Erractic Heater Temp Control Fix (3 Viewers)

Joined
Jan 18, 2006
Messages
834
Location
Michigan/Costa Rica
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.

Diagram.jpg


Cover.jpg


bad joints.jpg


cracks2.jpg


fixed.jpg
 
Last edited:
Joined
Apr 29, 2010
Messages
5,506
Location
Langley BC
Fantastic! I had the exact same problem with mine right down to the giggle to get it to go from hot to cold. Mine does it even with the A/C off, vents all of a sudden blowing super hot air.

I will definitely fix mine next time I'm in there.

Thanks! :cheers:
 
Joined
Nov 17, 2014
Messages
22
Location
Oahu, Hawaii
I need to do this fix. Same exact symptoms. The soldering part looks straightforward. Does anyone have any tips to get the heater control panel out of the dash?
 
Joined
Nov 17, 2014
Messages
22
Location
Oahu, Hawaii
Thank you. Dash came off pretty easy. Seemed like one of the larger connections on the back of the AC panel may not have been all the way seated, which could have caused the symptoms I had. Took everything out anyway. Solder joints all looked okay. No cracking. Used the compressed air on back of the board and switches. Put it back together and seems better.
 

lockd80

Totally right
Joined
May 2, 2007
Messages
313
I am having a weird problem with the 20 year old heater control in my 80 so i hop on mud and do a simple search. BAM there is a five year old thread describing my exact problem with a detailed explanation of how to fix it including pictures!

Mud is by far the best automotive enthusiast site ever! I own many vehicles and I have never come across a better site.

Where do i renew my star?

Sorry about the old tread resurrection but i just had to vent.
 
Joined
Feb 22, 2018
Messages
84
Location
Winston Salem, NC
Im glad you brought it back up. I am having a similar but different issue with my heater control. If I am one side of my 65 deg mark it is cold, a very slight movement towards the hot side and it is hot. There is no in-between for the temp setting, no adjustability. I wonder if it is caused by the same issue?
 

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