HVAC control head swap, "Max A/C" Style - Wiring Diagram Needed.

DerekTheGreat

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This should do it..
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Macfluke

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Good Luck! I spent time trying to figure this thing out in the past with no luck and if I ever get more time I may go have another look at it...
 

DerekTheGreat

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What did you figure out and then what was your conclusion?

Also, you wouldn't happen to have anything like the first picture I uploaded, do you? Something which shows me what each pin (A-M & 1-12) is would be extremely helpful in comparison. The diagram you provided has them listed differently and I'm too dense to see if I've got it right.. Although from what I've deduced last night, the wires are in the same spots for '94 as they are in '89 and the colors are 95% correct, just different shades of DK BLU or LT GRN so that's my mistake there. Only thing I wonder is the reference wires and if the module is responsible for stepping the voltage down to 8 or 5V and not some outside resistor.

Anyone happen to have the GM Electrical Troubleshooting manual from '94?
 

Macfluke

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What did you figure out and then what was your conclusion?

Also, you wouldn't happen to have anything like the first picture I uploaded, do you? Something which shows me what each pin (A-M & 1-12) is would be extremely helpful in comparison. The diagram you provided has them listed differently and I'm too dense to see if I've got it right.. Although from what I've deduced last night, the wires are in the same spots for '94 as they are in '89 and the colors are 95% correct, just different shades of DK BLU or LT GRN so that's my mistake there. Only thing I wonder is the reference wires and if the module is responsible for stepping the voltage down to 8 or 5V and not some outside resistor.

Anyone happen to have the GM Electrical Troubleshooting manual from '94?

My conclusion... /haha this damn thing ain't gonna work!
the thing I really wanted to do was to take a closer look at my friends 94, but he didn't want me pulling his rig apart so I didn't get to do. I wanted to compare the wiring harness and relays,etc... to see if I got a 94 harness and relays,etc... if I could install it in my 89 and make it work... another friend told me to just install a switch on the dash and do a little re-wire to control the recirc door motor. After all the frustration I took a hiatus from the project...

The only other thing I have is a schematic of a 89... I downloaded the 89/94 schematics online but don't remember where because it was awhile back... one difference is my 89 schematic shows it a 5v, so I think I would use a voltmeter and verify the voltages, but I think its more than a voltage problem here...
 

DerekTheGreat

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Haha
Yeah I've got the wiring harness end from the '94, so I used that this morning to make a table to try and cross reference but like earlier, I can't make heads or tails yet since the colors described in your upload don't jive 100% with the colors I'm seeing on the harness. At this point, all I'd need is an illustration of the connector like I uploaded but with positive ID's as to which wire goes in which socket of the connector.. The stuff you provided calls out ID's like "C11" but that doesn't jive with the illustration provided by my '88 service manual. Again, I'm too stupid to put it together without a little more assistance.

From what I can see so far, it does appear that at least one of the reference wires needs to be switched. But since your schematic and mine don't ID all wires going into the harness, I'm not sure where to move them since it appears that only ID "5" isn't used.. And the big problem is there are a lot of wires with the same damn color. It might not even be possible since for '89, the system uses one wire that splits to feed reference voltage to all three motors whereas it seems the '94 uses three separate wires for each. More work could be necessary. I'm not about to hack into the factory wiring or add a switch to my dash though. Will most likely probably end up going back to stock haha.
 

Macfluke

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I'm not about to hack into the factory wiring or add a switch to my dash though.
same here... that's why I wanted to get the 94 harness a required pieces and do a switch if it would work... I'm not into hacking...
 

phaessler

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Yeah I've got the wiring harness end from the '94, so I used that this morning to make a table to try and cross reference but like earlier, I can't make heads or tails yet since the colors described in your upload don't jive 100% with the colors I'm seeing on the harness. At this point, all I'd need is an illustration of the connector like I uploaded but with positive ID's as to which wire goes in which socket of the connector.. The stuff you provided calls out ID's like "C11" but that doesn't jive with the illustration provided by my '88 service manual. Again, I'm too stupid to put it together without a little more assistance.

