my 1994 E-420 starts up fine and idles fine when first started but when engine warms up the idles is at 1400 rpms. No other symptoms other than cruise control does not work but it has had both these problems sincei got it so dont know if one is related to the other. any ideas on what this may be? i have seen some posts on cleaning the buterfly in throttle body. did they just spray it or take it apart or does this not apply to this case? thanks for any help
94 e420 idles at 1400 rpm
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ok i took alonger look at it today. the idle when warm actually fluctuates from as low as 900 to as high as 1500 just doent do constantly. it fluctuates about every 30 seconds or so. where is the dile contrl valve and any way to test it?
Rasputin
Senior Member
Idle speed adjuster valve
This article might help.
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.
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 smoothly and proportionally 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.
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.
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 embarrassed 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 totally overlapped 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.
This article might help.
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.
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 smoothly and proportionally 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.
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.
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 embarrassed 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 totally overlapped 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.
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- Thread starter
- #4
i can follow a long pretty well but now my problem is, this is the first merecedes i have owned . only chevy before so i am really just kinda wandering around the engin compartment and trying to determin what is what. i bought a service manual on cd but that was about a complete waist of money. doesnt really tell you squat beyond the obviose. I am used to haynes manualls. But as far as i see they are not made for this car. so any advice on how to obtain something that will educate me as to where parts are on this thing. i sure want to get the idle down for fear of damage to my tranny. thanks for the great info so far. if i knew where these parts are located i will do just as you say.
Rasputin
Senior Member
This is what the Idle control valve looks like and is located just behind the distributor lying horizontaly justs below the air filter housing:-
and this is the Idle control unit 21:-
and this is the Idle control unit 21:-
Smalley
Active Member
The W124 E420 has a throttle actuator, which takes the place of the idle control valve in this engine.
There can be only three causes of an idle that hunts up and down like you describe:
1. bad throttle actuator - the harness to the throtte actuators have bad insulation that flakes off due to heat. Throttle acutators also have potentiometers that wear out over time. Throttle acutuators are also called the EA / ISC / CC module: Electronic Accelerator, Idle Speed Control, and Cruise Control module. All three functions are combined in one part. The fact that your cruise control is not functioning is also a good sign that your throttle actuator is bad.
2. bad mass air meter - when faulty, a bad mass air meter will keep hunting for the proper mixture of fuel to air, and the idle will go up and down rhythmicaly.
3. vacum leak - a vacuum leak is false air, and will not allow the mass air meter to be able to figure out the proper air to fuel mixture and thus the proper idle speed.
For what it's worth, there is a part called a stop lamp switch, which is located above your brake pedal. It is a relatively cheap part, and will also cause the cruise control to not function.
There can be only three causes of an idle that hunts up and down like you describe:
1. bad throttle actuator - the harness to the throtte actuators have bad insulation that flakes off due to heat. Throttle acutators also have potentiometers that wear out over time. Throttle acutuators are also called the EA / ISC / CC module: Electronic Accelerator, Idle Speed Control, and Cruise Control module. All three functions are combined in one part. The fact that your cruise control is not functioning is also a good sign that your throttle actuator is bad.
2. bad mass air meter - when faulty, a bad mass air meter will keep hunting for the proper mixture of fuel to air, and the idle will go up and down rhythmicaly.
3. vacum leak - a vacuum leak is false air, and will not allow the mass air meter to be able to figure out the proper air to fuel mixture and thus the proper idle speed.
For what it's worth, there is a part called a stop lamp switch, which is located above your brake pedal. It is a relatively cheap part, and will also cause the cruise control to not function.
- Thread Starter
- Thread starter
- #7
thanks alot for the info guys. now i have a starting point. hey smalley, i thought about the brake thing but isnt that the same peice that operates the brake lights? and if so can this go bad with out affecting the operation of the brake lamps? i just figured since the lights worked it want going to be under the peddle. i think i am going to look around and see what the throttle actuater costs and maybe try that. since this is an throttle actuater and not the idle control valve does the idle control moduel still exist and if so is it still tested the way stated in the first reply? thanks agin for all the help
Smalley
Active Member
I don't know why, but a bad brake stop switch can fail without affecting the brake lights.
The idle control module is the throttle actuator. I don't know how it can be tested.
The throttle actuators are expensive, costing over $1,000 U.S. dollars.
The idle control module is the throttle actuator. I don't know how it can be tested.
The throttle actuators are expensive, costing over $1,000 U.S. dollars.
teky
Senior Member
control modual is the computer not the actuator/rotory valve..
if the break switch went then the light would either not work at all or stay on all the time..
if you did a search i posted how to tet the idle control valve/actuator anc conrol unit
if the break switch went then the light would either not work at all or stay on all the time..
if you did a search i posted how to tet the idle control valve/actuator anc conrol unit
try my best
have you looked for a narrow guage pipe that has come adrift from behind the carb and if you have a economy guage is this working if not look for another narrow gauge pipe coloured with a red stripe on your exhuast manifold on the left bank this may have come loose 8)
have you looked for a narrow guage pipe that has come adrift from behind the carb and if you have a economy guage is this working if not look for another narrow gauge pipe coloured with a red stripe on your exhuast manifold on the left bank this may have come loose 8)
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