KE-Jet TPI feedback pot question
 

Working on fine tuning my '88 300SE after a complete fuel system rebuild. The question I have involves the TPI pot. on the fuel distrubitor.



When looking at the feedback pot from the front of the car, on the drivers side of the pot, there is a very small bress screw head poking out.



What does that little screw adjust?


I have the pot adjusted so that with the key on, engine off, I get 0.8 vdc between terminals 1 & 2 of the pot.. Watching the videos from our Russian friend, the target is 0.7 VDC +/- 0.1 vdc. Is that small screw for fine tuning?

It's not clear to me what you are talking about. I'm not aware of any adjustable potentiometers on the KE system. The device with an electrical connector at the end of the throttle shaft is just an on/off switch to tell the KE system and EZL ignition module that the engine is idling, and I believe it has rotational adjustment, but I've never had to adjust mine. You can check that it functions via the three pin connector above the inlet manifold near the #3 runner.

There is a basic mechanical adjustment for the KE system. You'll need a meter with a duty cycle function. Look at the following link.

https://www.landiss.com/mixture.htm

Duke

I don't think so. I think the only adjustment is by loosening the screws and rotating the device. I have never done this so please don't take my word for it.

https://shop.carservice-kueppers.de/bosch/sonstiges/28698/potentiometer-3437224035

See this image.

Note the slots rather than circular mounting holes. They should provide room for adjustment, er, I think.

RayH

Do you note the three pins for the electric connections? Just to the left of thode three pins is a small screw. That screw is recessed into a small de[ression.



What is that screw used for?

Well, I put my extra A/F feedback potentiometer on the bench and tested that little screw.
With the pot in hand, between pins 1 & 2, or between 2 & 3, there is no resistance. This is to be expected as pin 2 goes to the ECU through the 'wipers; built into the air housing.
Between pins 1 and 3, I got 1,789 ohms resistance. By turning the small screw in question, you can fine-tune the resistance of the feedback pot.

So if my brain is still sorting all of this out, I'm shooting to 0.7 VDC across pins 2 & 3 of the feedback pot, installed. This is ignition ON, engine OFF. Hopefully, I can pull the 0.8 VDC back to the spec 0.7 VDC. With that little screw, you can tune down to 1 ohm.
*

~.7V idling hot, ac off, neutral

That's what I'm shooting for. ;) Trying to "tap" the housing seemed OK for a rough setting but I knew there had to be more.


Right now my AFS at rest gives me 0.8 VAC. When the O2 sensor kicks in and Lamda starts hunting, the RPM swing seem to increase. I'm hoping the getting the 0.7 VAC will calm this down some. With the ICV disconnected, the engine starts and runs in "limp-home" mode steady as a rock. So I'm accepting that the O2 system is fun****ing as advertised..

Doug

This adjustment is done while the car is running.

Make sure the you are reading dc volts. not VAC. Also, don't worry about the o2 now or revs.

Just focus on setting the potentiometer to .7 volts dc +/- .1. Measure pin the top pin and center pin. (pull out the connector 1/4 inch so you can get to the pins.) Car at warm idle 80C and AC, radio, lights etc. turned off. It will take some fussing and patience because when you have it set, then when you tighten the potentiometer screws down, the voltage changes.

Hope this helps.