Hello, I've been trying to figure out my A/C issue and any help would be appreciated. I'm not a mechanic, I know just enough to get myself in trouble.

So, driving home from VT on Thursday A/C cuts outs, I left it on for an hour or so while driving to see if it would come back on, fiddled with the dash switch a bit but nothing.

Friday, pull the switch out of dash, light still works and has a solid "click" still. I've replaced the switch before, that time you could tell it was dead.

Went to Napa and got a DIY refill. When I came out of store and started car A/C came on for about 5 seconds then cut out again. Tried filling refrig, nothing.

Went home, tried jumping low pressure switch harness with a paper clip, nothing.

Checked the two 7.5 amp fuses with multimeter, they are good.

Swapped relays under the hood (the two for cooling fans and the clutch relay), nothing. Jumped the clutch relay with paper clip and boom clutch engages and blows cold.

Too me this means the compressor is fine, the signal is just not getting to the relay so... bad switch at dash? To confirm this I tried using a wiring diagram and jump the white and brown wires in the switch harness, this immediately blew the 7.5 fuse under dash. I would like to be able to simulate switch function at the dash to see the compressor turn on before I buy another switch but alas I have no idea how to do that. Maybe I could ring out the prongs in the actual switch if I knew what state they should be in with switch opened and closed?

Could it be the thermal protection in the compressor itself is bad? If so wouldn't jumping the relay NOT work?

info: compressor replaced about 4ish years ago, switch sometime before that.

Thanks in advance, it's gonna be a scorcher in the northeast this week!

 

The brown wire at the switch just powers the indicator light. The A/C demand signal is a switched-ground system, starting at the curved-bar symbol on the fan switch. Connecting ignition switched power to a ground-signal system blew a fuse because there was no load in the circuit to resist the flow of current. Just switches, a few fuses and wire. In that situation, the fuse was the weakest link, so it blew as designed.

The bar on the fan switch (looks like switch terminal 6? connected to the wht wire) tells you that the wht wire gets connected to the fan switch common terminal (1, connected to the blk wire, ground) when the switch is in positions 1-4, but is open-circuit (no connection) when the fan is switched to off. The goal there is to only let the A/C system run if the fan is running. Fan off = A/C off. You can verify the fan switch bar is working properly by turning the fan on and off with the A/C button pressed. The indicator should turn on when the fan is on, and off when the fan is off - this is a function of that bar contact.
Similarly, the bar in the mode control switch tells you that the wht ground-when-fan-on wire gets connected to the blu wire (driver-wants-A/C signal) when the HVAC panel is set to either of the windshield defog/defrost positions. The A/C push button switch does the same thing when pressed: connects wht to blu. Meaning you can rule out a bad switch by turning the knob to defrost, or connecting a jumper between the wht and blu wires at the A/C switch connector.

After the climate control panel, the ground signal passes through the A/C pressure switch, which interrupts the signal if the refrigerant pressure is either too low (system leak) or too high (blocked orifice, system overheat or too much refrigerant). After that it's the thermal protector on the A/C compressor, which blocks the signal (shuts the system down) if the compressor is overheating. The signal then runs back to the MICU (multiplex integrated control unit, aka body control module) where it is turned into a CAN communications message. You can verify all of that stuff is okay by measuring the voltage at pin A21 (red) of the dash fuse box. It should read zero volts when A/C is requested and the protection switches are happy, something higher than that (12V or 5V or maybe 3.3V) when A/C is off or protectors unhappy.

After the MICU/BCM, the CAN message gets passed through the gauge cluster to the ECM. The ECM has a temperature sensor on the A/C evaporator (the finned air-cooling radiator part of the A/C system) which it watches to decide when to turn the compressor on and off. If the evaporator gets too cold (below freezing) then condensation from the air will collect on the fins and freeze into ice, blocking air flow. For that reason A/C systems should never run colder than about 40°F, something closer to 50°F is safer. The ECM controls ground to the compressor relay coil, which controls power to the compressor clutch coil, which pulls in the clutch to engage the compressor.

Since A/C demand and protector switches are all lumped onto one wire, the only A/C faults the ECM can diagnose (assuming it's programmed to look for it) are a short/open circuit on the evaporator temp sensor, a blown relay fuse, maybe a blown relay coil. If you have a decent scan tool, you may be able to find the status of the pin A21 as an A/C demand signal, which should toggle on and off when you play with the HVAC panel switches. You might also be able to find the evaporator temperature sensor value, to verify that the ECM is seeing a sane value there (as opposed to say 20°F on a 95°F day).

 

Great read. Both posts.

 

Thank you for the info. It's a little beyond my knowledge but I think there is some tests in your suggestions that I can handle. I'll try to report back if I make any headway.

Thanks again.