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Discussion Starter #1
Right I have a problem that myself and the garage (Simply Alfa's) have been unable to resolve. Before I strip what is otherwise a very nice 156 for parts I thought I throw it open to the resident experts on here for suggestions.The vehicle in questions is a 04 plate 156 1.9 JTD 16v Sportswagon.

The problem:

At random intervals the accelerator pedal stops responding completely until you lift off off the pedal and press it again. Once you lift off and press it again it will go again, for a random period of time from 1 second to 30 minutes. It started about 2 months ago, doing it once or twice on my 26 mile journey to work, it got progressively worse and now its virtually undrivable. The engine is not stalling, if you dip the clutch it is idling fine, it is just as if the accelerator is not being pressed at all. You can move the accelerator up and down the full travel and nothing will happen until you lift off and press again. It does not do it on cruise control! No faults are logged on the ECU, monitoring sensors at the time the issue occurs doesn't reveal anything either, in fact it shows that the ECU is recieving the correct throttle position from the pedal.

What has been tried:

- New throttle pedal.
- Clean up of electrical contacts at pedal and ecu.
- Replacement ECU.

If anyone has any suggestions before I strip it to pieces they would be welcome, although I am loathed to throw any serious money at it on off chances. Failing that anyone need any parts (including black leather, 17" multispokes and facelift front end!) :)
 
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Old school mechanical knowledge it is then:
Has the throttle pedal connector been checked behind the glovebox?
They have been known to go loose and to my knowledge don't throw up faults as the ECU quite rightly shows no change in the pedal position when the signal is cut... No amount of cleaning and re-fitting will help if the connector itself is faulty. It's simply something that can't be checked while driving, road vibrations and potholes can jolt the connection easily.
A permanent fix is to solder the joints to by pass the connector altogether... Again something that can be reverted by de-soldering if you ever need to replace the pedal assembly.

There could be an issue with the throttle body itself, not to sure on a 1.9JTD but the butterfly could be sticking in position for a period of time due to grime, again, common fault on the petrol models, a simple cleaning without touching the butterfly may suffice>

Worth trying to save your car mate:thumbs:
 

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Discussion Starter #3
Going by elearn I couldn't see any connectors between the pedal and the ECU, certainly wouldn't make sense for it to go over to the glove box when the ECU is in the engine bay next to the engine on the belt side. Also as I said when the fault occurs diagnostics appear to show that the ECU is recieving the correct input from the pedal.

Diesels don't have a throttle butterfly do they?
 

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Diesels don't have a throttle butterfly do they?
No.
They have a shut-off valve in the same place though, but that is always fully open when running and (hopefully) fully shut when you turn the engine off.
 
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Going by elearn I couldn't see any connectors between the pedal and the ECU, certainly wouldn't make sense for it to go over to the glove box when the ECU is in the engine bay next to the engine on the belt side. Also as I said when the fault occurs diagnostics appear to show that the ECU is recieving the correct input from the pedal.

Diesels don't have a throttle butterfly do they?
On a 1.9 I'm not sure mate. Someone will be able to confirm

As regards to the connector, its a multi plug so bear in mind that some connections will be perfect, others won't... So old school logic suggests that by what you say, the wire/s that send the input signal to the ECU may be perfect in that case... What about output return to the pedal? Worth soldering them all IMO what have you got to lose?
 
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have you tested the continuity of the 6 or so wires, form the throttle body to the ecu?
 

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Discussion Starter #7
have you tested the continuity of the 6 or so wires, form the throttle body to the ecu?
I haven't and I thought about running a completely new set of wires from the ECU to the throttle pedal and I might try it. However there are a few things that make me think this will be a waste of time:

- Why would lifting off resolve the issue. If there were a wire issue I would expect the throttle to cut out/resume at random, not require the pedal to be lifted, after all the pedal is nothing more than a potentiometer no?
- If connectivity to the throttle pedal were lost I should get a relevant ECU code.
- If connectivity to the throttle pedal were interupted, diagnostics would show this on a graph of throttle position. However even when the problem occurs you can see the throttle position readout from diagnostics correlating to the throttle position.
- I haven't been able to get the problem to occur when the car is stationary.

Thanks

Phil
 

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How about the brake pedal switch ? If you press the brake pedal lightly (enough to operate the switch) whilst accelerating the throttle gets cut after a second or so and ceases to operate thereafter. I'm not sure if you just need to release the brake pedal to get the accelerator to work again or if you need to release the brake and accelerator to reset everything. So you might have a dodgy switch perhaps. This doesn't however explain why cruise control would work since it should be cut if the brake pedal switch is triggering. Also doesn't explain why it fixes itself when you release the accelerator. Having said that the brake pedal switch has 2 sets of contacts and seems to work in mysterious ways, causing problems with no faults registering. Easy to check.
 

