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1982 380 SEL
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235 Posts
Discussion Starter · #1 ·
Ok so I've tried fixing this problem once before by resoldering part of the ICU circuit board. Didn't do much. I finally got around to trying it again tonight and I actually raised my rpm by about 500 after touching metal to metal on the back of the circuit board [|)]. Blew a resistor?!?

Anyways, is my high idle problem unique in that it only seems to be a problem in neutral or park and mostly after the engine has been running a bit?

I can start the car and it will idle around 900-975rpm. Then when I go to put it into gear it jumps up to 1500rpm (now 2000rpm! since my accidental increase) just for a few seconds while it's in neutral and then CLUNK!! into gear and back down to around 900rpm. I switch to reverse and there is the same pause during which it jumps to 2000rpm ....CLUNK back down to 900rpm. Put it into park and it soars to 2000rpm and settles down to around 1500rpm after a few seconds.

The weirdest part though, is that the rpm also goes up about 500rpm when I push on the brake when it's in park. I can literally rev the engine with my brake pedal. Release the brake and back down it goes. odd?!?

On top of that, I have recently noticed rpm flares when my car is shifting gears as I drive. I've only noticed this a few times in the past two weeks. I remember somebody else having this "flare" problem not too long ago but I don't remember the fix.

I plan to go by my shop tomorrow and finally have the brakes checked out. They have been rock hard since I had my rear control arm replaced. Hopefully the gentlemen will have time to check out the this idle mess or it will be another week of high rpm tranny smashing before I'll have time to drive to his shop again. I welcome any help or ideas that you MB veterans have to offer.
 

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'87 420 SEL & 300SDL
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18 Posts
Did you check the idle speed control valve? (the round thing next to the distributor with two wires running onto it). Mine went bad about the same time the black box started acting up. Disconnect the wires from the idle control valve and use a set of jumpers from the battery. With the engine off, make a brief (and I mean BRIEF contact) on the terminals with your jumper wires. You should hear a very clear "clack" sound as you BRIEFLY touch the jumper to the terminals. DO NOT hold the jumper on the terminal. Just brush the terminal with your jumper wire. If it doesn't go "clack" then it may be bad too. It's an easy fix, purchase a new valve. If you can't remedy the problem with a new valve and idle controller box (the two seem to compound the problem), really quickly, take your car to a dealer before you ruin your tranny. You can save a little wear by immediately shifting the car into gear as you start it up, but I'd get some professional help. If it were a disposable GM or Ford I say what the hell, but a Benz is a different story. I'm surprised MB hasn't issued a recall on this, because almost everyone has this problem at some time or another and it seems to have morphed into an even more serious problem with the newer models.

Good luck
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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5,461 Posts
What a pain!

Sounds like you now need anther Idle Control Unit to replace the one shorted, just for starters.

Having the transmission in gear will put a certain drag on the engine which will reduce RPMs, if that is what you are getting at.

**With the banging into gear from Park or Neutral, it is only a matter of time before damage is done to the transmission, mounts, flex discs in D/shaft etc. ** just a heads up........ in case needed.

The bake booster uses engine vacuum to reduce driver braking effort... Leak?

I think the brakes & high idle (at least part of it) are related.

If the brake booster has insufficient vacuum supply, there will be a rock hard pedal. If there is a leak in the vacuum supply line, air will be drawn into the intake manifold (bypassing the throttle plate).... Still with me, right? You can easily connect the dots here.

A common failure for this vacuum supply line is the plastic check valve. You should be able to see something there..

Less common is a failure of the brake booster itself and if you hold the brake pedal down and the idle goes up.. stays up till you release the pedal - the problem is likely there. The engine is getting more air (idle increase) with the pedal depressed, drops back when pedal released.

Good luck, MBL
 

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Tribute 500SEC AMG Widebody
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Here's are some tests to help narrow your search.

quote:
48hp - 3/4/2006 1:33 PM

Per Factory Service manual V-8 Engine 116..96 & 117.96:

Checking electric activation of idle spead adjuster (ICV)

Engine - operating temperature.
Ignition switched on.
Pull coupling from idle speed adjuster and measure voltage.

