Air / Fuel adjustment - Another way

[DAVID LEE]

A few weeks ago I had to replace my Air Flow Sensor Potentiometer and while trouble shooting my issue I did some O2 sensor tests. This was the 1st time I had ever messed around with looking at the o2 sensor voltages and I found it rather interesting. Unknown to me, I screwed up something while trying to take voltage measurements directly off of the wire under the passenger floor mat. Anyway, the result was that no matter how I adjusted the fuel distributor the EHA was always trying to drive the FD richer.

I finally got tired of doing troubleshooting on the system and adjusted the FD to a "runs good" setting and let it go thinking that I would eventually take it to a mechanic, or get back to trying to fix the problem myself. During this time of troubleshooting I had read many posts, Bosch "how it works" papers, the service CD, etc. so I was becoming pretty familiar with not only my car, but the k-jetronic system as well.

Well, Saturday I decided I would give it "one more try". So...after getting out my DVM and removing the floor mats I "remembered" something that I had "done" that eventually led to fixing the issue. During the time that I was looking through the Service CD, I had noticed that the cable from the o2 sensor to N3 was a coaxial cable, but didn't think too much about what I had observed in the schematic diagrams. But....Saturday the thought occurred to me "I wonder if while trying to take the o2 sensor voltage measurements I had "shorted" the sensor cable's coax sheathing to the actually conductor that was carrying the voltage signal from the o2 sensor to N3. After disconnecting the o2 sensor conductor from the coax cable, I noticed that my o2 sensor voltage returned AND the varying duty cycle at X11 returned. But when I plugged the two connectors back together, the X11 signal returned to its "let's get this thing running richer" reading.

Well, I fixed the coax cable and when plugged back together with the o2 sensor conductor, things started working again. I still needed to make my FD adjustment and so I wondered - If I unplug the o2 sensor from the conductor going to N3, and disconnected the cable going to the EHA could I agjust my FD setting so that I would achieve the same results as using the "measure duty cycle at X11", or the "measure current at the EHA cable" method.

You are probably wondering "why would he do that to begin with" - and that is a good question. By measuring directly at the o2 sensor (while it is disconnected from N3) and by adjusting the FD setting, I should be able to get the FD setting to a point that is "exactly" at 50% AND then confirm that the rest of the system, once reconnected, is working as it should. This procedure confirms that the o2 sensor is working, and by plugging back the o2 sensor and the EHA adapter cable it confirms that all systems are individually working. This trouble shooting process also taught me that the o2 sensor does not need any pullup voltage in order to produce an output voltage (I suspected that, but was not sure), and that the voltage produced when disconnected is accurate and representative of what is happening from a fuel / air mixture perspective.

So...that is exactly what I was able to do. I would slightly tweak the FD setting until I got the o2 voltage reading to go from 0 to .8 volts - meaing I had gone from a too lean mixture, to a too rich mixture. I don't believe it is possible to get a "perfect" setting. Once I achieved what I thought was a setting that was slightly rich, I then plugged the o2 sensor back to the coax, and plugged the EHA connector back and then went to X11 and measured the duty cycle at pins 2 and 3 and just as I had theorized, the duty cycle reading was just a tad rich - just where I wanted it.

Perhaps this procedure will help someone trouble shoot and perhaps not. In either case, I had quite a "high" just from figuring this out (my wife was totally unimpressed) and I guess that is much of the reason why us guys work on cars to start with - we simply like to "figure things out".

if you read all the way through this thread, then I trust you will find some value and application.

Regards