still working out the kinks of the carb conversion and have a question. speedo works, fan works, car drives good ,lumpy idle nice.. but the a/c is not working. spent some money on it last year pre carb cold as ice. now i cant even jump the pressure switch and make the clutch kick on. of course iv deleted the temp switch which is one function the ecm has to see befor it will engage the clutch. so what i did was jump the temp switch with a paper clip ( solved that input )? alldata is showing an a/c fuse below or around the coolant tank at the firewall pass side. going to look for it on friday. all the other fuses right side of the dash are good as some supply more than one operation. any suggestions about this? all i need for now is to be able to jump the switch to make the clutch engage. i can handle any other issues after that. thanks..

 

ECM drove or grounded the compressor relay on the '90. Find the Relay and splice in a switch on the ECM side and you should be able to get it to engage. Use the Alldata schematics to determine the circuitry (which the last time I looked at an Alldata scheme had them spread out between the HVAC section and Computer Controls - so you might have to hunt around for awhile before you find it). You will need to make sure that the low pressure switch is in line with power to the clutch (otherwise, there's a good chance it'll be making ice, but little cold air) and you are going to need to add a high pressure switch on the same circuit to keep it safe. The pressure switch that is on the high side now is a thermistor and the ECM removed ground from the relay when it's voltage signal was below .8 volts (about 80 psi) or above 4 volts (about 400 psi). You need one that is inline with the clutch circuit and opens at 400 psi (which was used on everything up until '90). You may also have to fabricate a new high line to get it to fit or rig up some sort of adaptor because I doubt it will fit the bung that's on it now or you could go with a different compressor where the switch is incorporated into the rear of the head or go with one of GM'S variable displacement compressors and at least eliminate the low pressure switch. Of course that will also mean new bracketry, but since you've allready morphed into a carb setup, maybe it isn't that big of a deal. Your only other concern will be how to handle compressor load at idle so that it doesn't stall or start rocking back and forth as the compressor cycles at idle and remembering to turn it off before any stop light drags so that you gain back the 3 or 4 hp it's sucking up when engaged.