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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #1
I recently experienced the high idle problem, and sure enough tapping on the idle control unit ('computer'/black box) caused it to settle down momentarily. Opened up the box and what I found was a broken soldered joint with a black circular mark around it almost like a scorch mark, i.e. not like a typical dry soldered joint. I subsequently re-soldered this and reinstalled and the idle was back down at normal revs, however it was still surging erratically (a problem I'd had before the high idle). I tried playing around with the connector to the idle control valve and found that the revs were highly sensitive to its movement. I pried open the plug connector and to my amazement found this:



It's hard to see in the photo however both of the wires were badly frayed where they connect to the brass (?) plug connectors, and were short-circuiting badly as they wires moved. I'm amazed the valve was still working at all.

it seems extremely bad luck to get both a faulty ICU and faulty connector at the same time, and it made me wonder whether this short circuit had contributed to blowing a joint on the PCB in the control unit? Given that it is an AC (inductive/capacitative) device I would have thought that an intermittent short circuit could cause this (and funniily enough the blown joint was on an inductor).

Fixing it was straightforward. I pried open the connector with a flat head screwdriver then used a fine-tip soldering iron to clear out the brass plugs (the base was full of solder along with the remnants of the old wires),and then soldered them back to freshly stripped wires. Plugged in and all is now well.

Another thing I found was that the old 'listening to your fuel injectors through a screwdriver' trick also works with the idle control valve. It should hum quietly when the engine is cold, and hum quite loudly when the engine is warm. You can also feel it with your fingers. If the valve isn't humming then you have a problem (and your revs are probably at 1200 in park). Remember that is is a closed when energised valve (try putting 12V DC across it and the engine should die in about a second).

If you're having high/surging idle problems I would suggest:

Clean and lubricate idle control valve and test with 12V dc (takes minutes. I used a battery charger). It's pretty obvious when it's working!
Check inside idle control valve plug connector for frayed wires (takes minutes).
Check inside idle control unit for blown/dry soldered joints (takes about 30 minutes).

You can do all of these in about an hour and save yourself 400 USD/GBP in replacement parts.

Hope this helps.

Cheers,

Lube
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
Good Info, Thanks. I too, had a very fast idle, caused by a bad transistor in the idle
control unit. Bought one at an electronic shop for $3.85, and soldered it in.
Works perfect. Don't forget to be careful with the soldering iron, and not short out
anything on the PCB.
 

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450 SLC 190D
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25 Posts
Thanks for the info.. you are more tech than I, But may call upon your knowledge for the following:
Perfect running 80 SLC but of late at 200/210 degrees temp, the idle falls from 700 RPM to 600/ 500/ and 400 then stalls. At any temp below 200 idle is constant.
Replaced cold start valve etc in the past, no over rev when cold.
The only prospect is a Temp Sensor... hmmm... which one ????
Thanks to all for the look.
Henderson
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
I am not a Tech Expert, but several things could cause this problem. Like vacuum hoses,
EGR valve, Torque Converter, Fuel System Pressure, and a plugged Catalytic Converter.
 

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R/C107 Moderator
1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
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32,169 Posts
Your location in your profile would help. The temp. here has been 113 F so I can expect to run a little warm. Your fan clutch may need replacing. You are running a little hot.
 

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82 380SL 96 SL500 03 SL500
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6,158 Posts
Your location in your profile would help. The temp. here has been 113 F so I can expect to run a little warm. Your fan clutch may need replacing. You are running a little hot.
He's still at less than 100C though barely. Does the '80 still have the mechanical idle air valve?
 

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450 SLC 190D
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25 Posts
To Jack, Rowdie and Abiby: Thanks for your imput.. No EGR, Replaced muffler recently, seems to exhaust as usual, Vacuum leak... They all seem to have a minor leak( door locks), converter???? Do not know.
Fan clutch seems OK.. Car cools down rapidly but will re confirm clutch. Has mechanical idle adjustment.
This SLC has always had a an occational lean miss when warm.....
Texas.. we are WARM
Fuel pressure great when COLD.
I have the book.......... but some issues on idle require testing to a greater degree.
Thanks again,
 

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82 380SL 96 SL500 03 SL500
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6,158 Posts
Then IIRC the only temp sensor controlling idle speed is in the idle air valve. When it reaches a high temp, it shuts off too much idle air for the engine to run. IMO you'll just need to open up the idle air adjuster to get a viable idle speed on a hot engine.