Readout approx. 12v then

Checking regulation of control unit

Engine at idle at operating temp. Plug on couple at idle speed adjuster in a manner that voltage can be measured on plug.

Readout 5v +-1 then

Check idle speed adjuster

Engine at idle.
Simulate coolant temp <42 degree C, for this purpose pull coupling from temp switch and bridge. (located at rear passenger side on top of manifold)

Idle speed increases to approx. 750/min.

End of test

Any other values then those described above suggest a faulty control unit (located at firewall)

If a faulty control unit is found, nowhere within the Mercedes Manual does it suggest this method but others have suggested this and it worked for me. Remove the control unit cover and lightly twisting from side to side while engine is running. Idle should drop to normal speed once twisted in a manner that effects the potential poor solder union. Heat each and every connection, adding a bit of solder to each point. My idle is now on the money.

Hope that helps.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Hey there 48,

Good synopsis of the test procedures there. Must have taken a bit it typing on your part. Don't you wish you could just drag n drop the text... well maybe you can from what you use, but i cannot from the PDFs on the CDs.

I just jumped right into replacing the control unit as he said he'd shorted something on the board & blew something.

Cheers, MBL
 

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Tribute 500SEC AMG Widebody
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Hi MBL,

The intitial typing took a bit of time, yes I have the soft cover version of repair manuals. But since originally posting this test a couple of months ago I've dragged and dropped several times as it is a common fault. I just added it to the DYI section as well.

Regarding the ICU, it does require a steady hand and a LOT of patience. I acutally went thru the entire board twice! It's definitely worth trying as the worse case you'd by another ICU. But it did work for me. I also purchased a remanufactured ICU just to have a backup. Out of curiousity, I plugged it in and it actually did not perform as well as my original self remanufactured unit.[:)]

Cheers!
 

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1982 380 SEL
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235 Posts
Discussion Starter · #7 ·
Ok I sorta follow some of that. Unfortunetly I did not get to my shop yesterday, so now I'm hoping to go by there on Saturday--the only day that I don't have school or work this week. From MBL's post I think I can gather that on top of needing a new ICU and likely a new idle valve, I could have a leak somewhere with my brakes that is feeding my engine extra air. You said this is likely due to the brake booster? Does it matter that the car only revs from the brake pedal when it is in park? The brakes are always hard, but they don't cause the rpm to rise when I'm gear--only in park.

Next, how would I check the plastic vacuum valve and where is that located?

et 48hp,
Thanks for taking the time to post all of that info, but I'm not sure how I could go about doing all of that. I don't have a voltmeter. Hopefully my mechanic can tell me for sure whether or not the idle control and the idle valve are shot.
 

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Tribute 500SEC AMG Widebody
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Bfam4t6 - 4/11/2006 1:08 PM

Ok I sorta follow some of that. Unfortunetly I did not get to my shop yesterday, so now I'm hoping to go by there on Saturday--the only day that I don't have school or work this week. From MBL's post I think I can gather that on top of needing a new ICU and likely a new idle valve, I could have a leak somewhere with my brakes that is feeding my engine extra air. You said this is likely due to the brake booster? Does it matter that the car only revs from the brake pedal when it is in park? The brakes are always hard, but they don't cause the rpm to rise when I'm gear--only in park.

Next, how would I check the plastic vacuum valve and where is that located?

et 48hp,
Thanks for taking the time to post all of that info, but I'm not sure how I could go about doing all of that. I don't have a voltmeter. Hopefully my mechanic can tell me for sure whether or not the idle control and the idle valve are shot.
You are welcome. But do pick up volt/ohm/cont meter it's a wise investment and very helpful!

The vaccum line leading to the brake booster (black approx 15mm thick) is very prone to cracking. Check that line and the grommet on the booster itself.

Good luck!
 

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1982 380 SEL
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235 Posts
Discussion Starter · #9 ·
Sorry, but where is the brake booster? Describing it should suffice, but any diagrams or pictures would help too. Thanks.
 

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Tribute 500SEC AMG Widebody
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The brake booster would be the large black, round (approx 10" dia), thick (approx 4"), disk located on the driver's side against the firewall. When you check your brake fluid, it's that thing it's mounted to. [:)]
 
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