940 Coolant sensor faults 1-2-3 and 2-2-4, diagnosing

Hey guys, I've just completed a bunch of stage 0 work on my (new-to-me) 1991 940 SE, and am getting faults codes 1-2-3 (socket 2) and 2-2-4 (socket 6), relating to a fault in the ECT sensor connection.

For a little background, here is my project thread that outlines the work I've done recently: https://forums.turbobricks.com/showthread.php?t=360385

I recently pulled the intake off, replacing heater hoses and the valve, throttle body gasket, intake manifold gasket, injector seals, etc. Of course, I decided to replace the ECT (the ECU/EZK sensor under #3 intake runner) since I had easy access. On reassembly of the fuel rail and manifold, I cleaned the grounds on the intake manifold/fuel rail bolts and used some ox-gard on them.

The new sensor was an aftermarket unit bought from IPD. When installing, I followed a Brickboard FAQ tip about using a thin coating of Ox-Gard on the sensor threads when installing into the block.

Old sensor removed:


New sensor installed:


With everything buttoned back together, it took a long while to start again...numerous cranking attempts and needing to give it some throttle to have it catch and run before it would settle into an idle. Once it runs for a minute or so it will hold an idle okay. Once up to temperature, it will fire up again without an issue, so it is struggling more so when cold. CEL same on, and throws codes 1-2-3 and 2-2-4 as mentioned above. I checked the connector to the ECT to make sure it was connected and seated, cleared the codes, and they come back after it is fired up again.

I've done a lot of searching and reading on the issue, and found numerous areas to look at for diagnosing whether it is a fault in the wiring or the sensor. First step, I decided to test the grounds.

I checked resistance between the manifold grounds (two ring terminals each, under both the forward and rear fuel rail/manifold bolts) and the battery negative terminal. Both read 0 ohms. Same with the grounds on the back of the valve cover to the firewall.

So then I pulled the ECT connector, and attempted to check resistance of the sensor pins to ground. No matter what I try, I'm getting an infinite reading on the pins...checked to both the manifold grounds, and the battery negative terminal.

So before going further, I'm looking for some feedback on what to do next. Given I can't get a resistance reading from the sensor pins, does this indicate a bad sensor or no grounding to the block? Do I focus on that first, without needing to check voltage or resistance back to the ECU and EZK pins?

Appreciate any help or support on this topic! As I said, I've done a lot of searching and come across a lot of info for diagnosing ECT issues, however a lot of the threads end up with at least some kind of resistance readings from the sensor and focus on the ECU pins and wiring/grounds.

You using the grease on the threads of the sensor insulated the sensor from the engine block. This makes the sensor read infinite ohms which can cause the fault you see. First I'd measure the sensor out of the car. It should be two separate circuits that measure from the contact to the body of the sensor. Reference the temp charts online to get an idea of the resistance you should measure. A a guess it would be something like 2-3k ohm ambient air temp. Clean the threads completely and also clean the threads in the head. Then when you install the sensor you can measure its resistance again to make sure it's reading properly.

Reading infinite ohms tells the engine you are in Siberia which can flood the engine and causes the fault in the circuit.

dl242gt said:

You using the grease on the threads of the sensor insulated the sensor from the engine block. This makes the sensor read infinite ohms which can cause the fault you see. First I'd measure the sensor out of the car. It should be two separate circuits that measure from the contact to the body of the sensor. Reference the temp charts online to get an idea of the resistance you should measure. A a guess it would be something like 2-3k ohm ambient air temp. Clean the threads completely and also clean the threads in the head. Then when you install the sensor you can measure its resistance again to make sure it's reading properly.

Reading infinite ohms tells the engine you are in Siberia which can flood the engine and causes the fault in the circuit.

Click to expand...

Thanks for the suggestion, with the sensor installed could I check resistance from the pins to the brass hex of the sensor, would that tell me if the sensor itself may be okay but its simply the grounding to the block? And regardless of why it is returning an infinite reading, the original process I described in measuring from the pins to the grounds should be returning a resistance if the sensor and grounding was working, correct? I just want to confirm that I am not taking the measurement incorrectly.

