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73 climate control - in car sensor testing?

Started by pordy, June 01, 2022, 09:21:19 AM

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pordy

Hi- my mechanic has been slowly going through my ac that isn't blowing cold. I got a rebuilt programmer from Tim Groves that solved most of my issues- everything seemed to work as it should, just no cold air.  He narrowed it down to needing a new ac compressor.

AC compressor was just installed but for some reason it isn't getting power/never comes on (which wasn't a problem with the old one). Now, defrost correctly blows high, but any other setting that activiates the compressor gets nothing (bi, hi, auto, low).

Then I remember that over the winter I had the dash off and had to disconnect and reconnect the in-car temp sensor!

If that weren't reconnected right, would that break the loop and cause my problem? 

I reached up from the dash board and it seems well secured.  I tried flipping the plug - would that matter? it seems to go on either way. Could the sensor be bad?

I also have Tim's manual though I left it with my mechanic to read over the weekend!

Thanks for any ideas,
Pat

TJ Hopland

Just going from memory here...  Tims book I'm sure covers everything you would need to know in much more detail than I can remember but here are my thoughts anyway.....

I don't think the sensor connections matter.  As you noted its just a loop of variable resistors. If the loop was 'broken' you would not get any change when you spin the dial.  Does anything change when you go for 65 to 85?  Even on vent it should redirect the air to come out the floor at 85 or the dash at 65.  If you are not getting any signs of change with the dial the loop could be broken along with a dozen other issues. 

I'm pretty sure Tim's book does give ohm readings for all the sensors in the loop individually as well as the complete loop.  I think there is a chart of ambient temps to ohms so you look at the chart for your ambient temp as well as a chart for the dial so you can come up with what the whole loop should read in ohms at a given ambient temp and dial setting. 

Going from memory I don't think 'the loop' would effect the compressor clutch.  The compressor on these runs all the time unless you are in off or vent or the ambient temp is too low like below freezing. I think the compressor clutch circuit either gets power or maybe a ground from the mode selector lever in all but off and vent, temp selection doesn't matter. 

Have you or your tech checked the thermal fuse?  These didn't just have the simple high/low pressure switches inline with the clutch like slightly newer stuff did.  The pressure switch(es?) connected to what they called a thermal fuse which was basically a one time time delay buffer sort of thing.

It was normal for these systems to momentarily drop pressure during normal operations and having the clutch drop out every time would not let things work correctly so they came up with this thermal fuse idea.  Basically when the pressure drops it starts heating this fuse that is normally passing the power to the clutch.  I don't remember how long it takes but if the pressure stays low for an extended period of time it eventually blows the fuse which then disconnects the compressor.  I'm thinking it takes a minute or two to blow the fuse.  Normal operations the pressure drops are like seconds.

Maybe during the compressor service the fuse got blown? Its a dodad in the top of the heater/ac box under the hood.  From memory I think its 3 pins and held with 2 screws.  Its kinda angled pointing out to the passenger side and maybe 2 inches in diameter?  I assume its in the air box similar to the blower resistors because they wanted the air flow to cool it so during service and testing it could be easy to damage it?

They were used on most GM's in that mid 73-76 period so should not be too hard to find even if you had to go to a F body or maybe Corvette parts place.     
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

"Cadillac Kid" Greg Surfas 15364

Pat,
The first thing I would check is to see that your ground wire to the compressor clutch is connected and then if you have 12 volts to the compressor coil connections with the key on and the selector lever turned to anything (say "auto") but off.
If you do have 12 volts then the fault lies in the compressor coil. If you do not then check/replace the SUPER-HEAT THERMAL OVERLOAD. The electrical connection at the rear of your compressor is for the superheat sensor.  This is activated when the for an extended period of time the temperature is elevated above the saturation pressure of the refrigerant. Typically this indicates a shortage of refrigerant, and if the charging process was extended or the system run short of gas this will cause the switch to close, sending a ground to the thermal element and causing it to blow.
All that said If after replacing the thermal element you still do not have power to the compressor, check all wire connections,
To answer your question regarding the in car sensor, if this were disconnected the system would go to full heat, which, unless the outside temperature was below 32 degrees F, still has the compressor energized.
That's my story and I'm sticking to it.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-