940 High Idle after WOT

Venom00 · Oct 2nd, 2023, 19:28

Hi,

Can anyone point me in the right direction,

Got a high idle rpm that only appears after WOT. If I turn car back off and back on, it solves the problem.

I've checked TPS Sensor, MAF, Calibrated Idle to 500rpm with IAC clamped. Benchtested IAC Valve and cleaned.

Not sure how to proceed. Tested wiring and wiring is all good to the IAC Valve.

Thank you

Bugjam1999 · Oct 2nd, 2023, 19:59

Hi,

The issue is the ecu is no longer seeing the speed signal from the speedo to pin 34 on the ecu- that signal tells the ecu to activate the idle circuit, without it the car will idle at high rpm as you’ve found. Does your speedo work?

Cheers

Venom00 · Oct 2nd, 2023, 22:53

Quote:

Originally Posted by Bugjam1999

Hi,

The issue is the ecu is no longer seeing the speed signal from the speedo to pin 34 on the ecu- that signal tells the ecu to activate the idle circuit, without it the car will idle at high rpm as you’ve found. Does your speedo work?

Cheers

Hi, thank you.

Yes, speedo currently works. But have not investigated the wiring to the ECU telling vehicle speed.

How do you suggest I proceed?

Thank you

Bugjam1999 · Oct 4th, 2023, 19:40

Hi,

Whilst I know what the fault is, I’m not sure how to fault find it on a 940 - I’ve got a 240 with a 940 engine in it, so my wiring loom and speedo etc. is different.

Hopefully someone else will have some advice.

Cheers

TonyS9 · Oct 8th, 2023, 16:02

Yep had the same problem with 940 engine in a 360 with no speed sensor. Had to make one, luckily its an option in the speedo so there is a place for it.

I would say its not precisely as described above. The idle mode is triggered by the throttle closed sensor. The ECU just doesn't like not have a VSS signal with WOT, and creates some sort of limp mode, so far noone knows why.

Just trace pin 34 (assuming thats the right wire), normally comes from the ABS module to the speedo and then on to the fuel ECU on 34. I've had the rear axle speed sensor in the diff stop working due to chaffing and shorting on the body making the speedo fail, but speedo working and no signal to ECU is quite rare.

Oct 2nd, 2023, 19:28	#1
Venom00 New Member Last Online: Oct 3rd, 2023 19:55 Join Date: Oct 2023 Location: London	940 High Idle after WOT Hi, Can anyone point me in the right direction, Got a high idle rpm that only appears after WOT. If I turn car back off and back on, it solves the problem. I've checked TPS Sensor, MAF, Calibrated Idle to 500rpm with IAC clamped. Benchtested IAC Valve and cleaned. Not sure how to proceed. Tested wiring and wiring is all good to the IAC Valve. Thank you

Oct 2nd, 2023, 19:59	#2
Bugjam1999 Master Member Last Online: Apr 26th, 2024 12:00 Join Date: Nov 2012 Location: London and Cambridge	Hi, The issue is the ecu is no longer seeing the speed signal from the speedo to pin 34 on the ecu- that signal tells the ecu to activate the idle circuit, without it the car will idle at high rpm as you’ve found. Does your speedo work? Cheers

Oct 4th, 2023, 19:40	#4
Bugjam1999 Master Member Last Online: Apr 26th, 2024 12:00 Join Date: Nov 2012 Location: London and Cambridge	Hi, Whilst I know what the fault is, I’m not sure how to fault find it on a 940 - I’ve got a 240 with a 940 engine in it, so my wiring loom and speedo etc. is different. Hopefully someone else will have some advice. Cheers

Oct 8th, 2023, 16:02	#5
TonyS9 Premier Member Last Online: Apr 9th, 2024 21:44 Join Date: Jul 2009 Location: Holywood	Yep had the same problem with 940 engine in a 360 with no speed sensor. Had to make one, luckily its an option in the speedo so there is a place for it. I would say its not precisely as described above. The idle mode is triggered by the throttle closed sensor. The ECU just doesn't like not have a VSS signal with WOT, and creates some sort of limp mode, so far noone knows why. Just trace pin 34 (assuming thats the right wire), normally comes from the ABS module to the speedo and then on to the fuel ECU on 34. I've had the rear axle speed sensor in the diff stop working due to chaffing and shorting on the body making the speedo fail, but speedo working and no signal to ECU is quite rare.

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