A/C Request Output (To ECM) wire
Thread Starter
Instructor
Joined: Oct 2007
Posts: 156
Likes: 1
A/C Request Output (To ECM) wire
Hello,
I finally had the A/C charged on my swap car and am now trying to get the A/C clutch to engage.
This car has the C68 electronic HVAC controller. When the HVAC controller is in Auto, Recirculate, the front and lower vent option, and defrost the Gray/Red "A/C Request" wire from terminal C6 on the A/C Programmer to R15 on the L98 ECM has 0 volts. When I select Off, Front vent, and lower vent the A/C Request wire has 5 volts.
I'm assuming the request wire is supposed to supply 12 volts when the A/C is on. Is this correct?
Because I no longer have the L98 I currently have the request wire wired to the high pressure cutoff switch which goes to the pressure cycling switch and finally the A/C Clutch relay.
I finally had the A/C charged on my swap car and am now trying to get the A/C clutch to engage.
This car has the C68 electronic HVAC controller. When the HVAC controller is in Auto, Recirculate, the front and lower vent option, and defrost the Gray/Red "A/C Request" wire from terminal C6 on the A/C Programmer to R15 on the L98 ECM has 0 volts. When I select Off, Front vent, and lower vent the A/C Request wire has 5 volts.
I'm assuming the request wire is supposed to supply 12 volts when the A/C is on. Is this correct?
Because I no longer have the L98 I currently have the request wire wired to the high pressure cutoff switch which goes to the pressure cycling switch and finally the A/C Clutch relay.
Melting Slicks
Joined: Aug 1999
Posts: 2,240
Likes: 42
From: Baltimore, MD USA
According to the service manual if you use a test light from pin C6
of the AC Programmer to ground or pin R15 of the ECM and set the
Control Head to AC or Defrost mode the test light should light.
It doesn't say what the actual voltage should be. Only that the test
light should light.
Even if the signal voltage is 12 volts, tying that wire to the high
pressure cut off switch probably won't work. The circuit can't provide the
necessary current needed to drive the clutch coil.
In the original factory configuration, the ECM would ground the AC Clutch
relay and the AC 25 amp fuse provided the 12 volts for the Clutch Coil.
If the AC request signal is 12 volts, rewire the AC Clutch relay so the
12 volts triggers the primary coil of the AC Clutch relay.
If the AC request signal is 5 volts, purchase a relay that can be triggered
by 5 volts and can handle an output of 12 volts @ 25amps.
of the AC Programmer to ground or pin R15 of the ECM and set the
Control Head to AC or Defrost mode the test light should light.
It doesn't say what the actual voltage should be. Only that the test
light should light.
Even if the signal voltage is 12 volts, tying that wire to the high
pressure cut off switch probably won't work. The circuit can't provide the
necessary current needed to drive the clutch coil.
In the original factory configuration, the ECM would ground the AC Clutch
relay and the AC 25 amp fuse provided the 12 volts for the Clutch Coil.
If the AC request signal is 12 volts, rewire the AC Clutch relay so the
12 volts triggers the primary coil of the AC Clutch relay.
If the AC request signal is 5 volts, purchase a relay that can be triggered
by 5 volts and can handle an output of 12 volts @ 25amps.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Huh? L98 ECM uses Pin B8 for the a/c request which only controls idle and WOT cutout. It's 6 or less volts for Electronic Air; Battery for Manual and can handle either input. Is this some other ECM?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
PS: Electronic Controls that use the Dash as the Processor has to sense a completed circuit on the ECM feed (it's a separate circuit) in order to power the compressor. If it doesn't the signal isn't sent. Simplest compressor operation would be out from the Panel to High and Low Cutouts and then onto the Compressor (which is what the Manual Setup does), but early Electronic Controls doesn't and uses 3 circuits to accomplish this. It's a complex and crappy design but if it doesn't have all the parts, it ain't going to work. I'd find a Manual Panel in a boneyard and start over.
Thread Starter
Instructor
Joined: Oct 2007
Posts: 156
Likes: 1
Thanks for your help so far. 
Unfortunately I don't have the FSM but I do have the factory electrical supplement.
Here's what I'm looking at.
Right now I'm not understanding why the A/C request voltage drops to zero when I turn on the A/C and is at 5 when the A/C is off. Do you think the FSM would provide more detail on the operation of the electronic climate control over the electrical supplement manual?
I have the L98 ECM installed in the rear compartment. Currently it only has power, ground, and the two tan serial data wires connected to it. Would it be possible to have the A/C request wire and the Gray A/C clutch relay wire (999 on the diagram above) connected to the L98 ECM so the A/C functions as factory?
If anyone has any ideas they would be appreciated!
Here is the new motor for anyone that's curious.
Unfortunately I don't have the FSM but I do have the factory electrical supplement.
Here's what I'm looking at.
Right now I'm not understanding why the A/C request voltage drops to zero when I turn on the A/C and is at 5 when the A/C is off. Do you think the FSM would provide more detail on the operation of the electronic climate control over the electrical supplement manual?
