The Case of the Vanishing Temp Sensor

[BoundForGlory]

I am beyond lost on this one! 
Just a new guy trying not to burn his truck up! 
A little backstory that might help things, otherwise I’ll put a TLDR at the bottom

I’ve got my first Comanche, which I bought over spring break, but haven’t had time to dig into for a while. The truck has a 4.0, ax15, transfer case, and axles all recently installed out of a 1991 xj, while the truck itself is a 1989. This wouldn’t be all that complicated if we were running the 1991 system, but nope! We’re on Renix, baby! So 1989 CPS, and everything is hooked up to that.
Some other parts from a renix system are left over too, specifically the thermostat housing, so there’s nowhere stock to hook up a temp sensor. Despite this, the nickintime design renix engine monitor that came on the truck shows me a temperature readout, which starts at air temperature, and then creeps up to 248 degrees (the max temp it can read) and stays there, but the engine doesn’t boil over, making me guess a faulty sensor.

Where this reading comes from, I have absolutely no clue. I have only found one temp sensor in the engine bay, and it’s on the radiator (open cooling system conversion by the looks of it).
Here is a picture of the wires that it connects to (yellow and grey). They break out of the loom in the front, drivers side corner.

But when I unplug it and turn the engine on, I still get a readout, which rises as the engine gets warm. No dice.

As I looked around on the engine, I did find another mystery wiring harness break out that looks like it should hold a sensor, but if that’s the input, then how is ecu getting a temperature readout?! 
I did a little testing, and when I plugged in a spare, cold sensor into this plug on a warm engine, the sensor Renix monitor continued to read 248, so clearly this plug doesn’t effect that ecu, it’s probably for the efan? Which has never turned on for me.
here is a picture of the mystery plug, on the drivers side of the engine block:

And a picture of the wiring coming from the plug:

The thermostat itself is pretty much new, and a leaky radiator verified for me that it is working (radiator only leaked when engine got warm). So, I’m pretty confident that my cooling system is good to go, but I could stand corrected. The truck survived an hour drive without boiling over, and the idiot light never came on for temperature, which I think is hooked up, but I could stand corrected on that too. 

 

 

TLDR: 

1991 engine in 1988 Comanche, running a renix ecu, can’t find temp sensor, only one in engine bay seems to be on radiator, plugged into harness, but doesn’t affect the ecu. Readout on engine monitor starts off accurate, but quickly climbs to 248 degrees and stays there. Electric fan does not come on, may or may not be hooked up. Idiot light is hooked up but does not turn on even on 15 min drive. 

Any ideas as to where it could be? Could the Renix engine monitor be faulty? 
Do I just trust the idiot light and forget about it?

[fiatslug87]

In an HO the temp sensor is in the thermostat housing, in RENIX it’s at the driver side rear of the head next to the valve cover. The renix wiring isn’t long enough to reach the tstat housing.

Please add it to the registry when you get a chance. https://comancheclub.com/forum/32-1989/

 

[BoundForGlory]

I’ll get er added to the registry when I have a little daylight tomorrow!

As for tonight, I couldn’t help myself from trying unplugging that sensor to see if anything would change on the monitor, and, nothing. The sensor reads 69 degrees still, same as air, and warmed up to 75 degrees after a few seconds of idling.

Is that sensor for the idiot light or the ecu? Because I’m concerned about the ecu readout that I’ve got, which I thought was supposed to come from the engine block, but of course, HO block, there is no place for one.

 

I’m just completely lost as to where

this readout is coming from, maybe it’s just a weird renix thing that is causing my monitor to freak out because no sensor is installed in the block location? I’ll check out some more stuff tomorrow, but for now I’m at a total loss

[fiatslug87]

Sorry, I brain farted. You are correct, the ecu temp is from the sensor in the block. I would assume HO also has the sensor in the block. @Gjeep may help.

[Ωhm]

Check out Pg78. (Note the Note)

 

Jeep_Renix_Fuel_Injection_manual.pdf

 

 

 

 

10 hours ago, BoundForGlory said:

here is a picture of the mystery plug, on the drivers side of the engine block:

 

What are the wire color codes for this mystery plug? Looking for TAN & BRN W/TR.

[Gjeep]

HO era has temp sensor/sender back of block just like renix era. This changed/ disappeared OBD2 XJs.

 

[Eagle_SX4]

Renix era 4.0's have 3 separate temp sensors. The one in the radiator is the e-fan control switch. The one at the back of the cylinder head is for the gauge on the dash. It could be a switch for the dummy light or a sensor for the temp gauge. The one under the intake manifold towards the front of the engine is the one the ECU and the REM use for engine temp. This is the one your are looking for.