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Old 10-18-2010, 06:31 AM   #1 (permalink)
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300SEL Idle Control Module Location

I have searched the Forum and came up with four possible locations; none of which reveals the module: (1) Under the passenger's carpet access panel where the anti-theft module is located - not there; (2) Under the driver's side dash on the firewall near the steering column - not there either; (3) Under the driver's side footrest - nothing there; and (4) Between the two firewalls in the engine compartment where the ECU, OVP, etc. are located - not there.
So where is it? Is is possible that there is no ICM like on the W124s? I checked this out by seeing if either of the two sockets on the Idle Control Valve connector are connected to the big connector on the ECU. No evidence of connection, so that would indicate that there is indeed an ICM somewhere!
I have a 1991 300SEL with a rough idle caused by a defective PWM control signal from the Idle Control Module.
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Old 10-18-2010, 09:29 PM   #2 (permalink)
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There isn't one

What makes you think your rough idle is related to the PWM?

Perhaps the ECU's output to the ICV is trying to compensate for an actual problem?
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Old 10-19-2010, 07:10 AM   #3 (permalink)
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Based on your description, I'd check the EZL and the CPS first

EZL = driver's front wheel arch on LHD cars, just ahead of the wheel and behind the headlamp. CPS connects to that unit and wires back to where the block meets the transmission, towards the driver's side near the firewall. I'd start with checking connections to the EZL, vacuum to the EZL, cleaning the CPS head, etc. If that doesn't work out you might need to test your TPS and throttle microswitch - the latter being infinitely easier than the former to replace. The ECU is somewhat failsafe, I'd only check that if literally everything else doesn't check out. There's also the MAS to test, plus the OVP and its' associated fuses (worth putting in the "check this out first" category too). Good luck!
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Old 10-19-2010, 12:15 PM   #4 (permalink)
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It's incorporated in the MAS. That should be next to the OVP module, inboard from the fuse/relay center.

That could cause a fluctuating idle speed but for a rough idle I'd look elsewhere as suggested above.
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Old 10-19-2010, 01:12 PM   #5 (permalink)
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methinks in this order:

Vacuum leaks
Vacuum leaks
Vacuum leaks
Oxygen sensor (if this occurs only while it's warmed up)
Fuel distributor/EHA

But an unstable idle is very often a result of a vacuum issue on these models.

I had a bad idle and wound up having a bad camshaft on my 300SE. It's nothing to pull the cover and inspect it.

The idle would fluctuate because the exhaust gases were erratic, due to the engine not running properly. It's all a chain reaction.

If your Catalyst is clogged up, you'll have similar results.

You can check the EHA duty cycle at the diagnostic connector, also. You can search for it on the forum. This test can help determine the condition of the O2 sensor.

It's all related. The hipbone's connected to the... etc.

The ICV's signal is derived from engine load. It just wants to maintain a preset speed. If it's varying, that's because your engine isn't running properly, and there are many things to check before condemning any of the control circuits.
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Old 10-20-2010, 06:09 AM   #6 (permalink)
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My thanks to all who replied. The reason I think that it is the control signals to the ICV is that the valve does not vibrate when the ignition is ON or when the car is running like it used to. My idle was extremely smooth prior to getting caught in a heavy downpour. Now the next day I changed the header pipes. I had installed home made stainless steel flexible header pipes to replace the corroded pre-cats when I first began work on the car last May. Two weeks ago I obtained a pair of stock header pipes and gutted the insides of the chambers to make them free-flowing. The rough idle began right after I made that change; but I don't think that is the problem and I checked carefully to make sure I did not disturb anything else when the I replaced the header pipes. I am going to remove the MAS and check the circuit board. If it controls the ICV, then there should be some suitably-sized switching transistors on the board that might have gone bad. The heavy rainstorm could have also gotten to that module. I will keep everyone posted. Thanks, Sheldon.

P.S. Yes, I already checked the duty cycle at the diagnostic connector with the oscilloscope.

Last edited by Rubytron; 10-20-2010 at 06:29 AM. Reason: Hit posting key before completed message.
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Old 10-20-2010, 07:16 AM   #7 (permalink)
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There shouldn't be a constant vibration from the ICV, just action when it opens or closes. I'd check the plug on the wiring harness to the ICV with a voltage meter to be sure that the issue isn't simply a cooked ICV. Also, anytime you do exhaust work, O2 sensor gets bumped way up the possibilities list - a harsh vibration or a relatively minor impact can send it out of whack. I assume you would have mentioned if your CEL was on?

Another thing to check out is whether the engine ground strap is damaged. It's right near the headers, so if you had a rot issue in that area it may be worth making sure the engine is getting proper ground. Easiest way to test that is just run a jumper cable from the negative battery terminal to a connection on the block, preferably one of the crane hook loops. Bad ground = very bad idle.
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Old 10-20-2010, 07:25 AM   #8 (permalink)
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I thought that the ICV is opening and closing at a 100 Hz rate, i.e. 100 times a second controlled by a variable duty cycle PWM wave. That should produce a detectable vibration at the valve. I already tried cleaning my ICV and it tested good. I also bought a used one on eBay and tried that one without any different results. No Check Engine Light. The car performs very well off idle. My highway mileage has not been affected and I get 20 mpg. Car has 270K miles and reconditioned cylinder head I installed this summer.
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Old 10-20-2010, 08:54 AM   #9 (permalink)
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Definitely sounds like time to check the TPS and throttle microswitch

If you can fool the car into thinking there is light pressure on the gas, that will help isolate the issue. Try jumpering the microswitch (just two terminals, it's an on/off switch) and see if you get any noticeable result. I'd definitely also clean contacts on the MAS and OVP and check that OVP relay.
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Old 10-21-2010, 07:54 AM   #10 (permalink)
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Idle Control Module not in the MAS Module

Using my VOM, I checked out continuity between the two wires at the ICV connector and the connector that mates with the MAS Module. There is no direct connection which means that the Idle Control is NOT located within the MAS Module.
I had already checked the TPS and the Idle Microswitch as was suggested. As long as the ICV is not vibrating or humming, then the problem is probably in the module that controls the ICV or connection thereto. I scoped the connections to the ICV and found a very low duty cycle waveform. That would seem to indicated that there is a connection from the ICV Control to the ICV, but that the switching transistors, i.e. the drive circuitry, that causes the motor in the ICV to rotate back and forth at the 100 Hz rate are not working properly.
Does anyone know a good source to get an Electrical Diagram for my 1991 300SEL (W126)? Or do I have to buy a WSM?
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