Can't get fault codes on 87 420SEL
 

Help! I've read the artical on this site pertaining to checking the fault codes with an oscilloscope. I have the MB service manual and it eludes to an "X92" connector near the battery on the passanger side of by the firewall. I cannot find any such connector. The only connector I have is a round connector on the drivers side fender. I've connected the scope to pin 3 and ground clip to positive on battery and the ignition turned to on (just before cranking) All I get is a constant square wave 12V. Even after grounding pin 3 for 2-4 seconds. Am I using the wrong connector? Have I got it connected incorrectly? I'm trying to use the scope so as not to spend the $350 for the code reader.

The only time this signal changes is when I start the engine, then the square wave slowly becomes a flat line as the engine warms. I think I'm see the on/off ratio readout of the lamda system. Any help would be greatly appreciated.

use "SEARCH" to pull up a thread on code reading done a couple of months ago. There is an excellent article on making a cheap led code reader in one of the threads and what pins to use. The problem will not be getting the codes with the scope or homemade reader - but finding out what the codes mean. Once you find the codes - post your codes on this site and someone with the book may be able to post the meaning of the codes you get.

The only diagnostics on your car is the off-on ratio read out that you are looking at. To read it properly you should put your scope in the duty-cycle function and read the duty-cycle. A proper working car will have the off-on ratio (duty-cycle) varying back and forth in feedback (lambda) control. A proper working AND adjusted car will have it swinging about 50%.

If you have a fixed duty-cycle at even multiples of 10 (10, 20 ,30 ,40 etc.) then you have a current fault displayed. Fault memory is not maintained and if a fault goes away so does its indication.

Hi Steve
I saved some of the thread I talked about that pertained to my car.
It states:"The example I will use is an 88 560sel a/c system as that is what started this whole thing.
locate x11/4 connector(on the firewall near mono valve)
Plug the black connector in #1 and the red into#7,with the key "on" touch the exposed end of the wire to ground for three seconds and release.the led will blink any where from 1 to 57 times.
One blink means no fault,if it blinks more than once write it down
check for the next code,keep doing this until the first code is repeated.
Touch and hold the wire to ground for seven seconds to erase codes.
Instead of listing all the different codes on this forum I recommend that anyone intrested in using this tool order the DIAGNOSTIC TROUBLE CODE REFERENCE GUIDE through M.B. publishing."
Would not this fellow's 87 420 have the same circuits as 88 560? Please advise if they are different or am I confused as the above quotation refers to AC codes??? If I was confused I want to apologize to Jherrera for the advise I gave.
Thanks
Maurice

Maurice, no appologies required. Both you and steve are helping me out, and I really appriciate all your help. This is my first MB and I'm still learning the ropes. I was totally confused by the MB service manual referencing the X92 connector and I can't for the life of me find it. Any clarity on this subject would be very helpful, even if it means no fault code reading capabilities. Thank you both very much.

Jorge.

Your advise is fine for the systems that it applies to after the onset of onboard diagnostics.

Unfortunately this one squeeked in under the wire. There are NO impulse diagnostics on ANY system untill 1988. ABS didn't get onboard diagnostics until 1992 and never appeared in 6cyl 124 cars (did appear on 124.034 and 036 in 1992)

Thanks Steve - I got it now!!!:)

Knowing that it doesn't have impulse fault codes saves me a lot of time looking for connectors that don't exist. So I've worked with the on/off ratio and this time I used a dwell meter (in the absence of a DVM with duty setting) and I see the same thing I observed with my oscilloscope. I get 40% for about a minute and it steadly decerases to 0% over aproximately 30 seconds. Now what I read in the service manuals tells me that Lamda should be aproximately 50% after warmup. I'm thinking of replacing the O2 sensor and trying it again. Can you suggest additional or an alternate course of action? I never stated the reason I'm running these checks, which is hesitation on part throttle acceleration from idle. Additionally, the spark plugs are white which indicates lean mixture. Full throttle acceleration is OK.

Jorge.

The first thing to do is to disconnect the O2 sensor. In this position you should read 50% constantly. If you do not get this we need to calibrate your technique or pursue the electronics.

The O2 sensor is only a sensor if the car is off scale it is not likely that its the sensor. Don't blame the messenger for the news he brings.

I disconnected the O2 sensor and it did go to 45% dwell which is 50% duty cycle. I let it run for 15 minutes and it was rock steady at 50%. I grounded the connector to the O2 sensor and it slightly moved, then I connected a 1.5V battery with the ground on Chassis and the positive on the O2 connector. It immediately started going down to zero duty, just as it had with the O2 sensor. Looking at the schematic, the O2 sensor connects directly to the CIS-E control unit and that doesn't leave me with much to check. Any ideas? I'm leaving replacement of the CIS-E unit for the very last resort. It would be great if my testing method was wrong, but the fact that the oscilloscope and the dwell meter agree, somewhat leads me to believe my method is probably OK. Ideas?

You are doing the right things so far. Double check the action when you ground the lead. It should take the reading off scale in the opposite direction as the voltage method. BTW a simpler way of biasing the O2 signal positive would be to place one hand on battery positive and the other on the sensor lead to controller.

If you get response to both electric signals, I would suspect that you have too rich of a mixture. The on-off ratio is the result of the compensation necessary to move the mixture to lambda. If it is not 50-50 then the adjustment is wrong. Lean up the mixture.

Steve,

Steve, you had it right on the money. The fuel mixture was way off. I adjusted it while monitoring LAMBDA with the dwell meter and it swung right back to 50% and it would very between 45 and 55% depending on what it read from the 02 sensor. I couldn't have done this without you. My hobby is maintaining my own cars, but my profession is computer networks, so if you ever need any help with networks don't hesitate to drop me a message.


Jorge.