Idle suddenly increases - 380SL

Hello All,

I have had a strange thing happening on my MB (84' 380SL)
There doesn't seem to be any consistant condition that seems to causes it like low gas, temp, accessories on/off...

I can be at a traffic light and the idle will suddenly jump up to about 1700 rpm's from about 500. I am now keeping my foot on the brake regularly to avoid lunging at the car in front of me but obviously, this can't be a good thing not to mention the decreased gas mileage. I had an issue with the fuel relay in the past and (thanks to you guys) figured out it just had to be soldered. That was about a year ago I took the rely out and had it soldered by a MB mechanic for a smile and it resolved the problem. (car wouldn't start)
I am fairly mechanically inclined and would like to try to solve the problem if I can... Any Ideas? Is this condition destructive to my engine?

Thanks in advance for your replies...

Captgail

[ymsin]

My dad's 260e (w124) and my 300se (W126) had a similar problem. Our MB tech. said it was due to the idling valve which has to be replaced. A not-so-costly part of approx. RM150 or USD40.

Perhaps this could be your solution.

[stevebfl]

The intermittantcy of your problem suggests a bad solder contact in the idle speed controller. This is separate from the engine management controller on those V8s. Its somewhere on the passenger side under the dash. Maybe under the floor panel. SLs always give me grief on component locations.

This is a very solderable problem. We get very good results resoldering the pc board for symptoms like yours.

The problem was the idle speed control and I asked two different Mechanics if they could solder it but neither would - said it would just be a temporary fix. Cost about 420.00 all together but the car runs great and with gas prices the way they have been, I probably paid for it twice. Such is life in SFCA. Thanks for the replies.

[Q]

Soldering the board is not a temporary problem. I studied Electrical Engineering, so I ought to know. They just didn't want to do it, and I don't blame them, as it is easier to swap the part. I did mine and it worked great. Didn't cost me a penny.

Glad you got it fixed. I know exactly how dangerous it can be. I wouldn't let my wife drive the car for fear that she would forget to hold the brake and let it lunge into the car in front.

Thanks for your reply. I agree with you on the "temporary" fix. I have also had a similar experience with another relay - the fuel pump - I was able to get the info I needed right here on this forum. I pulled the glovebox and relay jumped two of the pins (forget the numbers) and the car started right up. I had a friend solder it for me (I am surrounded by high tech folk here in SF) It has worked fine ever since. I wish all my future repairs could be that easy! This Idle speed relay was a bit different. First, I could not locate it. I looked in the part shop and located the part for a visual. All I could get was that it was somewhere on the passenger side kick panel - I dis-assembled it and didn't see a thing. I don't have a manual. One other thing which I should check out -one of the guys also wanted to change another part (for another $500) that was under the hood. I can't remember exactly what it was but one said "they should both be changed at the same time" some type of temperature sensor that gets dirty? I am not sure if this is the case but would like to get some other opinions. The car has been working fine since it was replaced 2 1/2 months ago.

Thanks

Captgrail
1984 380SL
145,000 California miles

[stevebfl]

Time for a game. See how much money can be spent on two parts for a 380SL. The idle speed motor (most likely the other part that gets dirty under the hood) MB part # 000 141 12 25 and the Idle Speed Controller MB part # 002 545 33 32. I will tell you that current MB list is $428 for the idle motor/valve and $310 for the controller that was the fix here (did you say $400).

I have never looked a part up in partsshop but I Imagine that these parts can be looked up by number there. Don't look at rebuilts if they have them. New parts will be VDO and willnot have the MB # on them but will be the exactly same part (I'm fairly certain). See what they ask for these parts. I haven't looked but I did look at one of our mutual suppliers for my cost. It might be embarassing to see the difference in what they charge and the prices quoted here.

[Q]

I am sorry. If I had gotten to this post prior to your sending the car to the mechanic, I could have walked you through the whole deal. On my 380SE, the Idle Speed Control Module is located dead center under the hood between the two firewalls. It is a black box the size of a sandwich.

[stevebfl]

It is very easy on the 126 body but a little more elusive on the 107. It is either under the passengers feet (there is a false floor) or on the face of the evap housing below the glove box. I never remember which units are in which place.

My Euro 107 had the Idle Speed Control module behind the glove box on the left. It is marked as such. One bolt and the connector and it’s in your hand. I couldn’t get the cover off so I replaced it. I know I bought aftermarket parts and got an idle speed control and Air slide valve for about $400 (total). The air slide valve (on the intake manifold) makes a buzzing sound. A neighbor has an ’85 500SEL and he has the new MB part. His make a hum. The MB part seems to run smother but it was the difference between $400 and $150 for the part.

Steve
’85 500SL Euro

Thanks again for your posts guys.
>>The idle speed motor (most likely the other part that gets dirty >>under the hood) MB part # 000 141 12 25

I did NOT replace this part but I am wondering if I need to....?

The car seems to be running fine with just the new Idle Speed Controller replaced. If it aint broke don't fix it?..:

Tell you the truth about the part - I always ask for the old part for any replacement and I had contemplated seeing if I could have it soldered and keep it for a spare. The cover was really hard to get off but I did get it off and could not see any bad solders with a magnifying glass so I wrote it off and have been keeping my fingers crossed that the new one will last another 15 years but the outstanding question in my mind is what about the other part? should I or shouldn't I - That's something I think I can change myself if necessary. If not, I can move on to other repairs..

Captgrail

[Q]

I looked and looked too, but did not find any visible signs of a solder break. I went ahead and rewet all of the joints anyway and it did indeed work. What is wrong with these boards is called a cold solder joint. The pins from the components and the holes and pads they are supposed to connect to were not heated to a sufficeint temperature to make the solder stick. Over time, the pins, holes and pads all develop corrosion which creates an insulator. By rewetting (melting the solder) the connections, you can make a better connection. If you add a little bit of solder as you rewet them, the flux (acid) that is in the solder's core (get flux core solder) will help eliminate the corrosion. Once done, it should last the life of the components on the board.

That cover was hard to get off. The best way I found was to insert a flat screwdriver into the slots and pry back the tabs. If you pry them back far enough, they won't bend back into place very fast. Go around quickly and it will come right apart. The tabs will regain their shape and make a solid lock again when you are ready for them to.

I read here at Mercedesshop.com that if the Air Slide Valve is bad it will draw too much current and burn out the Idle Speed Control Module. Yours may still be good so this is not a condemnation of your part.
Steve
’85 500SL Euro

I have a 300E that does what your car did at stop lights, surge 1000 rpm. question did your car ever act up while driving it, bucking caused by surging? Mine does this also so I am not sure that it would be the idle controller?