From what I can see so far, it does appear that at least one of the reference wires needs to be switched. But since your schematic and mine don't ID all wires going into the harness, I'm not sure where to move them since it appears that only ID "5" isn't used.. And the big problem is there are a lot of wires with the same damn color. It might not even be possible since for '89, the system uses one wire that splits to feed reference voltage to all three motors whereas it seems the '94 uses three separate wires for each. More work could be necessary. I'm not about to hack into the factory wiring or add a switch to my dash though. Will most likely probably end up going back to stock haha.


The "recirculation" operation is somehow directly related to the "Air Inlet Pressure switch" (mounted on the compressor. It is available in various colors for different operating pressure ranges, best reference is from Four-Seasons A/C catalog. From what I can deduct is the system pressure drops, i.e. the ambient temp causes the refrigerant to stay below the say 170psi, then the system draws outside air over the evaporator, thus preventing a freeze up, however as the ambient temperature rises, then the switch sees 220psi, close and grounds the circuit (not sure what happens in the controller), but it opens closes the air inlet valve recirculating the cab air until the pressure drops again in the system. This is as far as I have gotten, my 1988 cycles the air inlet door while Temp= max cold; Fan = High. I would also like to control it manually, and have aquired a control unit from a 1992 with a "RECIR" button.
From what I can locate the 1994 wiring diagram book is not a "large" manual like the 1988-93's, however I have the 1988, and am awaiting the 1993 to compare.
Just a sidenote also , my 1988 has been converted to R-134a, evap/condensor flush, 8oz PAG150, new compressor, new lines, new acumulator, new o-tube. Syytem has been running for 14 years now trouble free and cold, charge weight was 85% of R12, 34oz. 8oz
Will poat as I find out more
 

studigggs

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Here's a writeup I posted on another forum regarding the auto recirc feature of 88-90 GMT400s (No max/recirc buttons)

"I wanted to add a writeup on the operation of the recirculate/fresh air door operation for 88 - 90 C/K models. This info only pertains to vehicles that do not have a 'Max' button on the electronic climate control unit.

I searched for this information on this and other forums and there seemed to be much confusion over the operation of these style units. The common misconception is that recirculated air is only activated when the fan is on Hi or Med with temp control full cold. If your system is stock and includes the high pressure sensor (actually called the Air Inlet Valve Motor Pressure Switch (AIVMPS)) on the back of the AC compressor, then the operation is as follows from the 1990 Electrical Wiring Manual:

"The Air Inlet Valve is normally in the position which allows outside air to enter the system. In this position, the Air Inlet Valve Potentiometer returns about 4.9 volts to the Heater and A/C Controller. When the A/C Button is depressed, and if the temperature setting is at full Cold and the blower is set to MED or HI, the Air Inlet Valve Motor will move the valve to the recirculating position 30 seconds after the Air Inlet Valve Motor Pressure Switch closes. The Air Inlet Valve is opened again, 5 minutes after the Air Inlet Valve Motor Pressure Switch opens. It opens immediately if the temperature setting is raised, the blower speed is reduced to Low, or the air flow is directed to the floor outlets"

Details on the AIVMPS are as follows: Closes at 210 - 255 PSI, Opens Above 165 PSI, Minimum pressure differential of 45 PSI. This is a single wire unit, so when the switch is closed at 210-255 ground is sensed on input 157 in the Climate Control head.

If you add a new compressor and do not install the old AIVMPS sensor, then recirculate mode will never be activated. To bypass this, I simply cut the vehicle side connector on the AIVMPS pigtail, added a terminal, and grounded it to the engine block. Now, recirc mode is enabled 30 seconds after A/C is turned on Hi/Med with the temp control on full cold. If I ever want fresh air on Hi or Med fan speed, I move the temp control one notch in the warm and fresh air is activated.

