Benzmac, Steve, someone, please share how an idle control valve fails. - PeachParts Mercedes-Benz Forum

Tinker · #1 07-10-2002, 01:55 AM

I know that when the idle air valve fails, it generally sticks open. Could it still move when twelve volts are applied, yet still be bad? When they get old/worn, can they get out of phase/sync with the volts delivered from the computer?

Would monitoring the milliamps tell you if its bad, assuming that it is still moving to twelve volts?

Thanks for sharing.

Tinker

mbdoc · #2 07-10-2002, 08:21 AM

There are about 4 different types of idle control valves on MB engines, they all use different controls. The last versions starting with the M103 engine pulse voltage signals, ie: duty cycle. The 1984 to 1986 190E have a 3 wire set-up on the idle valve.
Most common failure is aging of the solenoid windings.

lewise · #3 07-10-2002, 09:39 AM

for what it's worth, It was very easy and pretty cheap to replace mine on my M103 motor. It was the source and the solution to my eratic idle problem. I had tried the old "jump battery voltage to the thing to see if it clicks open and shut", but then I realized that the thing operates on more of a duty cycle at probably much lower voltage than 12-14 volts, so I just replaced it. Replaced the two little rubber hoses at the same time.

Good luck
Evan Lewis

Tinker · #4 07-10-2002, 10:22 AM

M.B. Doc and Evan,

Thank you for the replies.

M.B. Doc. How does the `aging of the solenoid windings` manifest itself? Can it still work marginally or does it fail completely and stick open?

If the windings age, would the resistance change when checking with an ohm meter and shouldn`t it draw more current when operating?

Mine (190e) open and closed with twelve volts and draws between 540 to 225 ma with the motor running, yet I am stuck with a 1500 rpm idle.

Thanks again.

Tinker

stevebfl · #5 07-10-2002, 10:35 AM

That valve should take over 600ma when pulling the motor to proper speed. Reduced current is often the fault of the power source, which is the OVP relay.

dpetryk · #6 07-10-2002, 12:38 PM

I have investigated this phenomeon very closely. I had a high idle condition on my 87 420sel. Applying a variable voltage to the idle control valve allowed me to control the idle. But when it was connected to the idle speed controller in the car it would not bring the idle low enough. It seemed like the valve idi not have enough authority to do the job. The idle speed controller was putting out the maximum current that it was designed to do. It was just that the valve would not close down enough.

I installed a new one and the problem was solved. However before I installed the new one compared it to the old one on the bench and did several measurements to find the differences between them. I measured current verses stroke and all sorts of other electronic measurements. I could not find any differences!. They performed the same on the bench. I cant explain why one worked and the other did not. I assume it was because of some mechanical wear related to the sealing surfaces or something like that.

In one case I tried to clean the valve using brake cleaner (a good solvent) but it didnt do any good. I ended up buying a new valve and the problem was fixed. However, these valves are way expensive. So on the other car I spent a lot of time cleaning the valve. This time I used carburator / choke cleaner. It did a better job of disolving the carbon junk. I kept working on it until I could not get any more junk out. I was surprised how much stuff came out. I spent almost 2 hours soaking, spraying, and repeating the process. To my surprise, it did fix the problem. I also replaced both of the hoses since they were hard. On hind sight, I should have spent more time on the first one and I would not have had to spend the $ to buy a new one. Live and learn.

However even if the electronics is OK the valve seems to have an authority problem. Let me explain what I mean. It started with my 87. The idle was always hi. About 1200 RPM. I could not get it to come down. When disconnecting the valve connector - the idle went up. When applying 12 volts to the valve - the engine died. I connected a variable voltage source to the valve and was able to regulate the idle speed quite nicely. I was then convinced the problem was in the control module since the valve seemed to work. NOT. The valve had the ability to control the idle, I demonstrated that. So why didnt it work?

The control module was fine. It seemed that it was not able to supply enough current to the valve to bring the idle speed down far enough. Thats what I mean when I say it didnt have enough "authority" to control the idle. It was trying but it did not have enough strength. I bought a new valve and installed it and low and behold everything was fine. So now I was determined to understand what the problem was.

