ac fans dont come on with ac

[jeff73]

Anybody know how to get the fans to turn on via the pressure switch. It is a 6.0 from a 09 truck with a e38 in a 73 Camaro. The ac is a vintage air unit and they are no help with this. Seem to only know about there own stuff. I have them to come on at about 170 psi on the high side to 100% in system fan desired% vs. ac pressure. Does it have to have a request to the pcm for this to happen. Switch is between the condenser and the compressor on the high side. I only have the binary switch in thinking the ecm switch would turn them on. The press. switch is the 3 wire switch from the doner truck. Any thoughts? Thanks.

[parsonsj]

I've been through this with a GMPP crate motor. What you are calling a three wire "switch" is actually a pressure sensor. Can you see the AC pressure as seen by the E38 via the scanner? What pressure is your AC generating? Is your setup for PWM fans? Or relay-controlled? Also, can you post your fan tables?

[jeff73]

Yes, it is a pressure sensor. The scanner shows no pressure. I let it run up to about 325 psi and nothing. The fans are relay controlled. I hope the I did the screen shot right.
Fan Screen Shot.JPGFan Screen Shot.JPG

[parsonsj]

Double check your wiring, I guess. If the scanner doesn't see AC pressure, neither does the ECM. You should have 5V from the ECM to the sensor along with ground, and a 0.5V to 4.5V signal coming from the pressure transducer.

Do the fans behave themselves with coolant temp?

Are both the ECM and the AC pressure sensor from the same donor vehicle? If not, the OS in your ECM may not support AC pressure input.

One note: your fan tables look ok, but you can probably keep your AC working fine with one fan. You probably don't need both running. Run one fan from 150 psi to 230 psi, then have the other one come on around 230. Your binary switch will cycle the compressor off at 406.

[jeff73]

I do have a 5v signal and the ground is good. I also ha

[jeff73]

I do have a 5v signal and the ground is good. I also have about 2.8v when I back probe the connector. Yes the ecm and sensor came from the same truck. Seems it would have to be the connection at the ecm on the signal wire. Don't know how to check this one. Thought it was something in tuners I overlooked. Fans work fine.

[parsonsj]

I agree... it seems the ECM isn't seeing the signal. Check the wire for continuity, I guess, if you haven't already.

[jeff73]

Yes, all wires are good. Gotta be the pin or something. Still working on it. You know how it goes. Other fish to fry. Thanks

[jeff73]

Does anybody know how to check the continuity between the pcm and the harness? Can I go from the pigtail at the ac pressure sensor to chassis ground? Will it show anything with a meter and will it hurt anything? thanks