MichaelBrown

OK just for clarification the check engine light is not on. If there is a different procedure for checking, error codes than pressing and holding the button and cycling through then I will have to learn that, otherwise, there are no error codes being reported.

MichaelBrown

The temp sensor that feeds the computer add the following readings at 20 K ohmCold 0.63

Operating temp 0.37

dolucasi

(1) Did you unplug it before measuring? You need to unplug it.
(2) Assuming you have the same 4 prong connector I have, can you measure both (one is for gauge and one is for ECU)
It should be like:
--1---
2---3
--4--

I made up these numbers for indexing. But in the above schematic 1-4 is for one (gauge or ECU) and 2-3 is for the other.
(3) They are both made of same thermistor material (actually all temp thermistor specs are the same for the W201).
(4) How long do you need to crank it when starting cold? How many seconds do you crank?
(5) So your CEL went away spontaneously without doing anything to clear it? Did you disconnect the battery lately? That will clear it.

The only way to read stored error codes is the push button led. You can read up on it on various forums but Kent has a good video on it and the also has a complete table of error codes.
I would highly recommend checking the codes, there is definitely something a little off with your emissions particularly running cold.
Also not all error codes will throw the CEL, so a no CEL does not mean the error codes are clear but then again, if you disconnected the battery they are gone.

A W201 should start and run super well cold, I mean even more lively than operating temp, especially in CA weather that is way over 32F. If yours is not like that we should fix it.
Also the idle racing you are having is not normal.

MichaelBrown

All right, I’ll have to re-run the test. I used the two pin sensor directly in front of the four pin sensor. I looked it up on Mercedes after sales to verify it was the sensor that fed the ECU and it said it was but I guess the four pin does as well

The 02 light is the light that will not go out. Unplugging the O2 sensor will cause the check engine light to come on but I can plug it back in and clear the codes and the check engine light goes away. I can also cause the check engine light to come on if I unplug the EHAA career but if I plug it back in the check engine light goes away.

MichaelBrown

Ok, here are the readings of the 4 pin connector. Both sets of pins are reporting the same value.Cold at 20ohm

4.24

Hot 0.25

unplugging this did generate an error code and check engine light

dolucasi

The 2 pin sensor is your sensor for the aux fan switch BTW.

Your coolant temp sensors both check out fine And the continuity of the wires to the ECU is fine too since it threw a check engine light when unplugged.

Some people use the two terms (02 sensor light and CEL) interchangeably so I was the one confused. Another reason is that my car only has a CEL, I have never seen an O2 sensor light up in mine.

The O2 sensor light you have on must be a specific code for the O2 sensor itself. The ECU is not liking the signal it is receiving from the sensor. How old is it and have you ever checked it for operability?

MichaelBrown

So a little history on the car. When I bought the car, it was actually headed to the junkyard not running, took me a few days to get it running that’s when that 02 light came on.

I replaced the sensor with a new sensor which did not fix the issue however, during those early days, the distributor was acting crazy and I have since had it rebuilt.

I suppose it is within the realm of possibility with all of the tinkering in the past the new O2 sensor became compromised. Maybe I should replace it again and see what happens?

MichaelBrown

I agree with you, I think that the O2 relay is receiving a value that it is not expecting. There’s very little info on that relay and they’re impossible to find.

dolucasi

Wish we had this info earlier. We could have shortcut some of the back and forth.

We need more details:

(1) Who rebuilt your FD?

(2) How many true miles on the engine/car?

(3) Did you put in a O2 sensor that is specifically made for your model?

190Efan

I really appreciate your efforts to try to save another 190 from the junkyard. I agree with Dolucassi that the FD rebuild might be a culprit. Non-MB parts have been known to cause all kinds of trouble. The loud racing that your 2.3 does when it's first started is also way off from where my 2.3 is when it first starts. The hunting for idle is also a concern as it leads me to believe that the ECU is receiving faulty information. At this point it's probably a good idea to recheck everything. I think all of the tinkering through the years by previous owners who likely also used non MB parts has brought us to where we are now. The problem might not actually be the O2 sensor itself but the result of other factors that then cause in the O2 sensor light to be on.