89 420 SEL...

I recently gave my car a full tune-up and checked the specs on everything...

The idle was fine at starting 650/700 rpm when cold, but as the engine warmed up, the idle would slowly creep up to 1000/1100/1200 rpm.

[Note I've only had the car for a year or so and think it has had this problem since I bought it - this is my first time working on the engine as it was in fairly good shape when I bought it.]

After a LOT of electrical troubleshooting to check all the inputs to the Idle Control Unit located under the passenger floor (all working and to spec), and checking for engine air leaks - none... I narrowed the high idle problem down to the ICV as being the ONLY possible cause of the problem.

Although the old valve seemed to be functioning properly??? I also tried cleaning the old valve, but it still had high idle on warm-up.

So I paid the 2 cents and bought a new Idle Control Valve.

Note the old part number (car came with) was...
VDO 408.202/010/002

And the new part number I purchased is...
VDO 408.202.010/001

They both look identical to me. But the new part stays at 650/700 rpm as it should.

So do these valves go bad with time/use?

Or perhaps a previous owner installed the wrong part number valve?

I've found these various ICV numbers on the internet...

ISC 000 141 12 25 MERCEDES-BENZ 408-202-001-001
ISC 000 141 12 25 MERCEDES-BENZ 408-202-001-003Z
ISC 000 141 14 25 MERCEDES-BENZ 408-202-004-002
ISC 000 141 14 25 MERCEDES-BENZ 408-202-004-003Z
ISC 000 141 16 25 MERCEDES-BENZ 408-202-010-001
ISC 000 141 16 25 MERCEDES-BENZ 408-202-010-002Z

And this is the only technical data I can find and only for two different part numbers (not my numbers)...

408-202-010-002C
working point at 1000 mA: 10 ± 1 kg/h
at 550 hPa ± 2 hPa difference pressure

408-202-001-003C
working point at 500 mA: 11.6 ± 0.5 kg/h
at 0.55 bar ± 0.01 bar difference pressure

I have had luck searching google.com using the word...

Leerlaufregler

Like...
Leerlaufregler 408-202-009-001Z

FYI - How I checked ALL the inputs to the Idle Control Unit located under the passenger foot well...

The various inputs to this little black box run ALL over the place! In one case there is one signal running from the CIS-E directly to the ICU and nowhere else, so you need to monitor that input at the foot well area on my car as the CIS-E is also located in that area (to the right).

But there is NOWHERE to clamp on test wires! So I had an extra ICU I found at a junk yard, removed the cover, then soldered on wires to every single connection from the inside of the ICU. Then taped each wire to a wooden ruler so they would not short. And taped a wire number on each one - backside of tape said what the signal was and what value it should be.

Then unplugged my ICU and plugged in the test ICU - Read the various inputs with a multi-meter and watched the values as the car warmed up - all to spec. (Measured ICV with amp meter and multimeter @ ICV.)

Here are the various ICU inputs/outputs for my car...

Pin#, AKA, Signal, Range
1. [V] Speedometer signal - Not moving 0 V. > 1 volt AC @ 40 km/h
2. [TF] TF - Temperature Signal from CIS-E: 3 - 5 volts DC with engine idling. Cold Start. **See below
3. [KL] A/C Compressor: 11 - 14 volts DC with compressor on.
4. [LL] Idle Switch Throttle: < 1 ohm foot off accelerator / Open when accelerator depressed.
5. ------------------- Not Used.
6. [87A] OUTPUT (and 7) Idle Speed Air Valve: 3.5 - 5.5 ohms disconnected and ignition off...
7. [87B] OUTPUT (and 6) ...700 to 1000 mA connected at idle.
8. ------------------- Not Used.
9. [15] Positive Voltage from OVP: 11 - 14 volts DC with ignition on.
10. [TD] EZL TN (RPM) signal: Should be feedback as to how fast engine going. Same as Tachometer.
11. [31] Ground.
12. [50] Starter Lockout transmission: 11 - 14 volts DC in park.

Note: I am not clear as to WHAT each input should do to the idle? Maintain the idle, reduce the idle, or increase the idle? For example if there is a large electrical load on the electrical system (every accessory electrical gizmo on - lights on - sitting at traffic light, etc.), and the voltage drops down, will the ICU increase idle to generate more electricity or not? Maybe not that smart?

Note as for checking for an air leak...

1st was to look at the dashboard Economy Gauge - this is basically a vacuum gauge and should be all the way to the left if there are no vacuum leaks and the car is at idle.

2nd - Vacuum gauge [tool] check connected to vacuum line of engine - to spec for my specific vehicle.

3rd, I removed the Idle Control Valve and installed a long rubber heater hose to its two connections, then squeezed the middle of the hose with a vice grips to close off about 4/5th of the opening (simulate the amount of air being allowed to pass as the ICV should do).

Then started the car and adjusted the vice grips to get 650/700 rpm. And readjusted after car warmed up.

I was able to maintain 650/700 rpm with this contraption - thus it was POSSIBLE to regulate the idle via the ICV - No other air leaks or problems causing a high idle. So this verified the problem was with the ICV (along with the above tests).

The ICV mechanism can get gummed up over the years. Even cleaning the thing did not help. I hooked up the ICV to my bench power supply and could observe the shutter stuttering as I varied the voltage. A replacement unit was nice and smooth.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Thanks!

If I have nothing better to do, maybe I will give my old ICV valve an "oven" bench test. That is warm it up in an oven a bit and then see if it starts to have "medical problems" with breathing and so forth (old age!)

That is... The problem could be due to the part itself warming up and not from the engine or an engine temperature sensor heating up - something I did not check.

Also now that I think of it... Perhaps at first cold air is entering the ICV... Then as the engine warms up, the air coming from the Air Meter could be warmed up and then you would get warmer and warmer air shooting into the intake of the ICV - Could cause the gradual warming problem with the part.

So an additional test might be to point a hair dryer into the ICV when bench testing. (Don't let the wife catch you borrowing the hair dryer of course .)