Thermistor broken - replacement available?

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AtlantaDan
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Joined: Tue Dec 19, 2023 3:51 am

Thermistor broken - replacement available?

Post by AtlantaDan »

1989 GW. Replacing the evaporator and the thermistor probe is broken. I cannot find a replacement to save my life. Does anyone have any suggestions? Also, I remember seeing a resistance chart but now I can't find it. If someone has a link, I would sincerely appreciate it
1989 Grand Wagoneer

joeyGee
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Re: Thermistor broken - replacement available?

Post by joeyGee »

Following along

sonoraed
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Re: Thermistor broken - replacement available?

Post by sonoraed »

Probably time for the group to devise a way to eliminate the thermistor and AC module, I don;t see any FSJ's in my shop so I looked at Ol'Tom's website for ac wiring diagrams.
Looks like there are two different versions of the way Jeep controlled the seperate heater and AC, 86 and newer had two seperate blower switch's and 1990 had one switch, resistor and a relay to toggle between the two blower motors.
I'm thinking mechanical ac thermostat with cap tube inserted into the evap could be used. there is few things I assume but not sure of such what the MODE switch does and how in defrost, other than sending hot air up the defrost ducts does the control head also turn AC to pull moisture out of the air?
Maybe someone with a working system can answer that question, also are there more than the two types of HVAC controls?
I have 1979 J10 and it was a separate manual system, since a ground up rebuild I used a 86 dashboard and Vintage Air Gen IV
evap.

runuover86gw
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Re: Thermistor broken - replacement available?

Post by runuover86gw »

sonoraed wrote: Mon Aug 26, 2024 8:10 am Probably time for the group to devise a way to eliminate the thermistor and AC module, I don;t see any FSJ's in my shop so I looked at Ol'Tom's website for ac wiring diagrams.
Looks like there are two different versions of the way Jeep controlled the seperate heater and AC, 86 and newer had two seperate blower switch's and 1990 had one switch, resistor and a relay to toggle between the two blower motors.
I'm thinking mechanical ac thermostat with cap tube inserted into the evap could be used. there is few things I assume but not sure of such what the MODE switch does and how in defrost, other than sending hot air up the defrost ducts does the control head also turn AC to pull moisture out of the air?
Maybe someone with a working system can answer that question, also are there more than the two types of HVAC controls?
I have 1979 J10 and it was a separate manual system, since a ground up rebuild I used a 86 dashboard and Vintage Air Gen IV
evap.
I am working on a replacement thermistor at the moment and I also have the capabilities to repair the AC control module. I just need to find someone that still needs a replacement thermistor to test fitment of a few different thermistors I am working with. I know they have the proper resistance range, but I just need to verify that they fit properly into the evaporator core. So, if anyone wants to reach out to me, I can send them one of these thermistors I have to decide which one would be the best for install purposes and feedback.

To answer your question on the defroster functions. It is a blend of hot air from the heater core and fresh air from outside depending on where you have the temp set to. The main differences between the two systems are the update to the Sanden compressor and the wiring is a little different because as you stated the 90-ish to 91 has the same controller, but it only has one blower switch for both functions and it only has one blower resistor for both the heat and AC. If everything is running properly and isn't damaged, this actually a relatively simple system to work on.
Josh's Classic Custom Auto Electronics LLC, Owner/Founder
https://www.facebook.com/JoshsFSJRelayKits
1986 GW 360/NP229/TF727/Dana 44's
1986 Comanche
1979 Wagoneer Limited 360/Quadra-Trac/Turbo 400/Dana 44's

sonoraed
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Re: Thermistor broken - replacement available?

Post by sonoraed »

https://www.toyota-4runner.org/4th-gen- ... 32929.html
might this interesting Toyota has used thermistors for years

sonoraed
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Re: Thermistor broken - replacement available?

Post by sonoraed »

https://www.uacparts.com/photo/WEBP/SW/ ... 1_336.webp

this has been used for years by the rest of the world, I think it would work as a replacement for the AC controller/thermistor

sonoraed
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Re: Thermistor broken - replacement available?

Post by sonoraed »

to find the resistance the module is looking for, and if the AC is functioning mechanically ie gets cold, I have a guage tester that can range from 0 to 500 ohms (OTC) in very small increments.
You can see where I'm going with this, with a set of guages attached and thermometer in a vent dial in different impedance till you find what turns on the clutch and what cycles clutch, in relation to ac temp and impedance.
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