That is what I assume I am pointing to. I was troubleshooting the ac system and it looks like this is the problem. The compressor was not coming on and I tried to fill it with r134a (I had it switched over professionally from r12 exactly a year ago), and the pressure gauge was reading high, so nothing came out of the can. I read on another post during a search (and can't find it again now, lol!) about how you can remove the plug from the condenser and jump the circuit with a paper clip. Well, with the paper clip in place, the air blows ice cold, so I can assume that the problem is this switch, right? If anyone has some insight (is the switch under pressure if I try to remove it, how do you depressurize if that is the case, etc.) or has done this before, please let me know... THANKS! --John

 

yeah thats the low pressure switch there. You can remove it by just unscrewing it and oyu shouldnt lookse too much if you get ot off there fast enough, it has a schrader valve behind it like on a tire to prevent it from completely draining.. If you jup the two wires and it runs that is probably the problem, either that or you dont have enough freon in the system but even if you do put a switch on it youll at least know its a good part seeing as how they are pretty cheap.

 

yup low pressure switch.


Does anyone know if the high pressure switch, (on the back of the compressor,) directly engages the fan relay, or does it go back to the computer, or what?

my fan does not come on with the AC, and I have to manually turn on the fan with a switch.

I may just wire up the high pressure switch manually to turn on my fan...

 

that high pressure switch is there to just turn the compressor off if the system pressure gets to be too high, nothing to do with the fan. the PCM itself is what controls the fans operation for the ac. It sees a request for AC operation and once it has determined that the compressor is running through a feedback from the compressor or relay(depending on system) it will then ground the fan control relay and turn the fan on so you have a seperate problem there.

 

so is the fan supposed to ALWAYS be on when the AC is engaged?

if that's the case, it would be simple to wire that up.

I figured that the fan would NOT be on when I'm on the highway.

 

The switch shown is the pressure cycling switch. The fan turns on and off as system pressure changes. Air flow across the condenser will also affect fan cycling.

 

so the fan shouldn't always be engaged?

 

scrapmaker
im not 100% positive on how the the ecm turns on the fan for the a/c operation, but my fathers electric fan on his carbuerated camaro comes on whenever the a/c is on. Also, on my car (TPI EFI with electric fan), i THINK the ECM energizes the fan relay when the a/c is on. I KNOW when i put in an adjustable thermostat/relay to energize the radiator fan relay, there was a wire that i connected directly from the Hayden relay to one of the wires on the LOW pressure switch ....whenever current flowed on that wire (ie when the a/c switch was on inside on the heater control) that baby fires up the radiator fan. I literally pulled that rubber covering off exposing the spade terminals and soldered that wire directly to the spade terminal and ran the wire down to the Hayden adjustable relay. I have the GM harness with the GM radiator relay hooked up and the Hayden (Oreillys) adjustable thermostatically controlled (with a probe stuck in the radiator) relay hooked up together so if one fails hopefully the other will kick in.

The a/c has a bunch of switches on it, the low pressure switch on the drier/receiver, theres a high pressure switch on the high pressure line and theres one or two the compressor itself. The a/c guy disconnected the high pressure switch and now the a/c wont cut off at WOT some im going to get after him.
Good luck

 

mine cuts off at throttle... and I couldn't find the 3rd pressure switch... only the low on the dryer, and the high on the back of the compressor...

I think I'll just wire up the fan to come on when the ac clutch engages... so I'll just splice into the wires there, then run it into my fan relay....

 

High pressure switch?

That beige colored thing on the high pressure line is what im talking about, in the middle. The orange/yellow blob on the top left is the overflow tank

 

Just in case you were wondering what area im talking about, heres a picture from farther away.

 

so this pressure switch goes off at a lower pressure, in time to activate the fan?

 

Scrapmaker, i think Meridius has a point, the ECM seems to sense the a/c on and energize the radiator fan relay. So whenever your a/c is running the radiator fan should be running. There is a dark green-white wire on my car that goes to the ECM as A/C on input.
I went to the library and pulled out the books for wiring diagrams and component locators for my year, those photocopies are very valuable.
The low pressure switch that coolcamaroz28 took a picture of, is the A/C pressure cycling switch, which turns the compressor on and off. The switch on the rear of the compressor is the high pressure cutoff switch and the one on the high side A/C line is the Fan Pressure Switch.
Thats on my 1986 EFI camaro, yours might be different. Fun huh?
By the way, i found the connector for the fan pressure switch disconnected and hidden under neath a flange of somesort.
I think the easiest thing to do is find the wire that tells the ECM that the A/c is on and make sure that there is voltage on that wire going to the ECM scrapmaker. Check your harness on your coolant fan relay for burnt spots and look for that switch on the high side A/C line and make sure it is connected and working. Pull the connector off and jump the two terminals. Good luck

 

well my ecm turns on the fan just fine... when my temp reaches 230F... but not for the AC...

so if I cannot find another fusable link of some kind, I'll just rig it


so is the general consensus so far that the radiator fan is ALWAYS on with the AC... even on the highway?

 

Yes scrapmaker, thats the whole reason i bought the adjustable relay. The ECM sees the coolant temp hit 240 and energizes the radiator fan relay. When its 100 degrees outside, thats really not going to cut it. All of GMs calibrations are based on the 195 degree thermostat. The A/C on should trigger the ECM to run the radiator fan (makes sense to reduce the head pressures).

 

you have a variable fan relay? what exactly is variable about it?

 

You turn a **** and you can adjust the fan on temperature, it bypasses whatever the ECM wants to do since the radiator (hopefully) never gets to that turn on point of 240. It has a probe that pokes into the radiator and then engergizes the fan relay. The other way to do it (since mine doesnt have an additional fan switch on the block like some engines) is to change the coding in the PROM in the ECM. I think i bought that adjustable relay for like $30 from Oreillys, i think there are other manufacturers that have something similiar, like in summit.

 

The high pressure sw is a normally closed sw and it feeds an input to pin C9 on the ECM. Many guys who remove AC on the V-8's forget to ground C9 and the cooling fan(fans) run all the time. The input at C9 controls the operation of the fan relay driver at I think, C2 on the ECM.

 

I know this is an old thread, but might be what my problem is. 87 Trans Am 305 TPI Auto. AC is still on the car but the belt is missing so assuming the compressor is bad. Both my fans run continuous. If the motor is running both fans are running. I have changed the sensor in the head and the fan relays but no luck??
Thanks