I thought the Ox-Gard grease, which is conductive, was supposed to help ground the sensor to the block, not insulate it. For reference, on this page under "installing the sensors" referencing the coolant temp sensors, is where I read the suggestion to use the Ox-Gard.

LH2.2 measure pin-to-pin. LH2.4 measure between each pin and ground (body of the sensor)

ZVOLV said:

LH2.2 measure pin-to-pin. LH2.4 measure between each pin and ground (body of the sensor)

Click to expand...

Exactly. So, yes you can check the sensor itself by measuring from each pin to the sensor brass body. One terminal goes to the ECU, the other, the EZK.

This tech article on the brickboard should help you get it sorted out.

https://www.brickboard.com/FAQ/700-900/EngineSensors.htm#DiagnosingECTFailures

So I checked the resistance between each pin on the sensor itself (still installed in the block) to the brass hex of the sensor. Still showing infinite.

Just to double check my meter, and that I was probing the right things, I grabbed the old sensor that I removed and tested it on the bench (roughly 20?C ambient). I get roughly 2400k and 2550k between each pin and the brass body of the sensor.

So, this tells me the new sensor must be faulty, correct? The pins don't appear to be grounding to the body of the sensor.

Yes, that new sensor can go in the bin. Your old sensor measures correctly and you can still use it. I too have received a brand new sensor that was defective in the last year. FYI these sensors are negative temperature coefficient and you can measure the resistance dropping as engine temp warms up. Another good way to test is to measure the applied voltage and watch that as the engine warmed up. Cleanflametrap has a good writeup on measuring the voltage on the sensor.

Correct. The sensor is dead right out of the box. That is not all that uncommon these days. Check everything before installing the part these days if possible. I've had dead sensors, relays, starters, alternators etc right out of the box. I couldn't check the alternator and spent over an hour installing it on a V90 and only knew it was dead when I started the car. That was a Bosch alternator rebuilt by Bosch, right out of the box. It wouldn't have been so bad except the customer brought her car in and waiting while I did the install and had to pick up her 1 year old son from day care.

Thanks guys, looks like I'll start pulling the manifold off again and put the old sensor back in. I've seen some suggestions that you can get the sensor replaced with the proper deep socket, but with the cold start injector it was tricky just to get the multimeter probes on it!

That'll be the last time I decide to replace something that is working fine just because I have everything around it apart

When you use your meter to check out the temp sensor. You can go to the ecu and Icu. Remove the connectors and you can measure on the connector. That way you check the wiring at the same time. This is also what to do when check the resistance as the engine warms up from cold. Check it cold then connect computers. Disconnect, then check warm and then reconnect, and finally hot up to temp.

If you're going to reuse that old sensor clean the thread sealant from the threads.

krymarchuk said:

Thanks guys, looks like I'll start pulling the manifold off again and put the old sensor back in. I've seen some suggestions that you can get the sensor replaced with the proper deep socket, but with the cold start injector it was tricky just to get the multimeter probes on it!

That'll be the last time I decide to replace something that is working fine just because I have everything around it apart

Click to expand...

I have changed many coolant sensors on my 88 765 with B230FT. Never removed the intake manifold. Of course mine doesn't have a cold start injector.

I got the sensor changed out again. Thanks lummert, yes I did clean off the old sensors threads before re-installing. Car starts great again and throws no codes.

I did pull the manifold off again, I just could not get at the sensor well enough with the cold start injector's fuel hose, and the injector itself, in the way. I do not have very small hands, so that wasn't helping.

All in though, considering I had just taken the manifold off and installed it again, everything was fresh in my mind. I kept the throttle body and cables all attached, and just pulling the manifold back a few inches was more than enough room to get the sensor swap done.

Thanks for everyone's help! Bummed that the new sensor was faulty, but I learned my lesson about installing new sensors without testing first.

Glad you got that figured out. Keep on rolling!