Hello Volks! After a long while of idle and stalling problems after warmup in my 2.1L European Digijet, with no solution from the dealer garage, I decided to take it head on! I thoroughly learned the system, and quickly found that both my throttle valve switch as well as Idle Stabilization weren't working properly. Resetting the switch seemed to improve matters, but still my Idle control valve (ICV) wasn't humming as mentioned in the Bentley manual. BTW, setting the switch as per bentley (0.05-0.1mm clearance) didn't work because the switch wouldn't close reliably at idle, so I had to back it off a little more. And since the ICV was within spec (4.5 ohms) I dug in deep and found the Idle Stabilizer Control unit behind the rear passenger signaling lights, and quickly opened it up for inspection, daring to do so thanks to all the informative posts on the list. That black box (and ICV) were always like black magic to me, so I decided to end the magic and solve those problems...I hereby declare war on the mystery of the ISCU and ISV! First off I fixed two clearly visible problems - Corrosion on the external connector pins and the disintegrating internal flat cable that connected the two boards (one track was broken and also shorting to the adjacent track). Then I also took many close-up pictures so I could later continue to analyze the circuit and even decode the circuit digram... Inside it I found a double-board with 3 quad opamps (LM2902), a dual comparator (LM2903), the power transistor everybody'd talking about (that goes bad) and another power device (probably a power regulator), with markings not visible because they're installed face down, as well as many descrete parts like resistors, caps, diodes, zeners and small transistors. BTW, my ISCU part number is 251 907 393C, made by VDO. Thanks to the info I found on the list regarding the St1 and St2 outputs that drive the ICV, I was able to continue the analysis and can provide the following info regarding the connection pins (useful for testing it standalone as well as reverse engineering it for the benefit of the list memebers who want to save those 350$ (ouch!) and repair their defective units: Pin Pin name in STAMPED wiring on ISCU diagram Pin function ----- -------- ------------- 15 14 Switched +12V from ECU power relay LH 15 Power steering pressure switch input 31 5 Ground ST1 11 Output1 to ISV (Transistor Collector?) ST2 4 Output2 to ISV 1 17 Ignition pulses from Ignition coil driver LS 8 Throttle valve switch input T 13 Coolant temperature sensor (TEMP2) input K 2 unused? 50 6 +12V while starter is turning This is only the beginning, because I can provide my photos to the list for a joint effort of reverse engineering project to get the schematic so we can easily repair these expensive units (rememeber, 350$...ouch!). Also, for debugging purposes, I need to know if the ISV indeed buzzes with ignition on with motor NOT RUNNING as mentioned by members on the list, because it has an input for ignition pulses (pin 1/17) and I'd expect it wouldn't "buzz" without those pulses. Can anyone shed light on that? Thanks and it's a pleasure being a member of such a nice community. Cheers! |
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