Daniel B
CCCUK Member
SHORT VERSION:
There's a diagram going around the internet (and I guess also paper copies) for the AC system which contains a mistake; a mistake which has unfortunately been copied by a lot of the replacement loom manufacturers and results in the fan continuously running at low speed when the fan speed selector is in the OFF position and the mode selector is in the VENT and HEATER positions (it should only enforce low speed in the AC positions to stop the condenser freezing). If this bothers you its easy to correct to get your controls working as intended.
LONG VERSION:
I've recently acquired a 72 and whilst the bodywork and interior was good, there were a few parts of the vacuum system missing and the wiring was terrible. As such I've been working my way through the car piece by piece getting everything sorted. One of the things on my list was the AC system which didn't really function properly at all so I began to tackle it.
I came across this diagram, checked it through and it appeared to match my car so I went through and sorted my wiring based on it including cleaning, re-calibrating and repairing the switches on the controls so they functioned correctly...
Upon completion I noticed the controls still didn't function according to the manual, as soon as you rolled the mode selector passed the OFF position the blower fan ran at low speed continuously even when the fan speed selector switch was in the leftmost OFF position and VENT or HEATER were selected. From what I can gather the correct method of operation is OFF on the fan speed selector should result in the fan actually being off unless the mode selector is in one of the AC positions (MAX/NORM/BI LEVEL/DEFROST). In the AC positions the fan should always run at low speed (or higher if selected) to prevent the AC condenser from freezing up.
To solve this I reverse engineered the diagram above and found it contains a mistake.
If we look at the compressor switch (top right) you can see there are 3 wires going to it.
LT GRN - Power Feed
The light green wire carries the power from the master switch (which is closed when the mode selector is in anything other than the OFF position) to the compressor switch.
TAN - Switched Output
The tan wire carries the power from the compressor switch (which is closed when the mode selector is in any of the AC modes) to the compressor via the discharge pressure switch (and in my case also a thermal fuse).
GRAY - Low Speed Fan Enable
The grey wire carries the power from the compressor switch to the fan speed selector switch to force low speed fan even when the fan is in the off position.
Based on this, logically the GRAY wire should only be hot when the compressor switch is closed. This means it should be connected to the same pin as the TAN wire NOT the same pin as the LT GRN power feed wire! By connecting it to the LT GRN wire they have basically made the fan run at (at least) low speed all the time disabling the ability to have the mode selector in HEATER or VENT without the fan on!
I rewired my compressor switch connector and now my AC controls function perfectly, with operation exactly as per the manual!
Just out of curiosity I googled for photos of heater switches to confirm this and I found a couple of pictures of cars with their original wiring still in tact showing the GRAY wire connected to the same pin as the TAN wire, but what was strange was I also found pictures of lots of cars with it connected incorrectly to the LT GRN wire. So I did some research and found photos of replacement looms with the wires connected incorrectly (as per the diagram above) instead of as per the original loom. So it seems the re-manufacturers have all been following this incorrect diagram!
So just to clarify...
This diagram above is basically wrong, it isn't for a different model year or anything like that, its just wrong.
The reason I believe this, is the wiring and speed selector switch could be dramatically simplified if the fan was meant to permanently run low and could never be switched off. GM didn't fit wires and add complexity for no reason - if they could save money by omitting a wire they would
This is my corrected version of the diagram...
Hope this helps some people!!!
Daniel
There's a diagram going around the internet (and I guess also paper copies) for the AC system which contains a mistake; a mistake which has unfortunately been copied by a lot of the replacement loom manufacturers and results in the fan continuously running at low speed when the fan speed selector is in the OFF position and the mode selector is in the VENT and HEATER positions (it should only enforce low speed in the AC positions to stop the condenser freezing). If this bothers you its easy to correct to get your controls working as intended.
LONG VERSION:
I've recently acquired a 72 and whilst the bodywork and interior was good, there were a few parts of the vacuum system missing and the wiring was terrible. As such I've been working my way through the car piece by piece getting everything sorted. One of the things on my list was the AC system which didn't really function properly at all so I began to tackle it.
I came across this diagram, checked it through and it appeared to match my car so I went through and sorted my wiring based on it including cleaning, re-calibrating and repairing the switches on the controls so they functioned correctly...
Upon completion I noticed the controls still didn't function according to the manual, as soon as you rolled the mode selector passed the OFF position the blower fan ran at low speed continuously even when the fan speed selector switch was in the leftmost OFF position and VENT or HEATER were selected. From what I can gather the correct method of operation is OFF on the fan speed selector should result in the fan actually being off unless the mode selector is in one of the AC positions (MAX/NORM/BI LEVEL/DEFROST). In the AC positions the fan should always run at low speed (or higher if selected) to prevent the AC condenser from freezing up.
To solve this I reverse engineered the diagram above and found it contains a mistake.
If we look at the compressor switch (top right) you can see there are 3 wires going to it.
LT GRN - Power Feed
The light green wire carries the power from the master switch (which is closed when the mode selector is in anything other than the OFF position) to the compressor switch.
TAN - Switched Output
The tan wire carries the power from the compressor switch (which is closed when the mode selector is in any of the AC modes) to the compressor via the discharge pressure switch (and in my case also a thermal fuse).
GRAY - Low Speed Fan Enable
The grey wire carries the power from the compressor switch to the fan speed selector switch to force low speed fan even when the fan is in the off position.
Based on this, logically the GRAY wire should only be hot when the compressor switch is closed. This means it should be connected to the same pin as the TAN wire NOT the same pin as the LT GRN power feed wire! By connecting it to the LT GRN wire they have basically made the fan run at (at least) low speed all the time disabling the ability to have the mode selector in HEATER or VENT without the fan on!
I rewired my compressor switch connector and now my AC controls function perfectly, with operation exactly as per the manual!
Just out of curiosity I googled for photos of heater switches to confirm this and I found a couple of pictures of cars with their original wiring still in tact showing the GRAY wire connected to the same pin as the TAN wire, but what was strange was I also found pictures of lots of cars with it connected incorrectly to the LT GRN wire. So I did some research and found photos of replacement looms with the wires connected incorrectly (as per the diagram above) instead of as per the original loom. So it seems the re-manufacturers have all been following this incorrect diagram!
So just to clarify...
This diagram above is basically wrong, it isn't for a different model year or anything like that, its just wrong.
The reason I believe this, is the wiring and speed selector switch could be dramatically simplified if the fan was meant to permanently run low and could never be switched off. GM didn't fit wires and add complexity for no reason - if they could save money by omitting a wire they would
This is my corrected version of the diagram...
Hope this helps some people!!!
Daniel