landwerlen,
is the ICV "centered" by having balanced PWM signals on each side or is there some sort of spring? Also, just curious...does the ICV go to the center or all to one side when the idle switch goes open? I'm going over to my mechanic this morning and see if he has an old ICV I can horse with. I have a PWM generator at work and may see what I can make happen.

I know that there isn't any sort of spring. If you unplug it, the valve just flops around.
-Chris

that's what I remember also... just double checking... thanks

+1 on what the dentist said...:D
I do however, think its important to realize that this one fix is not a cure all for surging idle problems. As initially posted, several possible culprits were checked, and logically dismissed. Case in point; my surging idle issues were rectified with an O2 sensor.

A great post. I'm a degreed electronics engineer with 25+ years experience and I wondered the same thing - why such a large transistor but no heat sink? Not only for heat dissipation but for mechanical stability. It is no big deal to wonder about things like that - free thinking is a good thing.

What should the three pins read if I ohm them out. Right now when I ohm out the two ON588 they do not read the same.

center pin to "right" side pin should read around 20-23ohms
center pin to "left" side pin should read the same amount.
across the two outside pins should read the sum of the other two readings

right now I don't remember what the "book" says.. what does it say?

try spinning the rotating part from one side to the other and make readings.. they should be the same regardless of the position.

when I had the car apart and the throttle body off... I think I got an answer to a question.
How does the ICV get gunked up?
When oil gets spit out the oil tank vent tube or maybe just hot oil fumes... some may be going through the ICV... when I looked in my manifold the ICV "outlet" had some black gunk that had come out.
You may think the intake manifold is a super clean place.. it's not.

ON transistors
 

Can anyone shed some light on how to crossreference ON transistors like T503 its ON946 in the porsche and ON608 in the BMW. Finding info on these parts are very difficult.

Here's a site
https://www.radiomuseum.org/tubes/tube_on946.html

Says you can replace it with a BD202

I've never seen that transistor go bad in a DME, I've fixed at least 50 or more DMEs over the years.

Also, the ON946 looks like a general purpose PNP switching transistor. You could try a 2N6125, it may work just fine. You can easily source a 2N6125 cheap.

I have the same probleam as Landwerlen,Bad transistor
 

Hi Guys,I have been having the same issue as described as above.I had done all the other things as the process of elimination to diagnose the problem,I came to the conclusion that it is probably the bad transistor.I had found on another post many moons ago a link to Mouser electronics and a Darlington transistor TIP 120,I can no longer find the post.Is it possible to replace the ON588 with the TIP120 ?,I have the motronic apart anyway so replacing 20 cent part seems like a no brainer.
Thanks. Andrew

Before you mess with transistors do this simple test:
- Turn ignition key to 'RUN' do not start engine
- In engine bay is the IdleControlValve ICV humming and vibrating?

If it hums and vibrates the transistors are most likely fine.

What may be wrong is base idle has not been properly set.

You can use a TIP120 transistor but you must replace them both with same TIP120

Try these guys, they likely have the TIP120:
https://www.futureelectronics.com

That's not true. The idle valve drivers are just NPN Darlingtons. The only key parameters are Vce(max) voltage & current Beta.
As long as the non-damaged driver tests OK, it doesn't require replacing.