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If I remember on my 16v JTD, when I pressed both pedals simultaneously the CPU registered a fault. Doesn't do that on the 20v.
 

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Discussion Starter #10
Certainly one I hadn't considered. Can I just disconnect it without any consequences (other than no brake lights)?
 
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yeah, modern diesel don't allow 'left foot braking' in fact it'll kill the engine power. The ECU can be fooled by lifting off the throttle & then re-applying it while holding your left foot on the pedal. So if the brake switch was sticking ON that simulate 'left foot braking' [ even though your are not doing it ] & kill engine power, by you lifting off the thottle & then re-applying the throttle the power is regained.
 

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Discussion Starter #12
Well it was a good idea but disconnecting the brake pedal switch did nothing but throw up a lot of VDC/ABS errors, made no difference to the fault in question :(
 

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Well it was a good idea but disconnecting the brake pedal switch did nothing but throw up a lot of VDC/ABS errors, made no difference to the fault in question :(

Hmmm, I still think it's got something to do with this, i.e. the ECU thinks the brake is being pressed. Certainly ties in with it resetting when you release the throttle pedal. May not be the switch itself but the wiring connecting it to the ECU. Also the switch has 2 sets of contacts so it's possible 1 set does the cruise and 1 the brake function that affects the throttle. For such a cheap little switch it seems to affect a lot of functions.

I guess the other option is to go to a main stealer and see if they have a fault tree that might identify this issue or, failing that, they could put a technical query back to Alfa themselves ... for a tidy fee no doubt.
 

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Discussion Starter #14
Car simply isn't worth enough to entertain me being bent over by the stealers. eLearn seems to show only 1 wire going back to the ECU, the other 4 go to various other places. I could try and cut that at the ECU end and see what happens.
 

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Looking at ELearn there are 2 connections to the ECU. The one to Pin 80 is related to cruise control and operates off the brake switch contact that is normally closed, i.e. pin 80 normally has power which is only cut when you brake. The other connection to the ECU is on pin 17 which comes from the other brake pedal switch contact that is normally open (contacts work opposite to each other). This puts power on to pin 17 only when you brake. I'm guessing this is the one that cuts the throttle when you brake since ELearn says that the connection to pin 80 is only used when optional cruise is fitted.

So I guess there would have to be a positive short onto the wiring going to pin 17. The wiring goes through the under dash fusebox and another connector that also connects to the ABS control unit. Hard to believe there's a short here somehow. I suppose the only thing to try is disconnect pin 17 and see what happens.
 

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I would have suspected another condition interfering, like the brake pedal being picked up, but if it this were the case then cruise control wouldn't work. No ideas about this, other than wire continuity or beyond the ECU.

The car must be worth a lot more investigation though; seems insane to scrap a car over this.
 

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I would have suspected another condition interfering, like the brake pedal being picked up, but if it this were the case then cruise control wouldn't work.
Not necessarily. The ECU receives 2 brake pedal position indications as I mentioned before, 1 for cruise and another independent of cruise. So cruise might still work OK even though the independent input is causing a problem when cruise is off. It all depends how the ECU has been programmed. Could be that with cruise on, the throttle cut on braking function is overridden, or at least the independent input from the brake switch is ignored.
 

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Ah, yes. I'm fairly sure but not 100% positive that you can interrupt cruise without triggering the brake lights, since I had switch problems for a while that prevented CC, and I don't think I was driving around with the brake lights on. That's not to say the ECU doesn't use that input for something else. I had thought it was the same physical switch assembly though - is it?

Rather than disconnecting it, which will probably cause all kinds of wrong, try lifting the pedal up from beneath with your foot when the problem occurs.
 

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Same switch but 2 independent sets of contacts. 4 wires into the switch 2 of which are always live with the engine running. So it's possible a short in the connector to the switch or it's wire loom could put a live onto pin 17 of the ECU without the pedal being touched or with the switch itself disconnected in fact.

Course it could be something else completely different and this is just barking up the wrong tree.:)
 

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Now that I think back further, I also had switch problems (before installing CC) that left the brake lights on after braking - noticed this at night. Lifting the pedal would cure it, until I eventually fixed it by bending the switch bracket slightly.

When CC was installed, it gave similar issues, presumably because the gap was enough for the brake input but not that. I taped a five pence piece to it for a while before replacing the switch. As I said I'm not certain that the brake lights weren't stuck on again, but I doubt it.

It may be different on diesels, but none of this gave power issues. Another point against it is that the conflicting inputs would probably log a diagnostic fault - I had one, but I didn't record the details. This only shows up when reading faults, i.e. doesn't generate a MCSF. I also found that dodgy switches are a common problem, so presumably lots of people would be reporting the same as the OP, and I've never heard of it before.

It does seem like a worthy avenue of investigation, especially since under normal conditions one is live and one is not.
 
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