining.

On the other end, same thing, could be either :) Interestingly, some diesels don't use a throttle body at all and just rely on the MAF :)

Reply to
Lee

Forgot the link :)

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There are plenty of others.

Reply to
Lee

Dunno about Porsche but the BMW system seems to use two pots (I get two values out of INPA on my M47) but, yes, pots are common. It's a generally reliable and long lived system so I presume other sorts of encoder are unnecessarily expensive.

The actuator end is usually just a servomotor arrangement. On my Jag S-Type, with the ignition on (engine not running) and the intake pipe off you could watch the throttle flap open and close with the pedal position.

Reply to
Scott M

I believe many VWs use two pots in parallel, running in opposite directions so they can rule out/detect noise on one track, and I'd not be surprised if this was common. The throttle body end is usually a stepper motor IME, and with some you have to let it re-learn it's end stops after working on it:

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Oddly, SWMBO's Lupo has a physical throttle cable, a sensor on the body, and a stepper motor on the body to adjust opening, which can be a bit odd when it adjusts it under your foot.

Reply to
Chris Bartram

I had horrendous problems with a worn track on the throttle pot on my 1993 Golf (petrol) many years ago. It was probably the first car with electronic throttle because my previous cars had all had carburettors, whereas this had fuel injection so could have electric rather than mechanic control. Very occasionally the car would hesitate and stall, or else fail to accelerate, when pulling out of a junction. It was in the VW garage several times and they couldn't find the problem. Eventually they detected the fault: I remember the mechanic phoning me in a state of delight to say "we know what it is and we've fixed it; bad news is there's a big labour bill for diagnosing" :-( Luckily I had kept the old garage bills which showed that I'd first reported the symptom while the car was still in warranty, even though it was now several thousand miles out of warranty. So the VW warranty paid up.

I've never had any problem with the throttle pot on any other car since - I've had several diesel Peugeots. Interestingly, the Golf (and maybe one of the Peugeots) still had a throttle cable which connected to a pot under the bonnet, rather than having the pot in the footwell by the accelerator pedal. I'm not sure why they do this when there's no mechanical connection to carburettor or fuel injection system so no need for a Bowden cable.

Reply to
NY

Honda jazz throttle position sensor is £400+. Actual part is made by Bosch, so probably common to many cars. Wiring loom connector has 5 or 6 pins, so god knows what it does. The only mechanical connection is the cable attached to the throttle pedal. One day I will try and take it apart to satisfy my curiosity.

Reply to
Andrew

CAN bus, maybe? Remember, your car knows more than you do!

Reply to
Davey

That sounds like daylight robbery to me, its probably an optical encoder of some sort to get away from the wear issues. Brian

Reply to
Brian-Gaff

?400 !!!

Corrr.

Reply to
pamela

Interesting. As far as I remember (and it was about 20 years ago) the part for my VW Golf was dirt cheap but it was the labour for fault-finding and diagnosing the fault which would have been the really expensive bit - if the warranty hadn't paid up when I produced the magic bit of paper proving that I first took the car to the garage while it was still within the warranty mileage.

Reply to
NY

Yes, I saw you post about that earlier but Andrew seems to be quoting a price just for a part. Amazing.

Reply to
pamela

I don't think they use a pot, more likely a Gray encoder, or some other rotary encoder ...

Reply to
Andy Burns

Dave Plowman (News) explained on 15/08/2016 :

The idle is entirely separate / independant of the fly by wire sytem, though both are controlled by the engine ECU. The throttle potentiometer just signals the ECU to indicate how much speed/ power is required. When at rest, the idle routine in the ECU takes over to hold the engine speed precisely at the correct idle speed.

Which means your throttle sensor might not be the cause of the problem, there are numerous other possibilities.

Reply to
Harry Bloomfield

Yes, but you're a Real Engineer. These guys understand price, and might not know a Gray encoder if it bit them.

Andy

Reply to
Vir Campestris

Most/ all use a pot..

Reply to
Harry Bloomfield

I've a feeling that is a different thing. Don't think 'drive by wire' was around in 1993. More like about 10 years later.

But virtually all electronic injection systems have a throttle position sensor (TPS) Basically a pot, the output voltage of which tells the ECU how wide the throttle is open.

'Drive by wire' replaces a mechanical connection between pedal and throttle with an electronic servo system.

Reply to
Dave Plowman (News)

That's not 'drive by wire'

Reply to
Dave Plowman (News)

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