Bizarre idle problems in a 1985 380se - PeachParts Mercedes-Benz Forum

troylatif · #1 10-30-2008, 01:52 PM

Hi all,

I've been experiencing a strange problem with my 126 body m116 engine 380se. I'm sure its a sensor/electrical issue, but here's how it started.

I was driving along when suddenly at the tap of the pedal the idle speed instantly jumped from normal speeds up to about 1300 rpm in gear. I got it home with all my weight on the brake pedal and found that it idles in neutral at about 2000 rpm. I went ahead and changed out the idle speed air control valve, but it didn't change anything. I checked the overload protection relay and it seems to be fine. I give it a shake and I hear something bouncing around. Could that be an issue? It seems to make a difference when I try running the car without that relay, so I'm thinking its functional.

I've checked the throttle linkage and it returns to the right position. The only few things I could conclude were that either the idle control unit is shot somehow, the throttle position sensor is stuck/broken/shorted, or there's a broken fuse or relay somewhere that's making all this happen.

At the moment, it idles at 1500-1600 rpm when its absolutely cold. When warmed up it hits about 2000 rpm. In gear it bounces between 800-1000 rpm betwen cold and hot operation.

Does anyone know if its difficult to get to the TPS or idle control unit?

Any advice is much appreciated,

-Troy

gmercoleza · #2 10-30-2008, 02:05 PM

Idle Control Module may need resoldering. It's in the bulkhead, easy to get to.

troylatif · #3 10-30-2008, 09:51 PM

I see. Is that simple or fine circuitry? My guess is simple.

-Troy

rg2098 · #4 10-31-2008, 12:14 PM

Simple. Its about 4x4x1" mounted in the center of the bulkhead. I'm waiting for a used one to arrive myself, its doing the same thing.

To test the idle switch, pop open the connector and check for continuity with the throttle closed, and at wide open. Its a 3 pin round (1" diameter) connector down on the drivers side behind the headlamp.

Chadahar · #5 10-31-2008, 02:59 PM

Easy way to detect a bad ICU is to rap it with a screwdriver handle while idling. If the car dies OR the idle changes with the rap, then the ICU is bad. That is how I found a bad ICU on my 380SE.

troylatif · #6 11-01-2008, 04:06 AM

Great! This is all good stuff, guys. I've been coming to this forum for some time now. However, I've recently relocated for work and I've never really appreciated how useful these DIY tips can be for even the road warrior such as myself.

Thanks to everyone for your input,

-Troy

Chadahar · #7 11-01-2008, 03:47 PM

You are welcome...from a former Road Warrior..i.e. CAD Mech. Designer Job Shopper.

troylatif · #8 11-02-2008, 01:46 PM

Here's an update. I've located the TPS plug. I've traced it back to this really well hidden spot under a bunch of vacuum lines and fuel/air injection parts (makes me wish the TPS isn't broken). I've unplugged it and have noted that the vehicle observes no change in idle performance or speed. It still has a high idle around 1600 cold to 2000 hot.

Another idea I had: take a jumper wire and close the leads to the Throttle Closed pair on the TPS leads. My only difficulty is determining which leads are which. The thought did occur to me, I could take a continuity tester and determine ground position, but if the ground position in the switch is broken, then it'd yield nothing on the continuity tester.

I suppose I could test the Wide Open Throttle position and just eliminate that option.

From experience, how difficult is it to remove and replace the throttle position switch? Are there any strange sockets or tools needed? Deep reaching metric hex sockets? Short head metric hex sockets? Specially angled wrenches?

I appreciate your input,

-Troy

Chadahar · #9 11-02-2008, 05:53 PM

I would try several things BEFORE I had to go into the throttle body location to repair the switch.

Try taking off the ICV the round valve in front of the air cleaner(it has two large black rubber hoses plugged into: 1, the rear, 2. the side.) and clean it with carb cleaner.
First, Disconnect the switch from the forward end and see if that makes a difference in the idle speed. (Or is this what you thought was the TPS?)
Spray it in the side port holding the valve with the end port pointing down. Hold your thmb over the rear port and slosh the carb cleaner around and spray until it runs clean. They do gum up over time.

It sounds more like Idle Control than Throttle Positioning Switch. Did you rap on the ICU between the firewalls? Result?
You have to give it a healthy rap.

The following is NOT Road Warrior friendly, especially if you have to use your car as a daily driver.

To get to the TPS you have to remove all the fuel lines from the fuel dist., leave the FD on the Air Metering Plenum, then remove the Air Metering Plenum then use metric Allen socket(7mm if I remember correctly) to remove the throttle body. While you are in there replace all the vacuum lines so you don't have to go back int there soon. I'd replace the 3/2 valve on the TB too along with the TPS.
I just performed this chore on my Euro 500 SEC and it's no fun, but doable DIY. My car had the manifold gaskets and donuts replaced 41,000 miles ago and I decided NOT to replace them at this time. However, if these parts had not been replaced I would have replaced those as well.

Pretty tough decision to go in there when I think your problem is either in the ICU, ICV the OVP (check it too). I would replace these or at least test them to see if they are OK way before I'd elect to "GO IN THERE"....especially if you don't have too.

troylatif · #10 11-19-2008, 03:27 PM

Okay, I finally got around to checking and replacing.

I tested the TPS with a continuity tester. It tested fine with the throttle both wide open and closed.

I removed the ICU (idle control unit) and found that a resistor had a hole burnt in the side of it. I bought another unit.

I replaced the ICV (idle control valve) with another.

The engine still idles high as it did before. I'm somewhat at a loss for words. It seems like I've traced down and resolved all potential issues. What else could there be?

-Troy

troylatif · #11 11-20-2008, 09:02 AM

Here's a thought, and maybe you guys can echo on this:

Could it be something offsetting the ignition timing? I've noticed a sharp drop in my gas mileage down from about 19-21 mpg mixed to about 14-16 mpg mixed. I checked the vacuum at the distributor and it seems to be strong. Could there be an electronically controlled valve somewhere that is poorly regulating this vacuum?

Thanks,

-Troy

troylatif · #12 11-21-2008, 09:21 AM

Still Idles high. Could there be an air leak somewhere? vacuum leak? A faulty ignition component making the idle ride high via bad control somewhere?

-Troy

troylatif · #13 11-21-2008, 10:53 AM

Thanks for your input. I'll check those potential issues. I'll post back with what I find.

-Troy

troylatif · #14 11-22-2008, 12:52 PM

Okay, an update.

I pulled the connector on the frequency valve. It does get 12 volts. Without the plug connected, the engine seems to want to stall and runs rough.

I'm going to check the other components. If the idle control valve isn't getting juice then is it safe to say that the idle control unit isn't either?

If either is the case and the overload protection relay is functional (i've tested it), could there be a ground fault somewhere? Or is there a box of fusible links I'm not even aware of?

Thanks,

-Troy

rg2098 · #15 11-22-2008, 02:18 PM

Not necessarily, my idle control valve was not getting power at all either. A new (used) controller fixed that problem. It looks like the one that was in the car was rebuilt at some point and not a very good rebuild. I had at least one bad capacitor inside. Now I idle, but I have several vacuum leaks to track down to get the idle stable.