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1991 420 SEL
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Discussion Starter #1
1991 420SEL:
I know this has been done to death but, with my limited electronical knowledge I’m stuck.
Trying to run down a high idle. Recently re-did the intake manifold. All the soft bits and vacuum hoses and ends. Replaced the throttle valve switch because you can’t get to it without removing the intake again. New injectors, holders, O-Rings. Cleaned the ICV.
Engine had been running rough and surging. This fixed that problem but, now I have high idle. About 1100 rpm.
Checking my lamda on/off ratio with a Fluke O-scope per landiss.com/mixture.htm procedure. 49 state 70% type.
What I’ve found.
Static Test:
1. Switch on: 70% duty cycle
2. Partial depress Air Flow Sensor plate: 40% duty cycle. Should be 10%
3. Fully depress Air Flow Sensor plate: 70% duty cycle. Should be 20%.
When Air Flow Sensor plate returns to full closed: 40% duty cycle. Should go back to 70%?
Step 2. Says to check Throttle Valve Switch. Job 07.3-121. Brand new.
Step 3. Says to check Air Flow Sensor Potentiometer. Job 07.3-121.
Question. How do you check this 3 pin connector?
Have copy of Job 07.3-121 but I don’t read Greek.
Could some electrical type please explain this in Neanderthal terms?

Thanks, Dave
 

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560SEC, 190E, Frontier, 240Z
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I think you are confusing tests.

Any deflection of the Airflow plate will return a Code 10%.
When it goes back to its natural state it might go back to 70%, but more likely code 40%. I think this because we are not testing with the fuel pumps energized and so the plate is not receiving pressure to go to its normal resting state. Mine does this same thing and I've replaced my Airflow pot with no change. The computer looks at its position as "improbable" and gives that 40% error which goes away when you recycle the key, or energize the pumps.

Fully depress THROTTLE and you will get 20%.
That tells you the throttle plate went fully open, the switch made and is sending the signal for Wide Open Throttle operation to the CIS brain.

I have tested that 3 prong connector on the airflow pot numerous times and then became convinced mine was bad so I bought a new one for $300 bucks and the brand new one performs the same as the 20 year old one.

This help?
Tj
 

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1997 Mercedes-Benz S500
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213 Posts
1991 420SEL:
I know this has been done to death but, with my limited electronical knowledge I’m stuck.
Trying to run down a high idle. Recently re-did the intake manifold. All the soft bits and vacuum hoses and ends. Replaced the throttle valve switch because you can’t get to it without removing the intake again. New injectors, holders, O-Rings. Cleaned the ICV.
Engine had been running rough and surging. This fixed that problem but, now I have high idle. About 1100 rpm.
Checking my lamda on/off ratio with a Fluke O-scope per landiss.com/mixture.htm procedure. 49 state 70% type.
What I’ve found.
Static Test:
1. Switch on: 70% duty cycle
2. Partial depress Air Flow Sensor plate: 40% duty cycle. Should be 10%
3. Fully depress Air Flow Sensor plate: 70% duty cycle. Should be 20%.
When Air Flow Sensor plate returns to full closed: 40% duty cycle. Should go back to 70%?
Step 2. Says to check Throttle Valve Switch. Job 07.3-121. Brand new.
Step 3. Says to check Air Flow Sensor Potentiometer. Job 07.3-121.
Question. How do you check this 3 pin connector?
Have copy of Job 07.3-121 but I don’t read Greek.
Could some electrical type please explain this in Neanderthal terms?

Thanks, Dave
I had the same problem on my 90' 560SEL. Measure the voltage between 2 pins at IAC valve, if it reads around 4.8v then your IAC valve is probably bad.
I noticed that If the idle control relay is bad then your idle would be around 1500 rpms. In my case it was 1100rpms. I cleaned IAC with no results, then ordered another used one, cleaned it as well, but all that was junk. Get a brand new one VDO and it will be fixed. I bought original VDO for 179 Dollars and it took me less than 5 minutes to replace and 3 weeks of hair pulling frustration was gone. after you replace it, then play/adjust your LAMBDA.
 

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1991 420 SEL
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Discussion Starter #5
When you refer to the IAC do you mean the Idle Control Valve (ICV)? Test with swithch on, or running?

Thanks.
 

