converted to an open cooling system...aux. fan issue

I have an '88 XJ that I converted to an open cooling system over the weekend. I installed a new 3 core radiator with a filler neck, '91 thermostat housing with the sensor, omitted the heater control valve, and an overflow bottle. I spliced the old sensor that screwed into the old radiator to the new sensor that screws into the thermostat housing. Now just after start up and the when temp. gauge slightly moves the aux. fan kicks in. I have a 180* thermostat, but the tempurature never comes close to 180* before the fan turns on. I posted this on jeepforum as well and got one suggestion.

http://www.jeepforum.com/forum/showthread.php?t=595290

Any additional ideas? Thanks in advance!

blk88 said:

I have an '88 XJ that I converted to an open cooling system over the weekend. I installed a new 3 core radiator with a filler neck, '91 thermostat housing with the sensor, omitted the heater control valve, and an overflow bottle. I spliced the old sensor that screwed into the old radiator to the new sensor that screws into the thermostat housing. Now just after start up and the when temp. gauge slightly moves the aux. fan kicks in. I have a 180* thermostat, but the tempurature never comes close to 180* before the fan turns on. I posted this on jeepforum as well and got one suggestion.

http://www.jeepforum.com/forum/showthread.php?t=595290

Any additional ideas? Thanks in advance!

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In my opinion you should have stuck with the closed system, but again thats just my opinion.

They said it correctly on JeepForum, the PCM is what controls the fan on 91+'s, so the thermostat housing and sensor is worthless in your application. You need the proper thermo switch.

I never really had major issues with the closed system other than the need to burp it when messed with. I had to replace the radiator and found a new open radiator for a good deal and decided to make the switch. With that said and the need to change the sensor...has anyone heard of the corvette sensor working? That seems kind of random, but I'm not one to know. I really don't trust myself putting the fan on a switch living in Phoenix. I'll not notice its hot and overheat so I don't want that as an option.

You got the right answer on Jeep Forum. Are you here testing us or them? I have converted 2 RENIX systems, so I speak from experience.

In the first one, I did everything in the MADXJ writeup, but the "switch" in the thermostat housing. I knew that was a sensor (thermistor) and wrong. I found a switch to fit the '91+ housing online, before I knew the Corvette one would work. With the last one took the fan switch bung out of the old radiator and soldered it into the new one, so the fan works just like it did from the factory. I just installed the new radiator and left the expanision bottle and heater switch to do at a later time. That time has not come yet. I am not convinced there is an advantage to the "open" system over the "closed" system, so no hurry on removing the bottle.

Not trying to bogart the thread, but I did this same conversion and my electric fan does not work. I was told by the jeep service tech that the sensor in the radiator was strictly for coolant level and had nothing to do with any of the cooling system operations.

It appears that sensor is for the electric fan, if thats true then how do I adapt that to the new setup? I read the corvette upgrade and was wondering if you use the bung on the water neck then where do you put the water temp sensor?

Thanks and sorry again for jumping in on this thread.

No testing...I really didn't want to go out and buy another sensor from one post. I wanted to see if others had heard of that before. From what I've read and talk to people the open vs. closed system is a preference. I never had overheating problems, I needed a new radiator and got a good deal on an open 3 core, so I decided to make the swithch. I just scratch my head why Jeep changed the system to open if there are so many that prefer the closed system. Maybe it just works better with the Renix motor. I'm no expert so who am I to talk. So far I don't have any issues with the tempurature, maybe running a little too cool, but the fan always runs and I can deal with that for now. Not a bad thing in Phoenix. I'll get the corvette sensor soon. Also, I enjoy wrenching so it was an easy and relaxing project. Anyways...not sure about the sensor in the closed radiator. It does plug directly into the electrical fan so I would think it would be for the fan. If it triggers a low coolant light I don't know.

I installed an '85 Corvette sensor last night and the aux fan turned on around 185* and cooled back down to around 150* and hadn't shut off. I just did a driveway test so after I get to drive it around I will let you guys know at waht temp it shuts off. The sensor was cheaper than the XJ sensor and it came with the electrical connector which was a bonus. Autozone $12.99. Thanks again!

Glad you got if working blk88.

For anyone else doing this, you really should consider taking the fan switch out of the old (closed) radiator and puting it in the new (open) radiator. The wire harness is already there and you don't need to buy any parts for the fan to work as it always has. The only expense is some solder, flux and gas.

The threaded bung that holds the temperature switch is steel and is soldered into the radiator just above the lower hose. All you have to do is heat that up with a propane (MAP is faster) torch to remove it. Cut a hole exactly 1" in diameter in the new radiator where the hole was in the old radiator. This is a little tricky, since the ATF cooler tank is just 1/4 inch below that surface. I ended up drilling a few holes close together and connecting them with diagonal cutters, until I got a big enouth opening for a 3/8" cylindrical bur and cut it with a die grinder. I found a tapered socket that I kept progressivly fitting into the hole to check for roundness, while measuring the diameter.

The bur on a die grinder will really cut that brass. Be careful!

I just picked up a replacement radiator today for my daughters 1990, to convert it from closed to open, and it's got a threaded bung just like the closed system radiator had. It's got plastic side tanks but, it is an immediately available part, and I can get it back on the road the same day.

Not trying to debate plastic vs metal tanks or open vs. closed systems. . . I'm tired of cheap failing footballs, and can't be waiting for mail order parts on some of these DD rigs. The rest of the stuff will come off a parts jeep, and I'll be done with all the hassles for about $100. . .

blk88 said:

I installed an '85 Corvette sensor last night and the aux fan turned on around 185* and cooled back down to around 150* and hadn't shut off. I just did a driveway test so after I get to drive it around I will let you guys know at waht temp it shuts off. The sensor was cheaper than the XJ sensor and it came with the electrical connector which was a bonus. Autozone $12.99. Thanks again!

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So what is the verdict? Still working? Happy with it?

blk88, Any chance you have a part number for that Corvette sensor?

blk88 said:

I installed an '85 Corvette sensor last night and the aux fan turned on around 185* and cooled back down to around 150* and hadn't shut off. I just did a driveway test so after I get to drive it around I will let you guys know at waht temp it shuts off. The sensor was cheaper than the XJ sensor and it came with the electrical connector which was a bonus. Autozone $12.99. Thanks again!

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are you concerned that those temps are too low? operating temps should be between 190 and 210. i would be concerned about running too rich(open loop) and in PA there is emissions testing and running too cool will most likely produce higher emissions.

blk88 said:

I never really had major issues with the closed system other than the need to burp it when messed with. I had to replace the radiator and found a new open radiator for a good deal and decided to make the switch. With that said and the need to change the sensor...has anyone heard of the corvette sensor working? That seems kind of random, but I'm not one to know. I really don't trust myself putting the fan on a switch living in Phoenix. I'll not notice its hot and overheat so I don't want that as an option.

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when i had my 87 i just installed a prestone flush system with the supplied "T" never had a problem.. even installed one on my 92.flush the system yearly and ad new coolant..

89xj said:

are you concerned that those temps are too low? operating temps should be between 190 and 210. i would be concerned about running too rich(open loop) and in PA there is emissions testing and running too cool will most likely produce higher emissions.

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Not to mention washing the cylinders down with gasoline and fouling out the plugs.