88 Fiero Duke automatic.

Is there a relay that I can jump that will turn the AC (air conditioning) compressor on?

I am under the impression that there is a seperate relay (not the main AC relay located on the firewall) that will specifically turn the compressor off if there is low pressure?

From what I read in the archives, if there is low pressure, a relay will not allow the compressor to turn on, I want to engage the compressor to start charging. What relay do I jump?


Thanks

You must disconnect the A/C Low Pressure Switch from the back end of the compressor and jump the two connector pins on the wire harness to each other. There are other ways, but none are as safe since this way keeps the high pressure switch connected which will shut off the relay in case you over pressurize the system. Here's why:

The power runs through a number of switches in SERIES, any one of which will prevent the A/C comp relay from turning on. Power runs through the HTR A/C Fuse on a brown wire to the A/C control head (pin N), then from (pin G) on a lt grn wire to the A/C Low Pressure Switch (which only closes above 8 psi so you are bypassing this switch), then on a gry/red wire to the A/C High Pressure Switch (which will open above 425 psi providing you a safe-guard), then on a lt blu wire to the A/C compressor control relay coil (pin A), then finally from pin C on a drk blu wire to the ECM. The ECM controls the ground for the relay's coil by sensing on another wire whether all the switches are closed, then the ECM waits about 2 secs to adjust the IAC before finally grounding the the relay coil, so don't expect immediate results when you bypass the low pressure switch.

(edited to change low pressure cut-off pressure from 55 psi to 8 psi)

[This message has been edited by Bloozberry (edited 09-08-2009).]

It's not necessary for the compressor to be running when you begin charging the A/C system. Refrigerant will quickly pressurize the system to ~80 psi, at which point the low pressure cutoff switch will close. The V5 compressor in the 4-cylinder Fieros may cycle on and off a few times early in the charging cycle, but it should eventually remain running as charging continues and the operating pressures stabilize.

quote Originally posted by Bloozberry: You must disconnect the A/C Low Pressure Switch from the back end of the compressor and jump the two connector pins on the wire harness to each other. There are other ways, but none are as safe since this way keeps the high pressure switch connected which will shut off the relay in case you over pressurize the system. Here's why:

Which one is the low pressure switch?

The one with the light green and gry/red wires (duhh. )

quote Originally posted by Bloozberry: the A/C Low Pressure Switch (which only closes above 55 psi so you are bypassing this switch)

quote Originally posted by Marvin McInnis: Refrigerant will quickly pressurize the system to ~80 psi, at which point the low pressure cutoff switch will close. .

So I initially need to charge the low side to 55 to 80 at the very onset of charging up the system. The compressor will kick in and take the pressure down and I can continue adding the R134 to the recommended 25 to 45.

quote Originally posted by jetman: ... and I can continue adding the R134 to the recommended 25 to 45.

NO!!! Except in very special circumstances, you cannot use pressures to determine when an A/C system is properly charged. The only way to properly charge a Fiero is by refrigerant weight.

Going from memory, the FSM (Factory Service Manual) specifies 42 ounces of R-12 for a Fiero with a DA-6 or HR-6 compressor (e.g. the V6 Fieros). The rule of thumb is that you would reduce that by 15% to 20% for R-134a, which would calculate out to 34-36 ounces of R-134a. R-134a comes in 12 ounce cans, so three of them would be just about right for most Fieros. Don't take my word for it; check this for yourself before recharging. Seriously overcharging an A/C system will usually damage the compressor.

[This message has been edited by Marvin McInnis (edited 09-07-2009).]

quote Originally posted by Bloozberry: You must disconnect the A/C Low Pressure Switch from the back end of the compressor and jump the two connector pins on the wire harness to each other. There are other ways, but none are as safe since this way keeps the high pressure switch connected which will shut off the relay in case you over pressurize the system. Here's why: The power runs through a number of switches in SERIES, any one of which will prevent the A/C comp relay from turning on. Power runs through the HTR A/C Fuse on a brown wire to the A/C control head (pin N), then from (pin G) on a lt grn wire to the A/C Low Pressure Switch (which only closes above 55 psi so you are bypassing this switch), then on a gry/red wire to the A/C High Pressure Switch (which will open above 425 psi providing you a safe-guard), then on a lt blu wire to the A/C compressor control relay coil (pin A), then finally from pin C on a drk blu wire to the ECM. The ECM controls the ground for the relay's coil by sensing on another wire whether all the switches are closed, then the ECM waits about 2 secs to adjust the IAC before finally grounding the the relay coil, so don't expect immediate results when you bypass the low pressure switch.