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1997 Mercedes-Benz S500
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Yes, IAC= Idle Air Control valve. Turn the car on, get it to operating temp. 80 degrees. pull the connector in a way that it will be on the IAC valve, but enough space to put the leads on the 2 pins. measure the voltage, should be around 4.8v.
Now, if voltage is good set you meter to measure DC current and try to measure the current flowing through the valve(the meter should be connected in series). that means that you have to put connector on one pin of IAC valve, then take first lead of the tester and put it in the other half of the connector socket.the other lead needs to touch other IAC pin. the current should be 850mA-1050mA if the current is correct, then you have a bad IAC valve. If you have current out of specs your Idle control relay or Overload relay (OVP) might be going ala shit. Let me know how it goes.
 

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1991 420 SEL
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Discussion Starter #7
abdimona
Car warmed up and running:
3.45V across posts of ICV. Connector partially off.
Meter in series.
Pulled ICV plug turned it 90 deg. Plugged one half into positive post of ICV and connected meter to other plug half and to other post on ICV. Correct?
Results: 60 kohms max. to zero. Jumping all over the place.
With car running checked voltage at ICV wire plug while disconnected. 1.5V
What should it be?
Soooooooooooooooo...ICV Toast?

Again, thanks for the help. Dave
 

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1997 Mercedes-Benz S500
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abdimona
Car warmed up and running:
3.45V across posts of ICV. Connector partially off.
Meter in series.
Pulled ICV plug turned it 90 deg. Plugged one half into positive post of ICV and connected meter to other plug half and to other post on ICV. Correct?
Results: 60 kohms max. to zero. Jumping all over the place.
With car running checked voltage at ICV wire plug while disconnected. 1.5V
What should it be?
Soooooooooooooooo...ICV Toast?

Again, thanks for the help. Dave
the voltage supposed to be 4.5v +/-1 v, so you good there. However, when you said you've measured the Kohm's that was wrong. connect the tester the way you did and measure DC Amps. DC current.
1.5v at disconnected connector is a bit too low i think. In my case it gave 4.8v regardless of the connector position (on or off the pins). What happens is that ICV valve burns out sonner than Idle control relay, when it burns out, it sticks open and idle control relay tries to feed more current through it to close ICV valve in order to lower rpms and eventualy burns itself out if the OVP fuse doesn't pop.
This explanation is my way of understanding this circuit.

One more thing, before you going to shed any money on all the valves. Pull the plug from icv valve and connect direct voltage from the battery 12v. + on one pin, - on another. if the car is running it will stall. if not you have a vacuume leak. Also, when you pull the plug off the ICV pins, is there a surge in the RPMS?
 

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1991 420 SEL
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Discussion Starter #9
the voltage supposed to be 4.5v +/-1 v, so you good there. However, when you said you've measured the Kohm's that was wrong. connect the tester the way you did and measure DC Amps. DC current.
1.5v at disconnected connector is a bit too low i think. In my case it gave 4.8v regardless of the connector position (on or off the pins). What happens is that ICV valve burns out sonner than Idle control relay, when it burns out, it sticks open and idle control relay tries to feed more current through it to close ICV valve in order to lower rpms and eventualy burns itself out if the OVP fuse doesn't pop.
This explanation is my way of understanding this circuit.

