Ph2 172 throttle circuit fault

[Thom_1991]

So I have a permanently lit electrical fault light and unresponsive throttle pedal. The error code indicates throttle pot circuit A.

Now a bit of history, for the last 5 years I've owned the car / known the previous owner, it's had an issue when ambient temperature is nearing freezing or below. Ignition on - electrical fault light lit - throttle input is ignored. Let the car warm up. Turn off and back on. Fault clears and car drives fine. Never a problem on warmer days. Previous owner replaced throttle body and pedal. Fault remained, but as it never left him or myself stuck it never bothered me.

Recently it was playing up more and more then one day threw the fault after starting with the car already up to temperature. Oh b****cks. Since then the fault has been present permanently.

I've measured both tracks resistance at 0 and 100% pedal angle:
Pin 3 1.27K - 1.7K
Pin 6 1.21k - 1.6 k .

Pretty close and I believe in spec?

Scoped the 5V supply, it remains solid irrespective of pedal position

Scoped both signals. Given the supplies are the same and the pots measured very similarly, I'd expect two almost identical signals, but no, one is half the amplitude of the other. I'd expect this if one of the pots was half the resistance of the other, but not when they both measure the same and receive the same voltage?!

I tied the two 5v supplies and two grounds together respectively in case one supply was dipping to 2.5V (they're not isolated so no problem) Clearly not as the signals remains the same...

You can see ignition on and then 0 to 100% pedal. Yellow trace is pin 3, the other pin 6.

Can anybody tell me if this is the output the ECU wants?

Edit to add I got as far as confirming continuity at the large white block connector in the fuse and relay box. Would have scoped the signal there as well but ran out of time. The connector pins and sockets showing no sign of corrosion.

 

[STEVE.M]

So I have a permanently lit electrical fault light and unresponsive throttle pedal. The error code indicates throttle pot circuit A.

Now a bit of history, for the last 5 years I've owned the car / known the previous owner, it's had an issue when ambient temperature is nearing freezing or below. Ignition on - electrical fault light lit - throttle input is ignored. Let the car warm up. Turn off and back on. Fault clears and car drives fine. Never a problem on warmer days. Previous owner replaced throttle body and pedal. Fault remained, but as it never left him or myself stuck it never bothered me.

Recently it was playing up more and more then one day threw the fault after starting with the car already up to temperature. Oh b****cks. Since then the fault has been present permanently.

I've measured both tracks resistance at 0 and 100% pedal angle:
Pin 3 1.27K - 1.7K
Pin 6 1.21k - 1.6 k .

Pretty close and I believe in spec?

Scoped the 5V supply, it remains solid irrespective of pedal position

Scoped both signals. Given the supplies are the same and the pots measured very similarly, I'd expect two almost identical signals, but no, one is half the amplitude of the other. I'd expect this if one of the pots was half the resistance of the other, but not when they both measure the same and receive the same voltage?!

I tied the two 5v supplies and two grounds together respectively in case one supply was dipping to 2.5V (they're not isolated so no problem) Clearly not as the signals remains the same...

You can see ignition on and then 0 to 100% pedal. Yellow trace is pin 3, the other pin 6.

View attachment 1355354

Can anybody tell me if this is the output the ECU wants?

Edit to add I got as far as confirming continuity at the large white block connector in the fuse and relay box. Would have scoped the signal there as well but ran out of time. The connector pins and sockets showing no sign of corrosion.

@MicKPM will know the answer.
Scope the large white block though, pins can look good, but loom to pin connections corrode.

 

[Thom_1991]

Bit more info I failed to put. The signal remained exactly the same when I chopped the two signal lines to disconnect them from the loom and ECU.

My fear assuming the pedal's outputting the correct signal and the remaining bit of loom from white connector to ECU is okay, is that the actual ADC in the ECU is at fault :weary:

Also it did briefly run without error after I'd disconnected, buzzed and reconnected the white plug under the bonnet. All be it idling like a pig initially then slowly better and better. After a few minutes of slightly improving idle, a stab of throttle threw it back into error state.

 

[Thom_1991]

With a house move in progress I've lost interest and have thrown this problem Diamond Motors way to see if they fancy a challenge :smile:

 

[Thom_1991]

So I never did get the car to Diamond. Had a brief conversation where I described the problem and he agreed it sounded like the ECU may be at fault. Problem was they were pretty booked up so would have been ages before they could look at it.

I've finally got round to replacing the ECU and UCH with a paired unit from another 172. All plugged in and ran with no problem, happy days.

Started this morning in the cold and the fault occurred again! Let it warm up and turned it off and back on. Fault cleared and it ran normally.

