Hi Peter, Picked up my multimeter that reads duty cycle today. Give it a quick try but don't really know what I'm doing, so have a few questions. Not really sure what you mean by "some read 20% adn MB duty cycle is upsidedown, os it may read 80%? I found the port and plugs it in. With a half turn of the key, it read a low number, 30% or something. As it started the car, it would constantly move between 37%-60%. Should it settle down at 50%, or is the movement just the O2 constantly adjusting the mixture? When you say "settle down", how much swing should I see no the meter? As you can see, this is all new to me. :-) Cool think, I was reading that I can use a similar technique on my old 911SC as it has CIS and a test port too. I think I need to rig some test clips for that one thougth.
Does Rusty sell the tubing for the idle control valve, or is it just basic auto tubing? Thansk Peter. Ed '88 300E gaser On 28/04/07, Peter Frederick <[EMAIL PROTECTED]> wrote:
You can test the O2 sensor with the multimeter -- inscrew the cap on the test connection on the fender (it's on the computer there, a round screwcap with a safty chain so you won't loose the cap). Put the red lead in the #3 hole and ground the black lead with the meter on duty cycle and switch key on, don't start. Should read 50% (some read 20%). MB duty cycle is "upsidedown" from US, so it may read 80%. Start and it should stay at 50% until the engine warms up a bit (you are on mechanical mixture, computer in open loop mode), then start to move and "float" when the heater gets the O2 sensor up to temperature. If it swings over to very low or very high duty cycle, the computer cannot control the mixture. This usually sets the check engine light, but not always. Excessive "hunting" along with an undulating idle indicates a vacuum leak. With the air cleaner off, gently press down on the air flow meter plate -- not too far, or it will stall! -- and the meter should swing over to 18% or so (full leanout). remove the pressure on the plate and it should come back to where it was. To adjust the mixture, you need a 3mm allen wrench, a long one if you can find one, but a standard will do. There is a small "tower" on the airflow meter horn that sticks up to a grommet in the air filter housing. It's blocked by a sheet metal plug at the factory, but I'm sure yours is long gone, no one ever replaces them. Down inside that tower is a spring loaded screw adjustment fitting. Get the 3mm wrench into it and press down to engage the screw on the adjuster, and turn clockwise to richen and counterclockwise to lean. You want the meter to read 50% at idle, engine warm. Make ONLY small (1/8 turn or less) adjustments and wait a minute or two between to let the O2 sensor stablilze. Pressing down to engage the screw causes temporary enrichment, too. Turning too far and not waiting long enough will get you bouncing around from way overlean to way over-rich. If you cannot get the mixture to settle down, you have either a vac leak, a bad electrohydraulic actuator, a bad O2 Sensor, or a bad fuel distributor. Hope it's not the latter, they are expensive. You can also have a bad temp sensort, or a leaking cold start valve. While you have the air filter housing off, you can check the idle control valve hoses for leaks, the idle control valve is the silver colored thing about half the size of a coke can with a two-wire connector and two large rubber hoses on it. I'm guessing they are rock hard and loose... Peter _______________________________________ http://www.okiebenz.com For new parts see official list sponsor: http://www.buymbparts.com/ For used parts email [EMAIL PROTECTED] To Unsubscribe or change delivery options go to: http://okiebenz.com/mailman/listinfo/mercedes_okiebenz.com