One more thing, before you going to shed any money on all the valves. Pull the plug from icv valve and connect direct voltage from the battery 12v. + on one pin, - on another. if the car is running it will stall. if not you have a vacuume leak. Also, when you pull the plug off the ICV pins, is there a surge in the RPMS?
When connected the correct way measuring current. Meter set to AMPs DC it reads .023 to 0. Jumps around crazy. Not steady. When I jump 12V from the battery to the valve, it has a good pronounced snap. Remove power and it springs back. With car warm and running the engine rev's up with plug removed. Checked fuse's on the OVP, mine has two red 10 amp spade US Type and they're both good.
Pulled the Idle control relay and popped it apart to look for any obvious signs of "smokage". Saw none. Circuit board is sealed with clear plastic. No "smoke got out of the wires" smell. Saw on another thread with a pic, and this guy says the transister usually burns out with a pronounced burn mark. Mine looks new.
Don't' think I have any vacuum leaks. Just had the intake off. Had intake hot tanked. What a total mess of varnish and coke. Replaced the eight rings between the manifold halfs. Hard as corn flakes. Removed and cleaned the throttle valve. Replaced the throttle switch just because it would be a bitch to get to later and they're only $35.00. Replaced ALL vacuum lines and rubber ends. Determnined the cause of the coked up intake was the EGR system. The EGR pipe was completly rocked closed. And, a huge blob of crap where it entered the manifold. Completely deleted the EGR system. Replaced all injectors, holders, seals, and O-Rings. Holders, seals and O-Rings brittle as toast. Dialed in the on/off ratio with an O-Scope.
Before this the engine was running very rich. Rough idle, surging. Had to pump gas to get it to start. Always on the verge of stalling. Runs very smooth now except for the fast idle. Which is where I'm at now.
I could just replace the ICV, OVP, and relay. But, as a Mechanical Engineer of German desent, I'm determined to figure out this clap trap, Rube Goldberg fuel delivery system.
Our mantra is Keep It Simple Stupid. And there comes a time in every design process when you have to shoot the Engineers and go into production. So sad they have strict gun control in Germany.
 

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1991 420 SEL
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Discussion Starter #11
I'm curious. What procedure did you follow to replace the $35 throttle switch?
I didn't. It's plug and play. Just two hex socket head cap screws hold it to the throttle valve. But, comma, however, you must remove the throttle valve from the lower intake to attach it. And, on my 420 I had to remove the intake to remove the throttle valve. No wrench space. Just make sure to align the slot on the switch so it ingages the corresponding knob on the throttle valve shaft or it won't move the butterfly. The other end is just an electrical plug connection. After I removed the intake and saw how crapped up it was from the EGR system, one thing led to another. The while your're in there's, and the ya might as well as's. The official name is Throttle Position Sensor/Switch. On my '91 420 sled it's part no. 2015405645. The 560 takes the same one. Also get gasket P/N 1171412680. Believe me, you don't ever want to revist this again. Cheap insurance.
 

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When connected the correct way measuring current. Meter set to AMPs DC it reads .023 to 0. Jumps around crazy. Not steady. When I jump 12V from the battery to the valve, it has a good pronounced snap. Remove power and it springs back. With car warm and running the engine rev's up with plug removed. Checked fuse's on the OVP, mine has two red 10 amp spade US Type and they're both good.
Pulled the Idle control relay and popped it apart to look for any obvious signs of "smokage". Saw none. Circuit board is sealed with clear plastic. No "smoke got out of the wires" smell. Saw on another thread with a pic, and this guy says the transister usually burns out with a pronounced burn mark. Mine looks new.
Don't' think I have any vacuum leaks. Just had the intake off. Had intake hot tanked. What a total mess of varnish and coke. Replaced the eight rings between the manifold halfs. Hard as corn flakes. Removed and cleaned the throttle valve. Replaced the throttle switch just because it would be a bitch to get to later and they're only $35.00. Replaced ALL vacuum lines and rubber ends. Determnined the cause of the coked up intake was the EGR system. The EGR pipe was completly rocked closed. And, a huge blob of crap where it entered the manifold. Completely deleted the EGR system. Replaced all injectors, holders, seals, and O-Rings. Holders, seals and O-Rings brittle as toast. Dialed in the on/off ratio with an O-Scope.
Before this the engine was running very rich. Rough idle, surging. Had to pump gas to get it to start. Always on the verge of stalling. Runs very smooth now except for the fast idle. Which is where I'm at now.
I could just replace the ICV, OVP, and relay. But, as a Mechanical Engineer of German desent, I'm determined to figure out this clap trap, Rube Goldberg fuel delivery system.
Our mantra is Keep It Simple Stupid. And there comes a time in every design process when you have to shoot the Engineers and go into production. So sad they have strict gun control in Germany.
Did your car stall when you applied voltage to ICV valve for more than 5 sec? if not you have a vacuume leak. Did you change the doughnuts at the lower intake? did you adjust the Switch that you changed? are you sure that the intake boot is not leaking and all the bolts between the boot and the aluminum gasket are in the same location, since they are different in hight? if your EGR valve is clogged maybe its time to change the valve seals? does your car consume any oil?
 

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w126
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Did your switch have the elongated holes for mounting? Kinda like you could vary the position of the switch relative to the throttle shaft?
 