I have the L98 ECM installed in the rear compartment. Currently it only has power, ground, and the two tan serial data wires connected to it. Would it be possible to have the A/C request wire and the Gray A/C clutch relay wire (999 on the diagram above) connected to the L98 ECM so the A/C functions as factory?
If anyone has any ideas they would be appreciated!
Here is the new motor for anyone that's curious.
Melting Slicks
Joined: Aug 1999
Posts: 2,240
Likes: 42
From: Baltimore, MD USA
Since it's obvious you've done some major surgery, here's
a couple of questions.
1) Is the CCM still working?
2) If so can you short pin A to pin G and display codes on
the dash cluster speedometer area?
3) Do the AC control head buttons function normally?
4) Does the inside and outside temp display properly?
5) Can you increase and decrease the temp display?
6) Has any electrical harnesses/wiring been removed?
7) Disconnect the negative battery terminal to clear any
codes that may be set.
Remove the end of the wire you have going to the high
pressure switch. Turn the AC on and check the voltage
of the AC request signal.
The manual does say it should be battery voltage (12 volts).
a couple of questions.
1) Is the CCM still working?
2) If so can you short pin A to pin G and display codes on
the dash cluster speedometer area?
3) Do the AC control head buttons function normally?
4) Does the inside and outside temp display properly?
5) Can you increase and decrease the temp display?
6) Has any electrical harnesses/wiring been removed?
7) Disconnect the negative battery terminal to clear any
codes that may be set.
Remove the end of the wire you have going to the high
pressure switch. Turn the AC on and check the voltage
of the AC request signal.
The manual does say it should be battery voltage (12 volts).
Last edited by Hooked on Vettes; May 19, 2011 at 01:03 AM.
Thread Starter
Instructor
Joined: Oct 2007
Posts: 156
Likes: 1
Since it's obvious you've done some major surgery, here's
a couple of questions.
1) Is the CCM still working?
Yes.
2) If so can you short pin A to pin G and display codes on
the dash cluster speedometer area?
Yes.
3) Do the AC control head buttons function normally?
Yes, everything functions normally.
4) Does the inside and outside temp display properly?
Yes.
5) Can you increase and decrease the temp display?
Yes.
6) Has any electrical harnesses/wiring been removed?
No, only the TPI wiring was removed... The LS3 harness was made to plug and play at the C100 and C237/8/9 connectors. Nothing else was disturbed.
7) Disconnect the negative battery terminal to clear any
codes that may be set.
I'll give that a shot tomorrow.
Remove the end of the wire you have going to the high
pressure switch. Turn the AC on and check the voltage
of the AC request signal.
The manual does say it should be battery voltage (12 volts).
Thank you for that!
a couple of questions.
1) Is the CCM still working?
Yes.
2) If so can you short pin A to pin G and display codes on
the dash cluster speedometer area?
Yes.
3) Do the AC control head buttons function normally?
Yes, everything functions normally.
4) Does the inside and outside temp display properly?
Yes.
5) Can you increase and decrease the temp display?
Yes.
6) Has any electrical harnesses/wiring been removed?
No, only the TPI wiring was removed... The LS3 harness was made to plug and play at the C100 and C237/8/9 connectors. Nothing else was disturbed.
7) Disconnect the negative battery terminal to clear any
codes that may be set.
I'll give that a shot tomorrow.
Remove the end of the wire you have going to the high
pressure switch. Turn the AC on and check the voltage
of the AC request signal.
The manual does say it should be battery voltage (12 volts).
Thank you for that!
Thread Starter
Instructor
Joined: Oct 2007
Posts: 156
Likes: 1
I checked the CCM and HVAC codes earlier and they had no fault codes. I checked the A/C Request wire again and it definitely has no voltage when an A/C function is selected and 5 volts when a non A/C function is selected or the climate control is off.
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Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Be nice to know what your trying to adapt to this LS motor which doesn't use a cycling orifice compressor. Might be easier to use the variable displacement compressor (which is far more compact) and eliminates the Low Pressure Switch, but you should then be able to use the LS PCM to drive the Relay.
Your a/c signal drops to 0 at the Programmer because it's grounded at the PCM which should then ground the Relay. The Relay will open if the Low Pressure or High Pressure Cutout is open or if it uses a Pressure Sensor, the return voltage is below .8 or doesn't rise with compressor engagement. Fault Coding for the Pressure Sensor is in the PCM, not the CCM.
Charging something that doesn't run is a bit difficult. Check the low contacts to make sure they're closed; ditto the High Limit because someone may have crammed too much into it or nothing at all if doesn't work.
Your a/c signal drops to 0 at the Programmer because it's grounded at the PCM which should then ground the Relay. The Relay will open if the Low Pressure or High Pressure Cutout is open or if it uses a Pressure Sensor, the return voltage is below .8 or doesn't rise with compressor engagement. Fault Coding for the Pressure Sensor is in the PCM, not the CCM.
Charging something that doesn't run is a bit difficult. Check the low contacts to make sure they're closed; ditto the High Limit because someone may have crammed too much into it or nothing at all if doesn't work.
