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Discussion Starter #1
Looking for some help with a 2008 Exige A/C issue, though system should be the same with Elise and Exige. Pushing the Air Conditioning button with the fan on lights it up but the compressor doesn't engage and RPM's don't change. Testing shows no voltage at the compressor fuse under passenger dash. I assume the issue is between the button and the fuse but was unable to trace wires from the A/C control panel to the fuse so I'm at a loss. I've reviewed the wiring diagram but unfortunately it's unintelligible to me. I've searched the forums as well and haven't found anything that covers this part of the troubleshooting. Any help would be greatly appreciated.
 

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There is also logic somewhere that will not engage the compressor under certain conditions such as max and min limits on the pressure in the AC system.
 

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Look at sheet 10 of the wiring diagram, fuse c4 7.5 amps only gets power when the compressor is called to be engaged.
It will also only get power if fuse F19 has power

ECU pin 53 has to tell the switch pack to fire the compressor
The ECU gets information from the trinary switch and the thermostat

check your pressures
 

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Discussion Starter #5
Thanks Gus. There is no power to F19. I've seen discussions of ECU control of the AC circuit, but don't see it in the wiring schematic, which seems to show constant, direct power to F19 - but I'm new to wiring schematics so I could be missing it. I pulled the trinary switch and ran 12v across pins to confirm the relays are closing and also confirmed continuity between the relays and the output terminals so trinary issues should be ruled out. Do you know what data the ECU pulls from the trinary? If there's no power to F-19 it seems it wouldn't be able to pull any info from there.

It sounds like I'm down to the thermostat and refrigerant charge level and I assume the charge level is the much more likely culprit. Should I jump the compressor at the trinary or the 7 amp fuse in the passenger compartment so it runs while I check pressures? Thanks for your help!
 

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Have it checked at a service station? I reckon the A/C service stations will check the pressure of the system in static (engine off) state first. That pressure should be above the lower treshold of the trinary switch so the compressor is allowed to run when asked. When activated the high pressure measurement must be way higher then static - but below upper treshold, and the low pressure measurent (suction port) will be lower than base static pressure. I bought some gauges which i plan to use for my A/C rebuild project on my eliseworks.com blogpage.

https://www.eliseworks.com/interior...eaterbox-and-airconditioning-system-hvac-a-c/
 

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you put gauges on the high and low fittings, it does not need to run.

this is the first thing to do before jumping anything.

There are charts on the web that will show you static pressures at temperatures to determine if there is any pressure in the system

do this before jumping anything

That said, there should be power to that fuse[F19] at all times
 

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Discussion Starter #8
No power to F19 just didn't make sense so I rechecked it. I had tried grounding it to the shiny fat torx bolt just under the windshield, which seemed an obvious path to ground. It wasn't! I checked to the windshield wiper shaft bolt and confirmed power to the fuse. I put gauges on the 134 fittings and found super low pressure. Adding some 134 kicked the compressor on and I refilled the system. I'll check pressures again tomorrow and look for leaks on the valves. I just bought this car so pressure may have been low from a leak or from the PO messing with the system. Thanks everyone for your help!
 
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