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1991 420 SEL
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Discussion Starter #15
Did your car stall when you applied voltage to ICV valve for more than 5 sec? if not you have a vacuume leak. Did you change the doughnuts at the lower intake? did you adjust the Switch that you changed? are you sure that the intake boot is not leaking and all the bolts between the boot and the aluminum gasket are in the same location, since they are different in hight? if your EGR valve is clogged maybe its time to change the valve seals? does your car consume any oil?
Applied 12V with ignition off. Did not check running. Pronouned "click" and snap back when removed voltage. Shake it, it rattles. Hums with ignition on.
Yes. The doughnuts were toast. When I squezzed one it crumbled to pieces. As did the injector holders, seals, and O-Rings.
Installed them with a very,very light schmeer of RTV.
There did not appear to be an adjustment on the TV switch. Bolts up solid no oblong holes. No adjustment screws. Did align the knob on the butterfly shaft to the notch on the switch. Is there an adjustment?
Made a card board template for the intake bolts. All reinstalled in original holes.
EGR system completely deleted. Fab'ed blank off plate in intake, and plugged hole in exhaust manifold. Plugged "a" port on TV. Removed and plugged thermo vac switch on intake. Replaced every vac line and rubber end.
Car burns about 1qt between oil changes. Every 3 to 4k. At 147k. Normal.
Bore scoped cylinders after sitting a couple of days to check for oil leak down when I got this sled about 4 years ago. Clean.
There is a little blowby from the PCV system detected at the cam cover hose to air filter housing. But, seems about right for the milage.
Also checked for pre-tension on the throttle linkages. None.

Dave
 

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w126
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I haven't seen one that didn't have the elongated holes before. I'm attaching the test procedure for that switch. Note that it switches @ 2 tenths of a millimeter.
 

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1991 420 SEL
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Discussion Starter #17 (Edited)
I haven't seen one that didn't have the elongated holes before. I'm attaching the test procedure for that switch. Note that it switches @ 2 tenths of a millimeter.
Aspect:

No I did not perform this test. I just looked at the old one and it indeed has slotted holes.
I'll need to get and I&C tech at work to interperate your attachment. I'm guessing I measure between terminals 1 and 3 for this? No idea what the LL and VL means or if it's important.
Oh, is this a bench test?
Thanks for the attachment.
 

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You can test it in the car. I don't know what LL and VL mean exactly. Something that didn't get translated no doubt. You test between 1&2 for one part, then 2&3 for the other. If the tests seem to provide 'backwards' results, reverse the roles. This file mentions renewing the switch if it's bad. I'm certain there's another procedure that says to replace the switch and the throttle housing as an assembly rather than the switch alone. I can't find that one. You can't really sense the 0.2mm point by feel so you really need to use a dial gauge to set it up.

Edit: In your case I wouldn't worry so much about the point it switches for now. Just make sure the circuit signals the controller that the throttle is at the idle position when it is.
 

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Discussion Starter #19 (Edited)
You can test it in the car. I don't know what LL and VL mean exactly. Something that didn't get translated no doubt. You test between 1&2 for one part, then 2&3 for the other. If the tests seem to provide 'backwards' results, reverse the roles. This file mentions renewing the switch if it's bad. I'm certain there's another procedure that says to replace the switch and the throttle housing as an assembly rather than the switch alone. I can't find that one. You can't really sense the 0.2mm point by feel so you really need to use a dial gauge to set it up.

Edit: In your case I wouldn't worry so much about the point it switches for now. Just make sure the circuit signals the controller that the throttle is at the idle position when it is.
Aspect:
In the FSM when they say RENEW, they mean REPLACE. Not REPAIR.
You can check this in place but, you can't physically adjust this on the car. You can't get to the mounting screws with the throttle in place. You can't remove the throttle without removing the FD assembly. You can't remove the FD assembly without removing all the fuel lines, etc, etc, etc,. That's the main reason I replaced the switch to start with.

Had the I&C guys look at your attachment and my old switch. They immediatley determined that all this really does is check that you have an open circuit at each butterfly stop (full closed or full open). And, resistance just a hair off of each stop thru the movement range. (As I recall) When reduced to Flange Head terms I got it.

Sorry other way around.
 
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