Jeep: 2000 Sport, 4.0, Auto. 225,000 miles. Stock

Background: This jeep doesn't belong to me. It's one of the Marines that work with me.

1. CEL is throwing P0118 code, engine coolant temp sensor, high voltage. When code is reset, it immediately comes back upon start up.

2. Temp sensor has been replaced along with t-stat and it's housing.

3. Tested sensor warm, measured 225 ohms, normal operating temp 725 ohms

4. When we started engine, e-fan immediately came on. Dash temp gauge stays around 130ish, fan turns off and it immediately shots up and reads right under the 210 line.

5. Checked on wiring and found nothing abnormal. Checking wires I notice in plastic channel that the brown/yellow wire feeds into either a connection or a diode and 4 same color wires coming out the other side. ( can anyone verify if it is just a connection?) I checked for continuity between plug end and that wire prior to connection (good) then check for continuity between before connection and all four on other side. (no continuity)

6. Jeep runs fine, no issues, no overheating.

Where the heck do I go from here? Any other tests I can run? Should I try to reset ECM? Everything seems to be normal other than the CEL will not stop coming on with P0118 code. His inspection is at the end of this month and any ideas will be appreciated.

 

There's a diode spliced into the wiring harness??? There should be nothing between the sensor and the PCM.

 

I don't think it's a diode but it's definitely wires in and Four coming out.

 

I didn't install the new t-stat but if it was put in backwards would it cause the fan coming on during start up? Would it cause issues with the sensor?

 

Backwards t-stat would mess with your coolant flow for sure, and you'd get an appropriate reading from the temp sensor... if it was wired properly.

At this point you need to figure out what the hell happened to the wiring. All of this stuff is computer controlled based on input from sensors. If someone's been in there hacking **** up your troubleshooting efforts are kinda going to be like watching Nietzsche argue with the Pope. Not going anywhere lol.

 

The wiring hasn't been messed with. The same connector is in my harness and it looks the same. It's in the plastic channel and looks like just a connection. There should be continuity between the sensor plug and one of the pins that connect to the ECM next to the air cleaner, right? Which pin, if so?

 

Checked my 2000 FSM and there's nothing listed between the ECT sensor and PCM except for a couple of splices on the ground path. These appear to be in that channel that runs along the fuel injectors.

Pin 2 of the ECT (right side looking at the plug face) should run back to pin 16 on the PCM's C1 (the black plug). Tan/black wire. The ground pin of the ECT sensor runs to a splice in that harness which is a brown/yellow wire. It hits another splice in that channel before going back to PCM C1 pin 4

I'll see if I can post a pinout for you.

 

That's splice S114. Several Sensor Ground wires (Brown/Yellow tracer wires) are soldered together at this splice.

Below is a pic of the ground network for the sensors (schematic is for a '99 XJ but the 2K and 2K1 network is similar). As you can see, there's several sensor ground circuits at splice S114.





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I really appreciate the posts/help. I thought it was just a connection. There definitely should be continuity on either side if that's the case. Since that is ground wire wouldn't that trip the code?