I took the "bad valve" apart - I mean completely apart. I could find nothing. I checked the stroke vs current & voltage signature from the new valve and checked it against the bad one. No observable difference. The bad valve had no observable difference to the new one except it was not shiny. So I accepted the fact that there was still so something I did not understand and I let it go at that until my 91 had the same problem.

This time I reasoned that there was something physical in the valve portion causing the problem. Thats when I got real agressive on cleaning the thing. I worked on that thing for hours until I could get no more carbon out of it. And it worked.

There is another phenomom used to drive these valves. The control module modulates the current applied to the valve with something called "dither". Dither is needed to overcome something called "stickshion" . Stickshion is best described as something like sticky friction. Its where the valve does not want to move shoothly and porportionally with the applied current. Because things get sticky it wants to jump rather than slide smoothly. So the electronics applies a low level rapidly varying current to it to constantly wiggle it and keep it free so it wont stick in one place. You can feel the dither when the engine is running. It feels like the valve is buzzing. This buzzing effect makes the valve move smoothly even though it is full of gunk and other sticky stuff.

My conclusion is that over time these valves get crudded up and wear mechanically and the result is that it takes more current to drive the valve shut. After a while the control electronics cannot supply enough current to bring the idle down and hence the idle creeps up. Cleaning it can cure it if it is not worn out. Dither makes the valve move thousands of times per minute and the resulting wear changes the stroke / current characteristic of the valve to something beyond what the electronics can compensate for. So try cleaning it really well and if that dont work, grab your wallet.

Tinker · #7 07-10-2002, 01:15 PM

Steve, thank you for the response. The OVP is new. Could it have failed so quickly? One of the tests in the factory CD states that voltage should be present between pins 5 & 6 of the OVP plug with the OVP removed. I do not have any voltage there, yet this point supplies the air idle valve and I am seeing approx. 5 volts at the valve when the motor is running?????

Dave, wow excellent info!!! Yes, I tried cleaning several times both with carb cleaner and soaking with thinner and again flushing with WD40. Sounds like I might be investing in a new one. When I unplug it while its running, it only makes a slight (100 -200 rpm) difference. Could be a result of low voltage as Steve stated. When I apply 12 volts with the motor running, the idle goes higher. It should go lower. ??????

Again, thanks very much.

Tinker

Cap'n Carageous · #8 07-10-2002, 01:35 PM

I'm following this thread with interest because I have a strange condition haunting my 300e. I have a starting/ stalling/ eratic idle/ check engine light/ overheating OVP and overheating power window relay problem. I've replaced the OVP, fuel pump relay, injector temp sensor and even the CIS-e unit!(talk about hanging on to your wallet!) When it chokes down I hear the idle valve clicking every few seconds sometimes and sometimes not. Is it supposed to click with the ignition on but the engine not running?

dpetryk · #9 07-10-2002, 02:23 PM

In my experience, when the valve is disconnected, the idle speeds up. When applying 12 volts to the valve the engine will die. Some voltage between 0 to 12 volts will control the idle.

engatwork · #10 07-10-2002, 03:40 PM

Can someone post a pic of the idle control valve for a 1995 E320 application?
thanks

dpetryk · #11 07-10-2002, 05:42 PM

You might not have one. Many of the newer cars use the throttle to control idle speed.

stevebfl · #12 07-10-2002, 05:53 PM

Yes, Jims E320 would use the throttle assy to control idle.

engatwork · #13 07-10-2002, 06:16 PM

Thanks Dave/Steve.
I still have a real slight "miss" at idle (since changing the head gasket) and I'm pretty sure it is a vacuum leak somewhere. It is very hard to tell when it happens, I'm not getting any codes and otherwise the car runs fine. I will get around to resolving it after it cools off some (ambient conditions).

Tinker · #14 07-11-2002, 01:16 AM

Steve,

I checked the idle valve ma`s again tonight and I am seeing around 600 ma when cold, but then it drops off as the temps rise finally settling around 200 - 300 ma. Too low according to your article.

If I blip the throttle, the amps don`t move much, maybe 100. If I unplug the OVP while its running and plug it back in, it jumps to 600ma and drops again to the 200 - 300 ma level. I tried the old OVP and got the same results.

