My heater was possessed. But after a simple exorcism, all is well.
I've had this problem with my heater on my '94 FZJ80 for some time. I would select a temperature with the slider and randomly the heater would go to full heat. I could jiggle the temperature slider and it would return to normal. This was particularly annoying in the summer when the temp slider was all the way to cold and I would get blasted with full hot air.
As I had my panel apart to add a trailer brake controller, I decided to check out the heater control. The temperature slider is connected to a linear potentiometer that is connected to the A/C amplifier (1st photo) and tells the amplifier where to position the blend door to achieve the desired temperature by mixing heated and cold air together. This potentiometer is connected to pins 1, 2 & 3 of the blue connector. The problem turned out to be cracks in the solder joints where the potentiometer is solder to the circuit board. I thought that I would write this up for others that might be experiencing the same problem.
To correct the problem, first remove the heater control from the panel, by far the hardest part. Remove the back cover by gently prying the three clips (Photo 2) to release the cover. Once the cover is off, you can see the four potentiometer connections (Photo 3). Only three are actually used but why not resolder all four. I've numbered the connections with the corresponding pin numbers of the blue connector.
In Photo 4, you can see the crack in the solder joint that goes all the way around the pin. All four of mine had similar cracks.
To fix the problem, just resolder all four connections, reassemble and enjoy! Photo 5 shows the resoldered joint.
Edit: Although my symptom was random shifts to full heat, depending on which connection failed, it could also result in shifts to full cold.
I've had this problem with my heater on my '94 FZJ80 for some time. I would select a temperature with the slider and randomly the heater would go to full heat. I could jiggle the temperature slider and it would return to normal. This was particularly annoying in the summer when the temp slider was all the way to cold and I would get blasted with full hot air.
As I had my panel apart to add a trailer brake controller, I decided to check out the heater control. The temperature slider is connected to a linear potentiometer that is connected to the A/C amplifier (1st photo) and tells the amplifier where to position the blend door to achieve the desired temperature by mixing heated and cold air together. This potentiometer is connected to pins 1, 2 & 3 of the blue connector. The problem turned out to be cracks in the solder joints where the potentiometer is solder to the circuit board. I thought that I would write this up for others that might be experiencing the same problem.
To correct the problem, first remove the heater control from the panel, by far the hardest part. Remove the back cover by gently prying the three clips (Photo 2) to release the cover. Once the cover is off, you can see the four potentiometer connections (Photo 3). Only three are actually used but why not resolder all four. I've numbered the connections with the corresponding pin numbers of the blue connector.
In Photo 4, you can see the crack in the solder joint that goes all the way around the pin. All four of mine had similar cracks.
To fix the problem, just resolder all four connections, reassemble and enjoy! Photo 5 shows the resoldered joint.
Edit: Although my symptom was random shifts to full heat, depending on which connection failed, it could also result in shifts to full cold.
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