I also found it interesting that the AIVMPS is not a high pressure cutoff switch (HPCO) for the compressor. As far as I can tell from limited understanding of wiring diagrams, the AIVMPS does not have any input to the compressor clutch to prevent overpressure.

One last note is that the fresh air door actuator on 88 - 90 C/K units has a 5 wire input rather than the 2 wire seen on '91 and later

Thanks for reading and I hope this helps someone"
 

phaessler

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Here's a writeup I posted on another forum regarding the auto recirc feature of 88-90 GMT400s (No max/recirc buttons)

"I wanted to add a writeup on the operation of the recirculate/fresh air door operation for 88 - 90 C/K models. This info only pertains to vehicles that do not have a 'Max' button on the electronic climate control unit.

I searched for this information on this and other forums and there seemed to be much confusion over the operation of these style units. The common misconception is that recirculated air is only activated when the fan is on Hi or Med with temp control full cold. If your system is stock and includes the high pressure sensor (actually called the Air Inlet Valve Motor Pressure Switch (AIVMPS)) on the back of the AC compressor, then the operation is as follows from the 1990 Electrical Wiring Manual:

"The Air Inlet Valve is normally in the position which allows outside air to enter the system. In this position, the Air Inlet Valve Potentiometer returns about 4.9 volts to the Heater and A/C Controller. When the A/C Button is depressed, and if the temperature setting is at full Cold and the blower is set to MED or HI, the Air Inlet Valve Motor will move the valve to the recirculating position 30 seconds after the Air Inlet Valve Motor Pressure Switch closes. The Air Inlet Valve is opened again, 5 minutes after the Air Inlet Valve Motor Pressure Switch opens. It opens immediately if the temperature setting is raised, the blower speed is reduced to Low, or the air flow is directed to the floor outlets"

Details on the AIVMPS are as follows: Closes at 210 - 255 PSI, Opens Above 165 PSI, Minimum pressure differential of 45 PSI. This is a single wire unit, so when the switch is closed at 210-255 ground is sensed on input 157 in the Climate Control head.

If you add a new compressor and do not install the old AIVMPS sensor, then recirculate mode will never be activated. To bypass this, I simply cut the vehicle side connector on the AIVMPS pigtail, added a terminal, and grounded it to the engine block. Now, recirc mode is enabled 30 seconds after A/C is turned on Hi/Med with the temp control on full cold. If I ever want fresh air on Hi or Med fan speed, I move the temp control one notch in the warm and fresh air is activated.

I also found it interesting that the AIVMPS is not a high pressure cutoff switch (HPCO) for the compressor. As far as I can tell from limited understanding of wiring diagrams, the AIVMPS does not have any input to the compressor clutch to prevent overpressure.

One last note is that the fresh air door actuator on 88 - 90 C/K units has a 5 wire input rather than the 2 wire seen on '91 and later

Thanks for reading and I hope this helps someone"


Ok, so my line of thought wasnt far off, and perhaps the "fix" is simpler than I was planning. Thank you
 

DerekTheGreat

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Oh I read it, thanks for the input haha. I still have the '93 head unit in my truck and it's bonkers now. I can't remember if I said something about it but I believe part of the problem is the head unit does not take battery voltage and step it down. Early crap ('88-'90) uses 8v to run the servo motors and the later stuff uses 5v. I haven't been able to see if the signal return wires are in the same spots '89 vs '93 but I assume they are since my system worked fine for a few days. Then the higher voltage must have popped something so now everything flashes like a Christmas tree. Been waaay too lazy and had other things going on to mess with it & finger bang it to see what's what and was trying to get a '93 EVTM before I tore into it, not to mention I've got another unicorn '91 tach cluster I might as well test out when I'm there and recalibrate my speedometer so it's no longer reading 2mph faster than it should be as well.
 
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