TECHLINE
COMPILED BY VERNE SIMONS EDITOR@FOURWHEELER.COM
MYSTERIOUS SENDING UNIT
I have a ’97 Wrangler TJ with a 4.0L that developed an antifreeze leak on the driver side of the block. Upon further investigation, we found the leak was coming from a sending unit (it looks like a pressure sensor) attached to the block with a 90-degree elbow. I believe the 90-degree elbow was added to keep the sensor from hitting the exhaust pipe. We took the pressure sender out and plugged the 1⁄8-NPT hole in the elbow, which solved the leak problem. The pressure sender was attached to a seemingly factory wiring plug that went into the wiring harness above it. The question is: What is the switch/sender for? I have looked at various wiring diagrams and can’t find a coolant pressure sender on it. I even contacted a national call-in show who replied that they had replaced over 500 Jeep 4.0Ls and never seen a coolant pressure sender. Even the local auto parts store doesn’t have a listing for a coolant pressure sender. The sender that came off looks to be a PS284 sender but is denoted as an oil pressure sender in the literature. I have read about dealer installed A/C systems and think this sender was used to turn off the compressor in the event of an overheating condition. The only time I have seen a water pressure sender is on my jet boat. I have looked at various Jeep blogs and can’t seem to find any reference to a water pressure sender. The engine didn’t throw any codes when the sender was removed and seems to be running fine. A real head scratcher! Your knowledge would be appreciated. DAN VIA EMAIL
I agree with your assertion that what you showed us looks like an oil pressure sending unit or switch, and how it got into the 1⁄8 -NPT port on the side of the block with an elbow is a mystery … and the wiring that you say goes into the harness is a real and true mystery. I had an initial gut reaction, and right or wrong, I am sticking with it. I’ve been wrong before, and by golly, I will be again!
But first, know that I did ask a few other well experienced Jeeple who might have some insight into this odd situation. One such person is Trent McGee, who is my pal and knows wiring (he runs Wicked Wiring in Glendale, Arizona) and he knows ’97 Jeep Wranglers well. He has a ’97 TJ buggy with a 4.0L and has spent time with it and a factory service manual (FSM) paring down the factory
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78 JUNE 2022 FOUR WHEELER
wiring, removing stuff not necessary on a buggy. He confirmed my suspicion that there is no indication of any kind of water pressure wiring in the FSM. His conclusion is the same as mine (below). Chris Durham of Chris Durham Motorsports in Pickens, South Carolina (cdmracing.com), has seen several East Coast ’97 TJs and found this as strange as the rest of us, but came up with another plausible explanation: That sending unit that sure looks like an oil pressure sender could be a temp sender or temp switch to control an aftermarket electric fan. That’s similar to your idea that it could be related to an aftermarket or dealer installed A/C system, but for the life of me I can’t find a temp sensor or switch that looks like the one you describe. Lastly, we chatted briefly with Jim Repp who worked for Chrysler to confirm our assumptions. He agreed with us that whatever this is, it is not from the factory and certainly added after the Jeep left Toledo. At the end of the day, I could imagine a situation where for some reason the 1⁄8 -NPT port in the driver side of the block started leaking and the oil pressure sender and elbow were the only items handy to plug the hole, but that doesn’t explain the wiring, and these threaded ports rarely leak to begin with unless messed with. Still, I’m gonna run with this as the most plausible explanation for what you’ve seen, a jury-rigged repair, though I could be wrong. The only other sound explanation is that the sender is a temp sender or switch related to an electric fan or some early aftermarket A/C system. Maybe a Four Wheeler reader has an idea about this and can tell us what the heck is going on here, but we can’t believe an oil pressure switch or sender would do much other than eventually leak when plugged into the water jacket. fourwheeler.com