Then again, there's that pesky WUR which could be doing it as well...

Of course I'd argue harder if it were an 81 or newer that I was more familiar with.:D
 

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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #10
AAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHH thought I had solved this but it's back, and now even more intermittent (mostly stuck at 1000 RPM, occaionally settles down for a few seconds). On the bright side I did start it for the first time in about 6 months today and man did it sound great to hear that old V8 again. Can't wait to start driving it again.

I'm going to work my way through MB Job 07.3-112 "Testing electronic idle speed control" until I solve this damn problem once and for all. My money is on the bloody dry soldered joints (AGAIN) or one of the sensors.

Will keep you posted.

Also apologies for my erratic appearances on this forum. I generally don't touch the car in winter as I'm too bloody busy to even think about it, but spring to autumn I use it a fair bit, and try to fix it! I'll try to check back here more often.

Cheers.
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
I think your guess about the solder joints in the idle speed control unit
might be correct. They could be partially shorting out. I'm assuming you
replaced the three-legged transistor. I would try redoing it. use a pen size
soldering iron this time, if you didn't before.
 

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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #12
Thanks. I've actually resoldered it twice now, but it could be that again. I didn't replace the transistor so I'll look into that.
Cheers

Sent from my MB860 using AutoGuide.com App
 

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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #13 (Edited)
OK did most of the tests as per that document, and the idle is still all over the place. Given how erratic it is I'm thinking it can't be a poor sensor since those generally either work or they don't. So removed the damn black box idle control unit again.

The transistor you refer to, is there any reason why it should be this and not the 100 or so other components/capacitors/resisitors etc?
Ans I assume you mean the BIG BD710 transistor right next to the two large green rectangular capapcitors, as opposed to one of the three smaller transitors.

What I'd love to do is hook this box up to a wave generator and an oscilloscope and test it PROPERLY!. I remember reading somewhere a guy managed to determine a test setup for how to do this - if anyone can find this please let me know. Or maybe I can figure it out.
Cheers.

EDIT:
First things first: Anyone know where I can find the pinouts / wiring diagram for my Idle Speed Control Box 002 545 40 32 (VDO 412.202/012/001) with 10 pins (pins 10 and 12 blanked off). This unit does not seem to be decribed in the 07.3-112 document as they all have 9 pins.

This looks like my unit: http://www.europartsshop.com/mercedes-benz-idle-control-unit-0025454032.html#

I have searched absolutely everywhere on the internet and cannot find anything that gives the pinouts for the 10-pin module.

Cheers.
Cheers.
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
The reason that it could be this one, is because it is the most likely one to
fail, not only in this ICU, but other electronic units a well. It is easy to identify.
It is the one with the heat shield. Why the heat shield? because it gets the
hottest. Why the hottest? because it does the most, or heaviest work.
But remember it has lasted over twenty years, which is pretty good.
The 002 545 40 32 unit is the only one that I have found that will work in a
Euro 1985 500SL. I have tried ICUs from a 380SL, a 450SL, and several SLCs,
and none of them worked. The transistor cost only $3.87, and is relatively
easy to put in. I would sure try a replacement FIRST, before buying an expensive
new ICU, or spending a lot of time searching for some other cause. If you are
leery of doing the job yourself, take it to an electronic repair shop. shouldn't
cost very much. Good Luck, let us know how you come out on this problem.
Cheers.
 

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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #15
Ok here's what I learned today:
Although there is a 10th pin on the box there are only 9 sockets on the connector so pin 10 does nothing, pins 1 to 9 are as per the manual I believe.
Hooked the thing up to a square wave generator and a scope, fed in a 12V square wave to simulate the distributor signal, what came out was a much higher frequency square wave. I hooked up two variable resistors in place of the coolant and oil sensors and twiddling them did absolutely nothing. The only thing that had any effect on the output signal was increasing the input frequency to about 35 Hz (1800RPM) at which point the output went to zero.
I guess that the coolant and oil sensor resistances should be doing something to change the frequency of the output wave and therefore regulate the engine rpm. So I think this is my problem.
Have ordered a new power transistor (BD910 which supersedes the BD710 original), will fit that and test again.
Cheers.

