Never goes closed loop
 

1989 260E Federal.
Car never goes into closed loop. No current through the EHA and O2 sensor does not oscillate. The O2 sensor was replaced and is good. O2 voltage moves up and down when I create a forced lean or rich condition. EHA shows 20ohms which is good. No duty cycle or frequency measured off of pin 3 of diag connector.
- I have a good OVP (fuses good too). Also checked with test lamp that the CIS is getting good power and ground.
- The check engine light comes on for 3 seconds as is normal during KOEO.
- I opened up the CIS case and nothing smells burnt, no cold solders and no obvious burnt components or traces.
- O2 signal is getting to the CIS computer but no closed loop (feedback) operation.
- With KOEO the O2 signal is 0V and it should be .4-.5V.

Please help.

Check fuses for power to ECU.
Spin them in their holders or test each one with a test lamp on BOTH sides to ground.

ECU fuse is the OVP fuse i thought. I opened the fuse box and none are labeled ECU or anything close to that. All fuses look good in the under hood fuse box. Any other suggestions please?

Looks good means nothing on Benz fuses..you want to test each side w/test light.

I will see if I can bring up schematic..........

I just tested all the fuses yet again with a test light. probed both ends with test lamp. They are all ok. In any case none of them are labeled for ECU or anything close to engine management. I thought it was the OVP that powered the CIS. Also please note that i tested the connector to the CIS for ground and power and it was good, power is getting to the ECU (CIS).

Fuse 7 OK ????

all fuses are OK. Fuse 7 as well. It's for climate control and backup lights.

According to my schenmatic for 88/89 Fed -124.026 #7 is coil feed for OVP.
Pin 1 of N3 have 12v ??

Thank you so much for your research Arthur.

Pin 1 on the 25 pin CIS connector (labelled N3 on the schematic) is 12V (batt voltage) and is powered from the OVP. The CIS is getting power and ground.

I also see the diagram that shows fuse 7 going to the OVP coil. Oddly it is not labeled in the fuse box as such.

I confirm, fuse 7 is hot on both sides, fuse holder is clean and pin 1 on the CIS is seeing power. Tested with test lamp.

OK

Test from bat + to pin #2.

And then check ohms at B11/2 sensor. [ Gn/Rd terminal to ground ]

Looking at another angle... Are you running NORMAL water temp? I have had a few American cars not go into closed loop because of a faulty engine coolant thermostat. They were running too cool.

Bat + to pin 2 lights up test lamp. So I have good ground on CIS pin 2.
B11/2 coolant temperature sensor has two pins with wire Gn/Bk and Gn/Rd attached to it. Ohmed it across the two pins of sensor. 4kohms.

Arthur did you also say to ohm the Gn/Rd wire to ground?

Running at normal operating temp (>80C)

Open between Gn/Rd wire at B/11 connector to ground.

<<Arthur did you also say to ohm the Gn/Rd wire to ground?

No.. i mean for you to ohm the sensor from it's terminal that the gn/rd wire hooks to ground.

That sesnor is 2 sensors in one, with each terminal using ground as a common..so, each terminal to ground should read the same value.
The gn/rd wire from the sensor will be the temp input value feed to the N3 module . The other side of that sensor is temp input value to ign module.
[ ICU]

Here is a trick
Take the green wire that goes to N3 and disconnect it from the 02 sensor at the 02 plug ..[ under rug, where it goes from Blk to Gn]
Now, you can give N3 an input from approx zero to 1 volt by holding that wire in one hand and putting you thumb of the other hand on the bat + terminal ..on/off with the thumb will mimic 02 sensor input to N3.
Try it.