560sl / Idle Control valve or amp? - PeachParts Mercedes-Benz Forum

msar · #1 03-03-2004, 06:26 PM

My '86 560sl occasionally idles at 1700rpm. The problem is intermitent & I've found that if I remove the wire from the idle control valve, and then put it back on, the car's idle returns to normal. Today the idle seemed to be OK until I would come to a stop. The idle would at first start out normal (10 seconds or so) & then bump up. I'm not certain if the idle increased when I pushed on the brake since I can't get the problem to repeat itself by design. Does this sound like a problem with the amp? Is the idle system a vacuum system and could a leak be the problem? The reason I don't think it is is that when I remove & replace the valve wire the idle goes back to normal. Thanks for any advice. Mike

RickM · #2 03-05-2004, 11:15 AM

Could be your ICV or the Idle Control Unit. Many here have taken out the valve and cleaned out with Brake Clean and then with WD-40. I have the same idle issue with my 380 and the cleaning didn't solve the problem.

I have troubleshooting info from Chuck Taylor (thanks again!) that I'll be utilizing shortly. My idle jumping after a few minutes of driving is no longer every once in a while, it's everytime.

msar · #3 03-05-2004, 11:23 AM

Thanks Rick, It's interesting that your idle problem started out infrequently and then worsened. I'm expecting mine will follow suit. What I can't figure out is what would change the voltage to the valve only on occassion. From what I'm learning the amp often seems to be the culprit but I'm doing some more tests this weekend & will update this post.

RickM · #4 03-05-2004, 11:40 AM

Is the AMP the Idle Control Unit?

I'm guessing heat is what triggers the failure.

msar · #5 03-07-2004, 06:09 PM

Yes. Not sure if amp is the proper terminology but I am refering to the control module.

txstinger · #6 03-07-2004, 08:15 PM

Chances are it is the control unit. As the unit supplies about 5 volt to the adjuster, the solder joint begins to heat up and expand. This becomes a loose connection that now requires higher amps to adjust the idle speed adjuster. This higher required amps slowly burns out the control unit transistor.

The Controller fails (SAFE). Meaning that it will not kill the motor, instead it puts out max idle speed.

Sometimes the adjuster is the root cause, meaning it just requires higher amps due to age.

Unplug the adjuster plug and ohm across the to pins - should be around 4 ohms. If tested OK, pull the controller unit and gently open box. Inside you should see a burn transistor on the left side of the board. You can try to reheat the joints with solder iron to re-make the joints.

I will take pictures of this and post tomorrow.

msar · #7 03-07-2004, 08:21 PM

Thank you. I'll test the valve & post its ohm reading.

msar · #8 03-07-2004, 08:32 PM

Pulled the valve plug & put my ohmmeter on it. It pegged the meter. I'm not sure I understand, what's in the valve that offers electrical resistance? Thanks.

txstinger · #9 03-07-2004, 08:42 PM

A simple coil. The adjuster should read about 4 ohms - put meter at lowest scale and read. Test the adjuster - male pins pointing at you. Then test the voltage that the controller is sending - to do this put the plug back on half way, so you can get the test leads on the pins while run the engine at idle speed. Should read about 5 volts.

RickM · #10 03-08-2004, 09:06 AM

TXstinger,

Thanks in advance for posting a pic. Is this transistor a replacable item?

Rick

txstinger · #11 03-08-2004, 02:20 PM

Open the Idle Speed Controller being careful not to break the knobies. I do this with toothpicks. Spread and hold with toothpicks until your able to seperate.

Pay attention to how the boards slides out (which side of box is facing up).

Notice the hot spot on board. This is the AMP driving the adjuster. You can try resoldering each joint on the chip and the trace from this chip to pin 1 or 5. If the chip is bad the replacement is TIP42. Many radio shacks sell them.

msar · #12 03-08-2004, 02:52 PM

Thank you very much for the advise & pictures. I'll check the board. I have read conflicting threads regarding the location of the module. If I remove the screws to the under dash cover underneath the glove box, does that enable me to take off the cover & get to the module? Thanks again for your help!

msar · #13 03-10-2004, 07:10 PM

Found the module & pulled it. There weren't any visible signs of loose or burned components on the board. The module is a bit different on the 560. Smaller & has more IC chips.
Anyway, I cleaned the contact pins & replaced it. Just by luck, tonight, while I was inspecting the distributor cap & rotor the car went into it's high idle, 1700rpm. I tried tapping the valve to see if it was stuck - no effect. Then I pulled the electrical plug out slightly & put my volt meter on it. The cars idle actually fluctuated between high & normal. Correspondingly, the volt meter read 5volts(when normal idle - the cars engine was warm by the way) to 0 volts at high idle. I know that tells me that the something is varying the input charge to the valve. Does this sound like the control module? I've read about an overvoltage relay but not sure a) where it is located or b) if it would trigger the symptoms I'm seeing. Thanks in advance for your thoughts & advise. Mike