Throttle Switch - 17% Duty Cycle - PeachParts Mercedes-Benz Forum

zorin · #1 05-07-2004, 12:25 AM

Hello,

I just replaced the O2 sensor on my '86 300E (Manual). Once the engine warmed I measured the duty cycle to check it out. It reads a fixed 17% . I guess this means we lookup 20% fault code which point to the throttle valve switch.

I stopped the engine and tested the Throttle switch by measuring the restistance between pin 2 and 3. It shows infinite . My understanding is that if I depress the plate I should get about zero ohm, but the resistance does not change.

Is this the proper way to test the throttle valve switch ?

Also, (this might be related), I am reading a constant 940 mv on the output of the O2 sensor. As far as I know this shoud fluctuate around 400 mV if the mixture is set right.

Any insight will be appriciated.
Thanks.

Duke2.6 · #2 05-07-2004, 12:27 PM

The thottle valve switch has two circuits - WOT and idle with a common ground. I don't recall which circuit is which, but with the valve closed one should be open and the other closed and the opposite at WOT.

If the idle signal circuit misbehaves, the car will not idle properly because the idle system never knows the engine is idling. It will usually idle high. When it misbehaved a couple of years ago on my car a thorough cleaning of the switch and connector restored it to proper operation even though neither appeared at all dirty.

The throttle valve switch has no effect on the EHA duty cycle as long as the switch sends the proper idle signal. The screw and locknut throttle valve stop can be adjusted if the idle control valve duty cycle is not in the correct range.

Duke

zorin · #3 05-07-2004, 01:28 PM

Thanks Duke.

The car idles OK. It's just that the duty cycle on pin 3 is stuck at 17%, and the O2 sensor output is too high (close to 1 Volt - fixed).

I looked up this code and it points to Throttle Valve switch.
Where is the screw and locknut throttle valve stop so I can adjust it.

Thanks

Duke2.6 · #4 05-07-2004, 03:05 PM

It's not clear to me what you are measuring. If you are measuring the EHA duty cycle according to:

www.landiss.com/mixture.htm

the throttle valve stop and switch have nothing to do with it. The adjustable throttle stop is on the opposite side on the throttle valve shaft from the switch as I recall, and has a yellow thread lock compound. Do not confuse the throttle valve stop with the air flow meter adjustment.

When the ECU receives the idle signal it activates the idle control system and the throttle stop should be adjusted only if the idle control system duty cycle is not in the proper range. This is a completely different measurement and system from the EHA.

If you ARE measuring the EHA duty cycle there is something wrong with your system or you are not measuring the duty cycle properly. The Landiss web page should help you to be sure that you are measuring the EHA duty cycle properly, and note that there are differences between CA and 49-state cars.

A VOM should read about 480 mV on the O2 sensor signal lead. With a scope it should cycle between about 0.2 and 0.8 volt. You can spy on the O2 sensor output by pulling the connector under the passenger carpet enough to attach the scope probe. The companion two lead connector is the heater and it should show continuity on the O2 sensor side if disconnected and system voltage on the chassis harness side.

Duke

zorin · #5 05-07-2004, 03:48 PM

I am reading the output of Pin number 3 on X11 diagnostic connector. Static test show %70 ducty cycle, so it is a standard controller. When engine running at operating temperature it reads 17% fixed. What does this value indicate? On my other car '88 260E this value oscillates at %50.

Duke2.6 · #6 05-07-2004, 07:32 PM

Your 260E is okay. A fixed 17 percent reading on the 300E indicates something is not correct - either with the system or test procedure - but I can't be of any assistance on what the 17 percent reading specifically indicates.

Duke

ctaylor738 · #7 05-07-2004, 08:21 PM

If your O2 sensor is working, and putting out nearly 1V, you are probably running rich. Why don't you try leaning the mix and see if the voltage comes down?

psfred · #8 05-07-2004, 10:41 PM

17% duty cycle means the fuel distributor is set so rich the EHA and computer cannot lean it out enough. If you check the EHA current, it will be negative.

Lean it out some and see what happens -- the O2 sensor is reading correctly if you indeed are rich -- the voltage will come down as the oxygen level goes up in the exhaust.

Peter

stevebfl · #9 05-08-2004, 10:36 AM

All good advise, I agree that what you are probably reading is the end of capability to correct a rich mixture. As was stated you are probably at the end of EHA correction at -10ma.

I just wanted to clear up this question you asked:

"My understanding is that if I depress the plate I should get about zero ohm, but the resistance does not change."

You were talking about the throttle switch and then made that statement/question. It sounds like you are talking about two different things. Depressing the plate sounds like you are refering to the airflow plate which is NOT the throttle and the potentiometer on the side of it is not the throttle switch. Just for clarification.

zorin · #10 05-09-2004, 11:47 PM

Thanks everybody. I appriciate your help. The problem is fixed.

Yes the mixture was too rich. I adjusted it and it is back to 50% now. It is just that I had to turn it about one turn and a half to get the proper reading. I was hesitant to turn it more than a quarter of a turn at a time before. I must have really screwed it up few weeks ago when I was trying to troubleshoot the system.

Thanks for the clarification Steve. I did miss-read the manual. I was pressing the Air Flow plate to see a change in the resistance. I pressed the throttle and I did get zero ohm on the switch.

Thank you all again. You guys are great. The car is running very smooth now.