So the pedal and now ECU have been replaced. I can see no difference on the oscilloscope between fault state and running normally. There's no sign of corrosion on the wiring or connectors. What is truly confusing is that if it was corrosion or a wiring issue, why on earth would the problem be temperature related.

To say I'm confused is an understatement.

 

[bloke]

chuck a new plug/connectors on the body and see how it goes.

 

[Thom_1991]

Need to get the fault code this time, but why the throttle body? The codes previously have always been pedal related.

 

[bloke]

am guessing it's a circuit fault it came up with?

 

[STEVE.M]

To say I'm confused is an understatement.

As bloke stated, and you have proved by replacing the hardware, it`s a circuit fault. (P0120) ?

I still have this as an occasional reoccurring problem, but have found that disconnecting & dousing the 90 pin connector for the ECU with Solvisol Super 10

(weapons grade) electrical cleaner provides an immediate resolution for around 6 weeks .

1st hint of reoccurrence, repeat.


Strangely, I find a build up of engine bay heat, sitting in traffic will induce the occurrence, as opposed to your cold start observation.

I cannot be sure if the problem is with the female connectors of the block connector, or the male pins of the ECU.

If you have replaced one 16 year old ECU with another of the same vintage, the surface oxidation of the pins will be about the same, even if they look good.

I will get round to polishing the relevant pins on the ECU eventually.

 

[Thom_1991]

Oh god losing your throttle while sat in a traffic queue, bet that's fun!

With the old ecu when it finally got stuck in this state it was pedal circuit A fault. Again though the throttle body is an entirely different circuit. Or am I misunderstanding what you're saying bloke?

I suppose all that's left is the wiring and connectors between pedal and ECU so will have to start getting deeper into that....

 

[Thom_1991]

Suspect I'll try just running new wires directly from the pedal to ecu end, that eliminates the three block connectors between

 

[Rystar]

Not sure if it helps at all, but I had a throttle potentiometer circuit fault on mine that came down to a wire shorting.

The ECU casing tab that bolts on near the suspension turret had broken allowing the wires to rub on the metal casing. This just wore through the insulation enough to intermittently short one of the trottle wires.

A bit of electrical tape to cover the wire and the ECU casing replaced with a non-broken tab seemed to sort it instantly.

Like I said, probably not on the flavour of what your issue is, but might be worth a mk1 eyeball test

 

[Thom_1991]

Be extremely surprised if it's a shorted wire, given how for years on end the problem was temperature related and never missed a beat once warmed up.

Think @bloke suggesting throttle body wiring/connectors may be onto something. Should have got the scope on there. Having read the technical manuals endlessly it would seem "Circuit A fault" could indeed be related to the body, rather than pedal. Both pedal and body have two pots.

Either way plenty more investigation required.

 

[@Josh]

I have similar issues, on my race car (not ideal when coming out of a corner and your car dies and goes into limp mode!!

History ok mine was.... electrical light comes on around 20/25 minutes of running. Limited to 3k revs.
Turn car off and kill it using kill switch, then turn it back on and it goes out (I guess it’s eiping the memory)

I changed throttle pedal, gave all the plugs a clean in white box in engine bay all sorts.

Still had the issues.

Recently change the throttle body itself (so now a new pedal and throttle body unit) and so far I have had the car running for over 1 hour on 3 different occasions without using the kill switch!

Only thing I have not tested yet is it under load so on track, this will be the final test.

I am no where near as technical as you above but just doing the basics seem to have worked for now.

Thanks Josh
@Thom_1991

 

[JamesBryan]

I have similar issues, on my race car (not ideal when coming out of a corner and your car dies and goes into limp mode!!

History ok mine was.... electrical light comes on around 20/25 minutes of running. Limited to 3k revs.
Turn car off and kill it using kill switch, then turn it back on and it goes out (I guess it’s eiping the memory)

I changed throttle pedal, gave all the plugs a clean in white box in engine bay all sorts.

Still had the issues.

Recently change the throttle body itself (so now a new pedal and throttle body unit) and so far I have had the car running for over 1 hour on 3 different occasions without using the kill switch!

Only thing I have not tested yet is it under load so on track, this will be the final test.

I am no where near as technical as you above but just doing the basics seem to have worked for now.

Thanks Josh
@Thom_1991

Have you tried the coilpack mate?

 

[@Josh]

Have you tried the coilpack mate?

Yeah coil pack, leads, plugs the lot I think I have sorted it now until I get it on track.

 

[Thom_1991]

Interesting. The opposite of my issue in that lots of temperature seemed to trigger yours. Will update if or when I get to the bottom of it!

 

[bloke]

Replaced the plug and connectors yet?

 

[Thom_1991]

No. Where could I source the bits?

 

[bloke]

Polevolt, cbs, autosparks, rd components etc. Nothing special about it, just a Bosch 6 pin plug.