3.4 DOHC swap A/C wiring

Aaron · Post by **Aaron** » Thu Dec 04, 2008 3:31 pm

I have a few questions on wiring for a 3.4 DOHC swap in regards to keeping the A/C. Using a 91-93 ECU, with a Sinister chip.

Ok, where to begin...So the ECU has these terminals for A/C wires:

A12: A/C Relay Control
C17: A/C Request
C21: A/C Pressure Signal

The Fiero C203 A/C terminals look like this:

D: A/C Request
N: A/C Relay Clutch 12v + power feed (hot when HVAC is on A/C)

This all seems pretty self explanatory. A12 controls the relay, C203 N provides power to the relay, power out of the relay goes to the clutch on the compressor, C17 splices with C203 D for the request. However, there's more to it. The 3.4 DOHC has a "A/C Pressure Transducer" that's wired in to give the ECU a 12v signal when A/C is on. I can do this halfass, and just give it 12v whenever A/C is turned on, but I'd rather it done right. So where do I get the pressure signal in the Fiero A/C system?

The car had A/C of course, but the 2.8l ECU did not have a pressure input that I'm aware of, and none of the C203/C500 wires were for A/C pressure. So I assume, if it had a pressure sensor, it was at the front of the car. Steven mentioned running wires through the firewall up front, but didn't go into quite enough detail.

Series8217 · Post by **Series8217** » Fri Dec 05, 2008 11:49 pm

The pressure transducer doesnt give 12V. It's a 0-5V pressure sensor. You do need it. You'll need to have the fitting for it welded on to a pipe anywhere in the high side of the system. Near the compressor works well. I haven't been able to find the correct fitting with the o-ring seal provisions, except on junkyard cars. You can cut it off and weld/solder it over a hole drilled in your AC lines. Make sure you clear all the chips.

whipped · Post by **whipped** » Fri Dec 05, 2008 11:51 pm

Since the ECM controls the AC, can't you use the high side(?) switch port that's in the front compartment?

Aaron · Post by **Aaron** » Sat Dec 06, 2008 2:27 am

Series8217 wrote:The pressure transducer doesnt give 12V. It's a 0-5V pressure sensor. You do need it. You'll need to have the fitting for it welded on to a pipe anywhere in the high side of the system. Near the compressor works well. I haven't been able to find the correct fitting with the o-ring seal provisions, except on junkyard cars. You can cut it off and weld/solder it over a hole drilled in your AC lines. Make sure you clear all the chips.

Thanks for the info. The FSM wiring pinouts said that the pressure signal is 12v when A/C is on.

Series8217 · Post by **Series8217** » Sat Dec 06, 2008 2:11 pm

Aaron wrote: Thanks for the info. The FSM wiring pinouts said that the pressure signal is 12v when A/C is on.

On the Fiero maybe. Not the DOHC; I have the manuals and they say it's a 0-5V pressure sensor, and I've confirmed it myself. Maybe you are thinking of the AC request signal?

whipped wrote:Since the ECM controls the AC, can't you use the high side(?) switch port that's in the front compartment?

No, it has the right threads but no step to capture the o-ring on the sensor so it doesn't seal at all. You have to cut a fitting off of a car that uses the same pressure sensor.

Aaron · Post by **Aaron** » Sat Dec 06, 2008 3:04 pm

Series8217 wrote:
Aaron wrote: Thanks for the info. The FSM wiring pinouts said that the pressure signal is 12v when A/C is on.
On the Fiero maybe. Not the DOHC; I have the manuals and they say it's a 0-5V pressure sensor, and I've confirmed it myself. Maybe you are thinking of the AC request signal?

Look at the pinout for the ECM C plug for a 91-93 DOHC. Pin C21 reads this:

A/C PRESSURE SIGNAL 314 DK BLUE 732 A/C OFF 0*, A/C ON 12V.

However, the reference signal to the pressure switch comes from C7, which is a 5v reference for the MAP sensor as well. So I'm sure it is 5v, but the FSM has a mistake in it.

Series8217 wrote:
whipped wrote:Since the ECM controls the AC, can't you use the high side(?) switch port that's in the front compartment?
No, it has the right threads but no step to capture the o-ring on the sensor so it doesn't seal at all. You have to cut a fitting off of a car that uses the same pressure sensor.

Now, will it hurt it at all if I just bypass the sensor and give the ECU 5v all of the time? Will that hurt the compressor if it is low on pressure, yet he runs the A/C?

Series8217 · Post by **Series8217** » Mon Dec 08, 2008 2:15 am

It wont turn on the compressor if you give it 5V. That's an over-pressure condition. It will not turn on the AC clutch.

Aaron · Post by **Aaron** » Mon Dec 08, 2008 10:46 am

Alright. I'll find a switch at the junkyard then.

Aaron · Post by **Aaron** » Mon Dec 08, 2008 5:30 pm

One more question about the A/C relay.

There are 4 wires, this is how I think it works, can someone verify/correct it?

Green: A/C clutch power out
Blue: Ground
Lt Blue: A/C clutch power in
Black/White: Relay Control

Seems weird that the blk/wht wire would be the control instead of the ground though. In the factory harness, the blue, blk/wht, and lt blue wires all went inside the cab, whereas the green wire stayed in the engine bay (So it must be power out to the clutch). But I'm not sue why blue would go to the cabin if it was ground. Furthermore, once inside the cabin, the blue wire goes to an ECU plug on the 2.8l.

Aaron · Post by **Aaron** » Mon Dec 08, 2008 6:19 pm

Ok I figured it out. The A/C Relay does not have a Ground.