OM601 Temperature Sensor?
 

Curious if anyone knows the resistance value for the sensor in the head on the OM601 2.2L for the 1985 190D. I’m trying to chase cold idle control that seems to not be working right.

I think it may be this sensor.

https://i.imgur.com/47TAUm2l.jpg

At 80C water temperature-ish, it is metering at 58-60 Ohm.

I only have a 602/603 FSM for diagnosing this, and it seems like the sensor isn’t right.

https://i.imgur.com/zb9OP2dl.jpg


FSM would imply I should see a few hundred Ohms. That certainly could trick the ELR to think it’s hotter than the engine is.

FWIW, when I check voltage at the Y22 (for the idle control) I get 5V while FSM implies I should see 12v.

The car idles reasonably well when hot, even in drive with ac on. But when cold it seems that the good is going to shake off. My w123/om616 is also rougher than my other 5/6 cyl diesels, so there’s something to be said about 4 cyl diesels being rougher. But I don’t want to use that excuse. Something is sporadic about the ELR at minimum.

Thanks!

Less research if you just use a 4k pot and find the value where the engine changes rpm. Then see if the temp sensor is similar.

Sure, but if it is wrong or failing, I need to replace it anyway.

I like the idea of running a pot to see if its operation invokes a fast idle though. Do you have a recommended model?

The problem is not electrical. It is mechanical IMHO.

I would do these tests and get a baseline. Pull the connector and run the engine, simulate hot. Gound the connector and run the engine, simulate cold. Plug in and run the engine. The engine probably will run the same under all 3 scenarios. Good luck.

I’m not going to disagree.

When I started the car last night after sitting for maybe an hour after a long drive, it started and RPMs were too low:

https://i.imgur.com/8bCZqXhl.jpg


Blip the throttle, and it relaxed to call it 720 which is correct per the underhood sticker:

https://i.imgur.com/E8ZTJ9wl.jpg

Which would imply something mechanical. Thing is, if I mess with the linkages under the hood, the only thing I can do that impacts a change is push things in the direction that opens the throttle. Which isn’t the right path with an ELR. The ELR should do that.

I do think the sensor lead I mentioned yesterday and showed in the photo above is for the gauge, not the ELR/EDS. That one is one cylinder forward per the EPC. So I’ll check that one cold if I can.

One thing I can’t grasp is how much vibration is normal for these?

I own a lot of MB diesels. Both 4 cyl are rough at idle even with new mounts. All my 5/6 cyl diesels are smooth as can be.

That said, low idle doesn’t help…

I found a bit more. I was correct, the temperature sensor I showed above is for the cluster gauge. The one for the ELR is further forward.

https://i.imgur.com/RD8EPeql.jpg

https://i.imgur.com/gbOFPeSl.jpg

https://i.imgur.com/23ceXsJl.jpg

I found the sensor is a bit off. It measured 1.353kOhm at 44F (6C) which is off of the FSM chart (granted it’s the FSM for 602/603 but I doubt it’s different)

https://i.imgur.com/zb9OP2dl.jpg

I also found that the connector for the temperature sensor did not show 5V per the FSM.

https://i.imgur.com/G3Pz6Wol.jpg

https://i.imgur.com/UXbdJI3l.jpg

So the temperature sensor is not working in the circuit.

That said, the idle control is working.

Full cold start, two prong control connector removed, the car idles below 500.

https://i.imgur.com/4aFvJ92l.jpg

However to the point that it is mechanical, giving some throttle returns it to much higher.

https://i.imgur.com/rvVidiql.jpg

I did check the two prong Y22 connector cold and hot, cold it shows battery voltage (13.9V), hot it shows about 5V.

I also gave straight 12v (via a fused connection) to the two prong connector on the disc, and the ending jumped. It definitely revved the engine.

https://i.imgur.com/84SYsfFl.jpg

So more to figure out mechanically and electrically. Can anyone verify which relay is N8?

https://i.imgur.com/kInHd7Bl.jpg