Dang Bloozberry,

I am coming to you if I have troubles getting my project car A/C going! (88 with V-5) Nice explanation!

Charlie

Marvin McInnis is right about this not being necessary though. You should be able to fill the low pressure side beyond 55 PSI which will flip the low pressure switch internally, then it's a matter of running the compressor and topping it up with the right weight of refridgerant.

quote Originally posted by Marvin McInnis: It's not necessary for the compressor to be running when you begin charging the A/C system. Refrigerant will quickly pressurize the system to ~80 psi, at which point the low pressure cutoff switch will close. The V5 compressor in the 4-cylinder Fieros may cycle on and off a few times early in the charging cycle, but it should eventually remain running as charging continues and the operating pressures stabilize.

V5 doesn't use a clutch switch. Usually doesn't cycle unless low cutoff is tripping. Use small refrigerant can? From empty, it usually uses 1.3-2 can before you turn on the system. Use can in upright, gas, or you can damage the system.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

Ok for a review, this is what i need to do,,,,

1) Disconnect the A/C Low Pressure Switch from the back end of the compressor and jump the two connector pins on the wire harness to each other.
OR,,,,
2) Fill the low presure side beyond 55 psi and that will flip the low presure switch.

It made me nervous that it took barely a 1/4 can to get the low pressure side to 50 lbs on an empty system, that's why I stopped to asked for guidance from the experts. I'll proceed tonight (weather permitting) with the small test of the system to see where I'm at on this. Thanks for the assistance so far everyone, all this is new to me.

1. No. Never do that.
2. Yes.

It will take plenty of refrigerant with system off. Them turn on system to add some more.

R134 12 oz (I think) can... Check at the end of 3 can. 3 usually does it. A ounce or two but not whole can for 4.

System "low" side still has room. "Low" side when the system is off, pressure going up to 150-180 psi.

Low cutoff work at 8 psi, 55 kPa metric.
Hi cutoff work at 425 psi, 2930 kPa metric.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

[This message has been edited by theogre (edited 09-08-2009).]

quote Originally posted by theogre: Low cutoff work at 8 psi, 55 kPa metric.

My mistake... service manual says low cut-off is as theogre says... 55 KPa not 55 PSI. Must've been late when I read that.

Back to square one. I filled the low pressure side to 62 lbs, nothing happened, no cold air, pressure didn't go down, I only have about 1/3 of a can in the system. I don't know if the compressor is cycling or not? Seems like all I did was to fill the accumillator. So where is this low pressure cut off switch at, on the accumillator?

Did you evacuate the system before attempting to charge it? In an earlier PM to me you said, "I know the system was open for a little bit ..."

[This message has been edited by Marvin McInnis (edited 09-09-2009).]

quote Originally posted by Marvin McInnis: Did you evacuate the system before attempting to charge it? In an earlier PM to me you said, "I know the system was open for a little bit ..."

I really didn't want to spend the money to do that if there's a leak or the compressor isn't working. I might try turning on the low pressure switch by jumping the connector like blozzy said. If not then I can duct tape the trigger on the R134 can while I open up the high side and wait until I see the stuff coming out, that'll force the air out. I looked and I didn't see any solenoid switch on the accumilator so I figure that it must be on the compressor, right?

Yes. The low pressure switch is on the back of the compressor.

quote Originally posted by Bloozberry: Yes. The low pressure switch is on the back of the compressor.

I just cannot get to the LP switch in the apartment parking lot, landlord running around, will drive it into work tomorrow. I did flush out the air by forcing the R134 through the system and then out of the high pressure outlet so I'm good to go now.

quote Originally posted by jetman: I just cannot get to the LP switch in the apartment parking lot, landlord running around, will drive it into work tomorrow. I did flush out the air by forcing the R134 through the system and then out of the high pressure outlet so I'm good to go now.

No, that won't get it but your system may work a little while before you need to replace the expensive parts. For short term you may have some success but long term, you are setting yourself up for failure. For now though, figure out why you cannot get the clutch to engage. Once you get the system figured out you can decide if you want to fix it short term or long term.

Charlie