1993 240 head scratching 1-2-3, 2-2-4 codes

Having issues with my new to me 93 245. Coolant temp gauge doesn?t work, car is throwing ECU 1-2-3, and EZK 2-2-4, both coolant temperature sensor related. I have replaced the 2 prong sensor with two separate new sensors (bench tested confirmed working), and a new single prong temp gauge sensor. Both codes persist after clearing through OBD1. (2-2-4) is new, it was only doing (1-2-3) before I replaced the 2 prong sensor again.

Wiring has been checked for continuity, and I?ve checked the pins at the boxes. The pigtail to the 2 prong looks clean also.

Pin 13 and 5 at the ECU deliver a result around 2000 ohms, which is correct based on temperature values. This means the wire to the ECU box is good, it?s making connection, and the sensor itself is good.

I tried to simulate an intermittent connection issues by starting the car on a cold start, turning it off, and repeating those steps dozens of times. It never failed to start, it never ran weird either. Runs like a top other than obviously running a bit rich.

Though, I did unplug the sensor on purpose to see how it would cold start.... it didn?t start. Plugged it back in, started right up.

Really confused at this point... the car does have an Ipd Cam gear, as well as a bunch of OEM style vdo gauges in the dash added by the PO. Overall the car is extremely clean and well taken care of, M46 w/OD swapped...even the underside is dry as a bone.

Check terminal tension at the ECU and the sensor. Try another computer, and if that doesn't fix it, run a new circuit.

Check the ECU/EZK fuse, and R&R the main relay. Maybe you have low +12volts on both boxes. You could also check continuity on the 2nd ECU ground, pin 17.

ZVOLV said:

Check terminal tension at the ECU and the sensor. Try another computer, and if that doesn't fix it, run a new circuit.

Click to expand...

If you've stuck your multimeter probe in the sensor connector you may have spread the contacts apart so that they're not making good contact with the sensor. Advance auto sells replacement connectors.

Yep. ^^^^^^

Some guys get away with lightly probing with a meter, or gently putting a needle in the terminal. However, the only correct way to front probe a terminal is with the proper test probe. You can make your own test probe with male terminals from an old component, such as one of your old sensors.

The proper test probe is also used to do a DRAG TEST of the terminal. You slide the probe in and out and feel for proper tension on the female terminal.

There is a procedure for testing the LH connector but it requires removing the cover and combs.