2005 Accord 2dr v6, 140k miles
AC was not cooling well, I noticed that I did not hear the fan/compressor cycling. Verified the compressor/fans did not cycle, compressor and fans were running constantly. Swapped relays around to see if that cured the problem no joy. Pulled AC clutch relay and manually pumped to verify clutch was engaging/disengaging - everything worked as expected. Re-installed relay and clutch will not engage at all. Replaced clutch relay with new - clutch will still not engage but the fans kick on with the AC switch. Ran diagnostic from control panel, no errors returned. Pulled single wire connection to compressor and verified no ground or voltage present. Pulled connector from pressure switch, verified fans do not cycle without pressure switch connected, cleaned pressure switch connections.

What should I check next?

 

Hi-Lo pressure switch has two sets of contacts. Pressure switch A is hi/lo switch (pins 1-4) and should be closed. Pressure switch B is mid level switch and Pins 2-3 should be open on a static system and close only when pressure is above 213 psi. A false closed switch might cause a problem. Pins 1-4 open should disable fans and compressor.

Jumping the compressor relay is dangerous as accidentally coupling 12V to PCM might damage the PCM. PCM provides a ground signal to pin 2 of the relay causing relay to close.

If AC is commanded on, pin 3 of compressor relay should show ground from PCM. If not, something is wrong w/ inputs (pressure switches, relay module, and gauge control module) to PCM, or PCM is faulty.

You need a wiring diagram to guide your work. If you don't have a shop manual suggest downloading one from automanualsource.com ($22).

good luck

 

Really Appreciate the reply, this is exactly what I need. Do you by chance know where I might find the pin diagram for the pressure switch and the relay respectively? I am assuming to test the pressure switch I am simply looking for continuity between pins 1&4 for switch A (unlikely since fan is kicking on) and continuity between pins 2&3 for switch B? What about the evap temp sensor?

 

The shop manual (see previous post) has pin diagrams in the Fuel & Emissions section. That and wiring diagram should help.

Switch B should be open unless system is running. It only closes at 213 psi, which can be reached in warm weather.

good luck

 

I found the attached diagrams of the relay, I am not sure the pin number you mentioned matches because pin 3 is always hot even with the car off. based on this diagram i'm guessing it is pin 1 that gets ground by the PCM.

The arrangement of the pins on the pressure switch is like below with * denoting the clip
-
| |*
-

 

That pin diagram is not necessarily for your MDX. It is impossible to say which is power and ground side of pins 1 and 2.

Compressor relay pin diagram and AC system circuit diagram are in the shop manual.

On pressure switch A (Hi/Lo) Pins are vertical looking at wire side of connector. Pin 1 over Pin 4. Pressure Switch B pins (2-3) are horizontal w/ 2 left of 3 as below. Again this is wire side view of connector.
1
2-3
4

For pressure switch, from wiring diagram pin 1 should have 12V and Pin 4 zero (ground) when AC is commanded On. Pressure switch pins 1-4 should be shorted when disconnected and tested.

good luck

 

Ok tested the pressure switch and it is as expected, white/grn(3) to black (2) = open, blu/red(1) to blu (4) closed.

The evap temp switch under the dash (red/blk to brown) is within spec at least as near as my cheapo multimeter can tell, ranges from 10k ohms warm to 30k ohms cold

Testing at the relay socket yielded some weird results, at least from an electrical novice.

I created a diagram of the relay socket, the numbers are my numbering scheme, the only thing I can tell you for sure is that jumping 1 to 2 locks the AC clutch. Based on my made up numbering scheme here are the values I got, mind you I don't have a high end multimeter by any stretch, quite the opposite.

1 - shows to be grounded and has 12 volts???
2- roughly 1.5 ohms to ground, 0 volts
3- shows to be grounded and has 12 volts???
4- 0 ohms to ground, 0 volts

The values don't change with the AC switched on/off. I am stumped but really hoping your not going to tell me my PCM is shot

 

Regarding AC compressor relay pins. Suggest mostly measuring voltage, not resistance.

Pin 1 should always have 12V. Measuring ohms to ground on live circuit is not relevant.

Pin 2 should show 0V relative to local ground either live or AC Off.

Pin 4 should have 12V w/ keyswitch On and 0V keyswitch Off. Don't measure resistance to ground. If not check fuse 30 (7.5A) in under dash fuse box.

Pin 3 should show grounded (resistance measurement) w/ keyswitch On and AC commanded On, and open (infinite resistance) w/ keyswitch off or AC Off.

good luck

 

Thanks, really appreciate the help. I've checked the fuses (several times), it seems like everything is present for the relay to close except the ground from the PCM. I was looking at the wiring diagram and I see another relay in play, I don't know where to find this relay along with the 'gauge control module'

 

Relay control module is in underhood fuse box. See pages 22-53, 22-91.

Gauge control module is part of gauge panel. Unclear what this module does to AC system, but it's an input for sure. See pages 22-66, 22-71.

In your previous post you say pin 3 shows grounded. If yes, then AC relay may be the problem.

good luck