W124 300CE-24 High Idle Broken TVS/TPS
 

Hi All,


I am the owner of a 89 300CE-24 5-speed with ASR!
Anyway I have the not unfamiliar problem of a high idle, ca. 1000-1200, when in either neutral or park.


This issue is often attributed to a faulty IACV. I have eliminated this as by testing the unit outside the vehicle and when voltage is applied the unit moves to the relevant positions.


Following this I tested whether the IACV was receiving any power which is was/is not.
Next step was to check the throttle position sensor or throttle valve switch. Having followed the KE-Jetronic throttle switch tests, see here https://www.youtube.com/watch?v=s9WjaO61QyI 1, I have found that my TPS sensor is broken, as when moving the throttle the resistance does not change when measuring across pins 1-2 and 2-3 as in the video.


Another thing I have done is check the duty cycle reading which shows 95%. According to this source this is an issue with the micro-switch of throttle linkage.


Now I am stuck what to do. Should I replace the sensor? Is this easy or hard?
Any advice appreciated,
Friedrich

Sounds like there is a problem with your deceleration shut-off microswitch, you should have continuity with the throttle closed (idle) and with the throttle open there should be an open, to eliminate wiring issues you can test it directly a the CFI module (pins 10,6). It's most likely no adjusted correctly.

Hi ytech15,


Are you judging that via the duty cycle reading?


The only issue is that my vehicle *has* ASR which means I *do not have* the deceleration shut-off microswitch.


What is the CFI module, apologies for my ignorance, and what do I do in said test?



Thank you for your reply :)

Although you emphasized it I missed the ASR part in you original post ;)

Since your car is equipped with ASR you would need to check that the ASR module is signaling idle to the CFI (cont. fuel injection module) to do that you will need to measure the voltage between pins 6 and 41 in the CFI module connector with ignition ON you should expect battery voltage with the accel. pedal at rest and <1 V with the accel. depressed.

The CFI module is located behind the battery and is labeled as N3

Ok great I will test this ASAP :)


If it is not signalling, which I suspect is not as the fault is analogous to having a faulty throttle microswitch, what am I to do?

We will check the TPS output to the ASR module. If that will be out of spec we need to make sure the TPS gets input voltage.

So I checked the CFI module pins 6 and 41, though I was somewhat unsure of the numbering, I assumed that they were ordered left to right...

Anyway the result was that with ignition on the voltage read approx. 12V.

However, depressing the accelerator had no effect on the reading.

So I guess I need to check that the TPS is getting input voltage, any other thoughts or guidance?

Continuing from my above post I have since done a number of additional tests since finding this resource...


1. Performed test 37.1 from the diagnostic manual, see link above, measuring voltage across pins 6 and 41 in the CFI module (labelled N3).


Result: with pedal in idle position read 12V, no change with pedal depressed.


The manual recommends testing the acceleration pedal position sensor if the test is failed, described in 3.0 of volume 2 of the manual - see here.



2. Performed test 3.0 from diagnostic manual measuring voltage across pins 5 and 9 of the EACC module (labelled N4/1).



Result: open circuit.


This test then recommends to check perform test 7.0 in volume 2 of the manual, checking the accelerator pedal position sensor R25. Here I had some trouble, was I to check the pins or socket? *If anyone has experience here please let me know!*


The test says:

As the socket is wired to the connector I believe it is the socket as opposed to the plug... but I am unsure.



3. Performed test 7.0, measuring the resistance of wires within the R25 accelerator pedal position sensor.


Note:I am going to repeat the test because I don't trust my first go.


Result: 3 combinations gave readings of approx. 1k Ohm. All other wires showed infinite resistance.



Does this mean I have to replace my entire wiring harness or can I just do this socket back to the EACC control module?


Furthermore does anyone have guidance how to perform this?


Regards,
Max