HVAC control unit diagnostics

[Tom]

Thanks for being here! So you are saying you had to keep turning the temp higher and higher over time, and now even if you set the temp to full-hot, the A/C remains on? That could be one of the sensors, so I'd check those for sure. If not that, then maybe built up resistance at the connectors somewhere, or the temp-knob pot in the head unit getting old, or the logic in the head unit. I'd start by checking the sensors (and the little tube that carries cabin air back to the sensor near the cigarette lighter) and see if that offers any clues. Let us know what you find and I'll help as much as I can...

[7b944]

Thanks for reply. That's correct, temp control has no effect on AC. Was hoping for defective sensor, but all 3 measure around 10K ohm at 75-78 degrees. Blowing on one in mix chamber, ohms decreased,(NTC). Sampling tube at dash pulls paper towel slightly so fan seems to be working at least somewhat. Tested flap motor units, all moved and pot values changed close to what has been published, except for footwell motor which did not move. Took it out, top mounting screw took me an hour to get out using small vise grips. Motor disassembled, badly rusted and locked up. Cleaned it up coated with light coat of rust converter, lubed bearings and now working, feedback pot rotates and reads within range. Some change in flap positions with CCU reconnected, but not as much as with 12 V applied, seemed to not move same amount each time. With car running, pressing defrost switch causes flaps to move, AC and heat come on full and fan blows max. That seems good, but still temp control has no effect on AC temp. Pulled unit and going to disassemble, check interconnect pins as you recommend, caps and voltages. Like "CJK" Dave, I also have EET degree from DeVry, from last century. Having worked on various equipment find that connections tend to often be source of trouble, so hoping for that and not to break plastic which is in remarkably good condition. Dave

[Tom]

Thanks for reply. That's correct, temp control has no effect on AC. Was hoping for defective sensor, but all 3 measure around 10K ohm at 75-78 degrees. Blowing on one in mix chamber, ohms decreased,(NTC). Sampling tube at dash pulls paper towel slightly so fan seems to be working at least somewhat. Tested flap motor units, all moved and pot values changed close to what has been published, except for footwell motor which did not move. Took it out, top mounting screw took me an hour to get out using small vise grips. Motor disassembled, badly rusted and locked up. Cleaned it up coated with light coat of rust converter, lubed bearings and now working, feedback pot rotates and reads within range. Some change in flap positions with CCU reconnected, but not as much as with 12 V applied, seemed to not move same amount each time. With car running, pressing defrost switch causes flaps to move, AC and heat come on full and fan blows max. That seems good, but still temp control has no effect on AC temp. Pulled unit and going to disassemble, check interconnect pins as you recommend, caps and voltages. Like "CJK" Dave, I also have EET degree from DeVry, from last century. Having worked on various equipment find that connections tend to often be source of trouble, so hoping for that and not to break plastic which is in remarkably good condition. Dave

The defrost switch overrides most of the other electronics on the board, so not a huge surprise that works with the rest of it doesn't. Since everything else checked out, I think it makes sense to check the board interconnect pins/wires. They are too spindly for the job and often corrode to the point of dust. Fingers crossed.

[7b944]

Interconnect pins have continuity. Power with 12V bench supply and have the both 10.6 or so and 6 V supply voltage. Symptoms seem to indicate problem with or near the LM2902 chip which I have on the way. Do you happen to recall which board that chip is on? Looking at old National IC book indicates those chips can be destroyed if shorted or overloaded, but can probably true for any chip. Thanks, Dave

[7b944]

To follow up in case someone finds this post. Seized, corroded Footwell motor cleaned up, lubed and now working. Climate Control Unit LM2902N IC , C12 (10Uf) and C13 ( 47uf) replaced while I was in there. Replacing LM2902 required a lot of careful soldering disassembly and reassembly. System now working as designed. I redrew the LM2902 part of the schematic which helped me visualize that the combined output of the sensors and Temperature pot summed with the mix pot, control the Mix Flap Motor and that in turn, the Mix pot position controls the heat solenoid.
A big thank you to Tom for his troubleshooting guides and the guys who took the time to create the schematics.

[Tom]

To follow up in case someone finds this post. Seized, corroded Footwell motor cleaned up, lubed and now working. Climate Control Unit LM2902N IC , C12 (10Uf) and C13 ( 47uf) replaced while I was in there. Replacing LM2902 required a lot of careful soldering disassembly and reassembly. System now working as designed. I redrew the LM2902 part of the schematic which helped me visualize that the combined output of the sensors and Temperature pot summed with the mix pot, control the Mix Flap Motor and that in turn, the Mix pot position controls the heat solenoid.
A big thank you to Tom for his troubleshooting guides and the guys who took the time to create the schematics.

Awesome news! Great work. Would love to see what you drew up as a schematic for the LM2902 portion of the schematic!

[7b944]

Don't have schematic drawing program anymore so is just hand drawing that I'll clean up and try to post.

[Tom]

Don't have schematic drawing program anymore so is just hand drawing that I'll clean up and try to post.

If you post your hand drawing, I'll try to add it to the partial Eagle schematic I did before.