When I checked the duty cycle at X11 #3, I get a fixed 50.7 cycle. All temp sensors are in spec. Could this be a ECU problem?

Where to start investigating? Any way to check an ECU?

Tinker

stevebfl · #15 07-11-2002, 02:51 PM

I couldn't answer this question yesterday as at the moment I was in the midst of having a 83 380SL totally whip my butt idling at 900rpm. I began to doubt all my knowledge and experience.

I gave up last night and couldn't look at the car this morning. I went in and sat down with my electronics guru brother and began explaining what I had done and how the conclusions kept telling me that I had a bad idle controller, but I already had a new idle controller and it was doing the exact same thing.

Now I'm embarassed as this is the height of simplicity in the world of electronic control, we are talking a system that is over 20 years old and I was already an experienced MB tech when it came out and I've been tangling with this one off and on for two days.

This is how it works: the idle valve is held to 650 rpms by 400ma of current. The MB book says a little more maybe. My brother about 15 years ago built us a transistorized power control for these idle valves as a test tool. I hooked it up and at 400ma the idle was at 650rpm. Don't you wish you could check your valve so easy. BUT, the darn system is running at 250ma.

First conclusion is that the circuit is poor due to high resistance or bad OVP relay. Voltage drop tests show the power and grounds to be OK.

Since the current was low and the power circuit good we got a controller. Same story. So now we need to look closer at whats happening. It is noticed that when the trans is put into gear the current drops quickly to 150ma maintaining the same 850-900rpms. I thought about how the speed signal could get misinterpreted, so I compared the square wave prepared speed signal (its on one of the pins of the diagnostic connector) to the primary signal at the coil. The signals totaly overlaped running at 57hz (855 rpm).

After watching this thing I become convinced that the car is idling at the low temp setting. It is strong and determined to run at 850-900rpm which is the cold (below 16deg C - 60deg F) range.

The idle control box only has 9 wires on it. It looks at the idle switch. When grounded the idle switch places the box in the control mode. Open moves the valve to a fixed current holding the valve farther open than at idle. This is because if the system worked otherwise, the valve would shut as the engine sped up. If the throttle was shut rapidly the engine would die as the valve couldn't open quickly. The unit views temp by looking at the 16deg switch on a wire that also carries the same signal to the lambda sensor.

Very early in testing I found that the 16deg circuit was grounded. Seemed the answer. I was checking the system at the controller under the dash. To isolate the problem, instead of finding the source of the short, I decided to just eliminate it for test purposes by pulling the pin from the connector. At this point there was no ground at that terminal since there was no wire at all on it. The circuit is only hooked to various grounds (both the full throttle switch and the snap acceleration vacuum switch also can ground the circuit). I probably should have paid more attention to the fact that plugged or unplugged there was no change.

At this point I was through (5:30 yesterday). This morning I talked the whole thing through in the A/C with my brother and we absolutely decided that the system had to be operating at cold setting. Since the wire was shorted we found the problem - the snap accel vacuum switch was a hang-on this car; it was an 83 and they were installed in production starting in 85. The harness ahd been tied to the airpump and was shorted internally. I unplugged the factory 16deg oil temp switch connector from the harness and plugged it back to the original point on the oil switch under the alternator.

The problem was solved. This should have been easy, I found the short quickly, but didn't believe it because nothing happened when I disconnected the short.

Here is the answer: normally a control unit expecting a ground signal has a small biasing voltage that gets pulled to ground by the ground. Evidently the idle controller is so small that they left the circuitry out and rely on the circuit in the lambda controller for the biasing. SOOOO. When I disconnected the short the voltage stayed the same. What should happen is that when the switch to ground (short in this case) goes away the biasing voltage of the lambda controller is seen (since it isn't grounded).

So after working on this system for 20 years I have now found a new way it can fail ( I am also wondering now if I have seen this before). The short I knew about, but a break in the wire between the two controllers will simulate the cold response since the idle controller is actually looking for a bias voltage that comes externally from the lambda controller.

I hope this makes some sense.

Anyway, finding this, is more fun than money. If one has never chased a problem to a solution one can never know how much fun it is. This makes the fifth time I have told the story. Everyone one of my techs has heard it.