Yes, if I remember correctly the 91 Vette ECM controls the A/C compressor clutch directly. The Vette ECM looks for an input signal from the heater controls and A/C pressure switch.
Ok, just looked at the diagrams for both cars. Here is the deal. The 91 Vette ECM looks for what appears to be a ground signal on PIN B15 from the A/C controls to determine when the A/C is being requested. According to my diagrams, it looks like when the wire that goes to PIN B15 is grounded, the ECM thinks the A/C is being requested. Now this is different from the Fiero in that the A/C controls for the Fiero supply IGN B+ voltage to the A/C "request" wire instead of grounding it so you will have to hook up a relay in the following way to interface the stock Fiero A/C controls with the Vette ECM...
Using a standard 5-pin relay, if the stock Fiero A/C harness and relay are not present:
Term 85 hook to GROUND
Term 86 hook to LIGHT GREEN wire coming from A/C controls
Term 87 hook to GROUND
Term 30 hook to Vette ECM term B15
If you still have the portion of the stock Fiero engine harness that has the A/C relay, hook the additional relay up like this:
Term 85 hook to GROUND
Term 86 hook to LIGHT BLUE wire that goes to the stock Fiero A/C relay Term A
Term 87 hook to GROUND
Term 30 hook to Vette ECM term B15
This should take care of the A/C request part of the Vette ECM but there is more...
If you are using the stock Fiero A/C relay and harness, you will need to hook term A13 of the Vette ECM to the DK BLUE wire going to PIN C on the stock A/C relay, which is located next to the Fuel Pump relay in the engine compartment. If you have removed the stock A/C relay and harness, hook up a standard 5-pin relay in the following way:
Term 85 hook to LIGHT BLUE wire coming from stock A/C cycling Switch
Term 86 hook to Vette ECM pin A13
Term 87 hook to Fused IGN B+
Term 30 hook to A/C Compressor Clutch +
The 91 Vette ECM also wants to monitor the A/C clutch status. Connect the Vette ECM Term C17 to the + side of the A/C compressor clutch.
This should take care if it. Hope this does not confuse you more... Let me know if you have any problems.
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