Worked on the Grandson's 1990 240 DL this weekend. Got it idling sweetly right about 750 RPM (thanks for the tips Hoonk). That's the good news......the bad news is I got a check engine light (yay!!) Read the codes and in Socket 2 I got code 1-2-3, 2-1-2, and 3-2-2. Socket 6 gave me 2-2-4. The 1-2-3 and 2-2-4 point to the Engine Coolant Temperature sensor, which is weird as I have checked the wiring and put in three different sensors and still get the code. The 2-1-2 points to a bad O2 sensor or wiring, and the 3-2-2 points to a Air Mass Meter wiring burnoff signal faulty.

Ok, Here is the link to the wiring diagram I used: https://www.davebarton.com/pdf/Harne...ctions2021.pdf For the 1-2-3 and 2-2-4 I'm at a loss as I have tried 3 ECT sensors and checked the wiring (all of it metered out good), but it still pops up. For the 3-2-2, all the MAF wiring checks out just fine so I'm assuming I have a bad MAF. The 2-1-2 is giving me fits. I checked the wiring between the O2 sensor connector and the ECU connector, and fuel pump relay socket with the relay unplugged. The single green signal wire checks fine between the O2 sensor connector and pin 24 of the ECU connector. On the two pin O2 Sensor connector, the brown wire checks fine to ground, as well as pins 5, 19, and 29 on the ECU connector. However, the red/yellow wire on the O2 sensor connector checks fine to the fuel pump relay connector, but it also reads 0 ohms to ground. Lastly, both pins of the O2 Sensor connector (firewall side) show continuity with each other.

It appears I have a short to ground, but I cant find where. Any ideas? I doubt this is normal, but if anyone is willing, could you unplug the O2 sensor connector and see if the two pins on the firewall side of the harness are shorted together or both read to ground?

Many Thanks for any help

 

On an early 3 wire O2 sensor - the 2 wire plug is for the O2 heater, and the single wire is the sensor signal. Should have 12volts at that two wires coming from the car/ECU. When disconnected the two black wires for the resistance heater should have close to 0 ohms resistance and should not be grounded to the outside of the sensor. With a hot running engine (in closed loop) you should find a sine wave on the unplugged signal wire. A small voltage fluctuating from .2-.8 volts will be seen on your Volt-ohmmeter. The voltage range is ~0-1.2 volts where 0 is lean.

For fun you can make the system think it's lean or rich to watch the voltage change. (and verify the O2 sensor responds properly to rich or lean conditions) With the signal wire disconnected and voltmeter connected between sensor and ground - ground the male connector to the sensor (green wire from the ecu) to the valve cover - watch voltmeter change (to 0 or 1.2, can't remember). Then grab the male connector with two fingers and touch the +battery terminal with your other hand (that sends a small voltage to that wire through your body) - watch voltmeter change.

Temp sensor codes - 2 temp sensors in the head. Single wire sensor near the front of the head is for the gauge only. The 2 wire sensor under #3 intake runner is for the fuel injection.
Which sensor are you replacing? - The fuel injection temp sensor is not easy to get to - so I can't imagine changing that 3 times! (there are 2 different color FI temp sensors, they are not interchangeable)

 

Yes, it is definitely the ECT, and yep, I'm dumb enough to replace it twice. You are right, it is a royal pain to get to. Anyway when we first got the car it had a reccuring code for a bad temp sensor so I replaced it. Sure enough after replacing the sensor the code was still there, so I pulled the plug off and metered out the two wires and they checked out fine, so I figured I got a bad sensor, picked up another one and tried again. Same code again.......Well, looks like I need a new MAF and possibly a new O2 sensor since it sounds like the wiring is ok. Once that is taken care of I will dig some more into the ECT fault......

 

[QUOTE=Psaboic;529268]Yes, it is definitely the ECT, and yep, I'm dumb enough to replace it twice. You are right, it is a royal pain to get to. Anyway when we first got the car it had a reccuring code for a bad temp sensor so I replaced it. Sure enough after replacing the sensor the code was still there, so I pulled the plug off and metered out the two wires and they checked out fine, so I figured I got a bad sensor, picked up another one and tried again. Same code again.......Well, looks like I need a new MAF and possibly a new O2 sensor since it sounds like the wiring is ok. Once that is taken care of I will dig some more into the ECT fault....

Also, when we got the car it had a black connector ECT so that is what I have been replacing it with....wonder if I need a blue connector one??

 

Are you erasing the codes and driving the car to see them come back?

Or are you assuming that by replacing a part the codes will go away? Erase the codes, drive then deal with the first code that comes back.

(there's only a few codes that will self erase, and that's after x number of drive cycles where the code is not detected - maybe only a gas cap code)

Test the O2 sensor as described earlier before replacement- you actually don't need an O2 sensor, the sensor can only change the mixture very slightly and the car will run just fine without it - but it will set a code if unplugged on a 1990.


Your car takes the black 1346030 temp sensor.

 

Yes I'm erasing the codes and they come back. Thanks for confirming I have the correct ECT. Going to check the ECT connector again. I cleaned it with contact cleaner and a toothpick earlier.

 

Another of your posts discusses your ECU - are you certain you have the correct ECU in the car?

 

Yes it is the same part number (0280000561) as the one I took out. The old one is a pink label, the new one is a white label. For the record the car starts fine and runs and drives well, just smells like it is running rich, plus the annoying check engine light.....

 

Voltage from the O2 sensor will give you a hint if it's rich or lean - (as long as it's really working)