Sent from my MB860 using AutoGuide.com App
 

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1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #16
Also if anyone knows the exact cold and hot resistances of the oil and temperature switches that would be useful. I read that the coolant sensor goes from 4kohm when cold down to 200 ohms when hot, and the oil sensor is 450 ohms cold and 750 ohms hot. Can anyone confirm these figures?
As for the throttle switch and the ignition switch, I guess those two need to be grounded?
Cheers.

Sent from my MB860 using AutoGuide.com App
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
I hope the new transistor you have on order is compatible with the old one.
You will find out shortly after you put it in. After you have everything ready,
and fire up, the cold engine should rev between 1000 to 1200 rpms.
As it warms up it will start to slow the rpms down, until it reaches operating
temperature, at which the idle should be between 600 and 700 rpms with the
shift lever in DRIVE. These are the figures on my car, after I installed the new
transistor in the ICU.
The tests that I did to find out if the trouble was with the Idle Control Speed
Valve, or the Idle Control Unit, was to apply voltage to the disconnected wire
at the Idle Control valve. By varying the voltage, the engine changed rpms,
with at about 6 or seven volts being right, going to the valve. It is a butterfly
valve controlled by the voltage to the solenoid, which normally gets the correct
voltage from the ICU. With the ICV working O.K., that meant the trouble was
most likely in the ICU. Old age and heat are the biggest enemies of this type
of transistor. when they start to go bad, the car will idle fine one day, then
a couple days later it will start revving high, and then be O.K. a while only
to start again, until it stays high. Since repairing mine about nine months ago, it has been idling fine. I hope yours will be doing the same soon. Cheers.
 

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1981 Mercedes 380SLC, 1977 Mercedes 280E, 1981 Porsche 911SC, 1963 Jaguar MK2, 1981 BMW 320is
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50 Posts
I hope the new transistor you have on order is compatible with the old one.
You will find out shortly after you put it in. After you have everything ready,
and fire up, the cold engine should rev between 1000 to 1200 rpms.
As it warms up it will start to slow the rpms down, until it reaches operating
temperature, at which the idle should be between 600 and 700 rpms with the
shift lever in DRIVE. These are the figures on my car, after I installed the new
transistor in the ICU.
The tests that I did to find out if the trouble was with the Idle Control Speed
Valve, or the Idle Control Unit, was to apply voltage to the disconnected wire
at the Idle Control valve. By varying the voltage, the engine changed rpms,
with at about 6 or seven volts being right, going to the valve. It is a butterfly
valve controlled by the voltage to the solenoid, which normally gets the correct
voltage from the ICU. With the ICV working O.K., that meant the trouble was
most likely in the ICU. Old age and heat are the biggest enemies of this type
of transistor. when they start to go bad, the car will idle fine one day, then
a couple days later it will start revving high, and then be O.K. a while only
to start again, until it stays high. Since repairing mine about nine months ago, it has been idling fine. I hope yours will be doing the same soon. Cheers.
Hi Jack,

This is exactly the same sequence and symptoms of my high idle issue. Did you repair the ICU to fix it and how was that done?
 

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1985 500SL. Previous cars-190SL, 230S, 250SE, (2) 300SDs
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291 Posts
I repaired it by replacing a transistor in the ICU.
Reading ALL the posts in this thread will give you
the information.
 

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Registered
1984 500SL (RHD) 160k miles
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186 Posts
Discussion Starter #20
OK I think I sorted it.
Late last night I decided to resolder EVERY SINGLE JOINT on the circuit board. Tested this morning, warmed up at 900 RPM, then nicely dropped down to circa 700 and sat there.
Even though I had closely inspected every joint on the board it seems it was still a bad solder joint that was causing the issue. Even though my unit was made in 1998!
So the moral of this story is, if in doubt resolder the board!
In the process of searching to find a solution I also found this very useful info (that applies to BMW's but by looking at the pinouts it seems their units are almost identical):
Mesa Performance Parts...Idle FAQ

I also found this thread which describes a simple test for the oil temperature switch:
1985 380SL idle high when hot | Mercedes-Benz Club of America
I still plan to get hold of a spare box and recondition it to have as a spare. When I get around to doing that I'll post anything useful that I find.
Cheers.
 
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