126 idle control unit operation
 

my car is a 86 560SEL with 190K miles on the clicker.


I am still trying to get to the bottom of a hard start/rough idle condition that has persisted thru all of my other troubleshooting efforts. I have checked all of the basic stuff per the archives and good maintenance -- new injectors and seals (tested with carb cleaner for leaks and found none) , ignition wires, plugs, rotor ,cap, OVP relay, fuel pump relay , bench tested and cleaned the idle air valve .

I am starting to conclude that the defect can only now be in the idle control unit. However, i dont have a spare and dont really want to buy one unless I am more certain the fault is there.

I have tested the wiring to the icu and it appears to be good. However, I can remove the ICU from the car and the engine still starts and idles as normal (rough) indicating to me that the icu isnt doing anything. Also, when i unplug the connector to the idle air valve there is no change to the idle. The voltage at the idle air valve when connected to the icu is only 1.25 volts. The engine is under lambda control based on the duty cycle at the diag connector.

Can someone give me an informed opionion if my thinking is rational here and some ideas on how to test the icu???

Thanks.

I would first test the idle valve control current. It should be above 600ma (don't remember actual value).

If you have no current be sure of power from the OVP. If you have poor current like below 500ma but above 100ma I would look for poor solder connections internal to the idle controller.

idle control unit
 

i just tested the control current to the valve and it is running about 180ma or substantially below spec of 600-1000 ma. just for kix i unplugged the ovp while the engine was running and the current as suspected went to zero. This indicated to me that power is getting to the ICU but the ICU is not putting out substantial enough current to drive the valve. I have checked the solder conns on the ICU and they appear to be fine.

Defective ICU???

Thanks.

My test was to hook up a model train transformer to the valve and see if I could vary the idle speed by varying the voltage in the 3 - 6 volt range. I found that it took almost 8 volts to get the idle down to 600 RPM or so, and the spec is 4 - 5 volts.

So I replaced the valve, which improved matters somewhat but I think the control unit is fried as well. I am looking for a used one to come up.

The actual control of the idle valve is a pulsed battery voltage. When you read a 50% duty cycle pulse with a volt meter on a 12v circuit you get 6v. It is my recollection that these devices will only go to 70% duty cycle in attempts to control. That would be 8.4v if you measure it that way.

Obviously the best control would have the nominal condition somewhere in an easy control band. Thus I would have to guess you are right about the required voltages as I would guess 50% duty cycle is the middle.

idle control
 

thanks for the inputs gents--interesting reading. My reading at the connector when attached and engine running is 1.25 volts or 2.5volts at 50% duty cycle as steve points out.

I hve placed full battery voltage accross the terminals of the idle valve and the idle REDUCED and eventually the engine STALLED.This sounds like the opposite of what Chuck was observing.

Chuck - did you introduce higher DC voltage with the ICU connected into the valve or just remove the connector and attached your vaiable dc there.

What input to the ICU determines the control current to be output to the valve ???I also note that when the AC in switched on there is no resulting increase in idle speed.

Thanks.

I think it's consistent with what I got - more voltage reduces the idles speed. I would expect that battery voltage would kill it.

I disconnected the ICU before connecting the transformer.

As I recall, the inputs (at least on the 107) are AC compressor, coolant temp >50, and the tach input from the ignition module.

idle control
 

Not sure i am understanding what is happening still. My symptoms are a slow and rough idle with a lower than spec control voltage and current. If raising the voltage and current slow the idle then I would be going in the wrong direction.

Back in my home office.

I took a look on the CD, and your 560 has a more complex setup than the 380. Send me an email (ctaylor738@aol.com) and I'll send you back the troubleshooting sections.

You want a current of 700 - 1000 mA to the valve, and the valve should have 3.5 - 5.5 ohms resistance.

BTW - your email rejected the email I sent via mercedesshop.

idle control
 

Yes I agree this is starting to make sense now. However, I hasten to add that in my previous troubleshooting I removed the valve, ran carb cleaner thru it, bench tested with 12 volts (understand that is testing the closure of the valve).

Oh yes, and while the valve was off the car I decided to start the engine. It barely started and the idle was even slower than with the valve in place. I am completely ignorant of how the valve works and where the air flow is coming from but it does confuse me that removing the valve didnt increase the air flow.

I am obviously missing something. I will check again and again check all of the hoses etc for restrictions.

Thanks for the help.

You can't imagine how many of these I have fixed and yet without them in my hands I don't remember the exact caricature of each.

If the v8 KE is like the 6cyl KE then you are getting the wrong idea here. Early v8s had two wire idle controls that would race at 1800+ rpm when disconnected. This was the first idle control system on MBs. In the interest of a better "limp home" situation ( hard to drive that 1800rpm plus engine around when the system goes dead) they built valves that go to an opening just greater than that which occurs on a warm idling motor in park with no assessories turned on.

I think this is the style kE idle control the v8 has as that is how the 6cyl works.

Now for how you get low idle. This is the tricky part and it confused me for a long time. I watched the idle valve current on a 300e that was periodically dieing at intersections. I had a long wire and multimeter in the car as I drove around. For most stops the idle current dropped to 6-700ma and the car did fine. But ocassionally the current would drop to 2-300ma and the engine would idle really slow and die. The problem was the OVP and what was happening was this: The system will default to a airvalve opening that sustains say 650rpm with no current. When there is current the valve moves to this same setting if there is say 650ma. If there is 850ma the idle runs unloaded to 850rpm and at 300ma it runs 300rpms. You might have to think about that a while I sure did. The hard part to grasp is that the zero current position is faster than the 300ma current and slower than the 750ma current setting.

This is how poor solder contacts cause the slow running condition on KE cars where the same problem will cause too high idle on the early K-jet w/lambda cars. The controller wants to give 650ma and the poor connection only allows 180 through. Could be other electronic ways for this to happen but 200ma will drive the valve closed on the KE cars (6's for sure, v8s I think)

idle control
 

Steve-
Thanks your experience . You suggest that the performance of the valve is not linear wrt ICU current - especially at lower current levels (or out of spec limits). Are u suggesting then that the ICU is still the culprit???

Keep in mind that I have taken the valve off the car and blown thru if and it is completely unrestricted.



Thanks.

It isn't so much that it isn't linear. It probably is linear. Its just that with no current it was made to go basically to the middle openning. I'm not sure how it works, but remember I am describing the KE system in the 6cyl. I'm not sure the v8 works exactly the same way.

Check the idle speed with the electrical disconnected. If it is just a little high in park with the accessories off then it probably is as I describe.

algae1


The source of the idle air comes from inside the air mixture box. If idle air is not restricted the increased air flow should pull down the air meter plate increasing fuel causing increased idle. Restricting air reduces meter plate deflection thus reduce idle.

With the idle valve removed air bypasses meter plate resulting in no fuel!

It is important to eliminate bypass air.

Why not try replacing idle valve with a tube making sure all air goes through the air flow meter. If that results in low idle I would suspect other causes of low idle. If the result is high idle the problem lies with the parts you are investigating.

The idle valve bypasses the throttle plate NOT the airflow plate. All air entering the engine must go through the airflow plate or it will be unmetered air.