Are two of the A/C relay terminals supposed to be hot when car is in off position, or one terminal hot (battery voltage) and one cold?

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Okay, terminal 29 takes power from the relay to the A/C clutch. Terminal 28 is constant power from fuse 22. Terminal 30 is the relay control signal from the PCM. Terminal 31 runs on power that comes from fuse 22 as well and from there through the A/C pressure switch, if the is a refrigerant charge in the system that switch should be closed. So, you should have power at terminal 28 and 31. as they get power from the same fuse. Follow the highlighted path.

Thanks, Steve.
Terminal 28 currently only has power when the ignition is in the 'On' position.
If I jumper 28 and 29, the compressor will run and blow cold.
Should I be looking for a short between 28 (in the engine bay) and fuse 22 (in driver side dash)?

Additionally, I back probed the ECM (engine control module) at the terminal for the AC relay and while it has battery voltage when the car is off (correct reading), it also has voltage when the car is running and the AC is switched on (bad reading, should be 0v when A/C is engaged). Trying to determine if the ECM is bad or I have a wiring problem between 28 and fuse 22. I appreciate your help.

I dug around a bit and found an error in the diagrams. Fuse 22 should be switched power So both pins 28 and 31 should have power only when the key is on. If you jumper 28 and 29 and the compressor works, I would check for switched power at terminal 31. That should have power at the same time as 28. If it doesn't go, find the pressure switch and jump between pins 1 and 2 in its connector. Do you have power at 31 now? If yes, the pressure switch is bad. If you do have power at terminal 31 but still no A/C then check if pin 30 shows a ground when the A/C is selected on at the control panel. If it doesn't then there may be a problem in the blue wire w orange stripe.

I have continuity between 28 and fuse 22 (0.4 ohms), when key is in Off position 28 has 63.5mV, and 31 has 62.8mV.
With key power On (ac off), 28 and 31 both have 11.94v. With power On(ac on) both show 11.7v.

30 shows no ground with power on and ac switched on.

In previous testing terminal 33/connector 135 on ECM (ac relay control) showed battery power in the Off and power On (A/C Switched On) positions. I assume when the AC is switched on 33/135 on ECM is supposed to ground?

Sounds like the power side is okay and it has some charge because if the system was empty, it would drop the power at terminal 31 as the pressure switch would open. So now you need to determine if the ECM is the issue. With the readings you had from testing, pull the A/C relay and use a test light connected to battery power. Turn the A/C on max cold and touch pin 30 in the relay socket. If the light comes on the ECM is switching properly, if it doesn't and your earlier test showed it has continuity then we need to verify the powers and grounds for the ECM. Attached are the diagrams for those grounds.

Battery-powered test light at Relay 30 is negative with car started and ac on max cold.

Continuity between Relay 30 and ECM B135 33 is good (0.9 ohms).

All ECM grounds listed show continuity (1.2 ohms), including GND Power Supply (B135 1 and 4).

Possibly not relevant here, but ECM B136 35 (Sensor GND) not showing continuity, but perhaps due to sensor not being on and system not plugged in?

So, pin 35 in B136 is a floating ground that goes to joint connector A and if it fails you will know it, the car wouldn't start or run.
So, you show continuity between 30 and the ECM but the ECM won't activate the A/C.
A couple more things to look at. There are two sensors (ambient air and interior air temp) that can tell the ECM not to run the /AC. Basically, those can tell the ECM that it's already colder than the temperature selected and not to run the A/C. The first two are the interior, it's in the dash next to the center stack. It can get clogged with crud or just fail. Testing it involves gaining access to it or to the AC control module behind the right side dash. To access it you remove the side panel of the console, and the sensor and connector are right there. Testing is simple, it's a thermistor with the attached temperature curve. Simply measure the resistance across the pins. An even faster way would be to use a scan tool to access the A/C module and see what it reports as the ambient and interior temperatures. For the ambient temperature sensor, it is mounted behind the front bumper, if your car shows external air temperature and it's close to actual temperature then it should be okay.
These two can stop the A/C if you selected say 72 degrees but the interior sensor was disconnected with it an open circuit it will max out and show -40C as the temperature! Probably don't need A/C at that temperature. Ambient would show the same.

Hi Steve,

For the interior air temperature sensor, I can access it directly, but if I can test via the A/C Control Module, I'd prefer that as it's currently accessible. What would be the method for testing the interior temperature sensor via the A/C Control Module?

Exterior temperature sensor is within 3° of the actual temperature.

Look at the diagram I attached above find the module and connector B282, in it are pins 5 (white with green) and 15 (blue) and measure the resistance between them. Compare that to the temperature curve above. So, say you measure 5200 ohms and it's 75 degrees out, the sensor is bad. Measure 1250-1275 ohms at 70 and it should be okay.
The issue then becomes that if the sensors test okay and the system pressures are enough that it should work but isn't then you will need a scan tool that can control the ECM and the A/C module, which would be the only way to determine which module isn't activating the A/C.

Thanks, Steve.
282 15 has no continuity to chassis ground.
Looks like 5 components directly connected to that ground:

Interior Temperature Sensor
Evaporator Sensor
Mode Actuator
Air Mix Actuator (x2)

Will begin diagnostic flows.

No rush. Oh, does everything else appear to be operating okay and it's just the A/C that doesn't? If so, I would sort of suspect the controller more than the ECM but without digging in with a scan tool and scope, I can't say that with 100% authority.

The weather is heating up!

Everything else runs great, aside from no compressor engagement.

I pulled the same ac control module out of a car at the salvage yard and plugged it in and nothing happened.

FYI, my driver side air mix actuator was clicking for 6 months prior to the A/C failure. I took the car in to the dealer to get the actuator fixed when the A/C went down, and they waived me off and told me I needed a new compressor...

Here's the current rundown:

Mode Actuator - Physically Operational
• Sensor Power Supply - (B282) 8 to 15: 4.9v
• Actuator Power Supply 1 - (B283) 7 to Chassis GND: 10v then cuts to 0.5v
• Actuator Power Supply 2 - (B283) 17 to Chassis GND: 10v then cuts to 0.5v
• All 5 harness connections btwn (B77) & (B282) have continuity
• Position Switch Signal - (B282) 3 to 15 shows appropriate voltage change when switching btwn DEF & FACE

Air Mix Door Actuator (x2) - Physically Operational (Both)
• Position Switch Power Supply - (B282) 8 to 15: 5v
• Actuator Power Supply 1 - (should be 7v)
Driver Side (B283) 9 to Chassis GND: Shows 10.7v then drops to 0.5v
Passenger Side (B283) 8 to Chassis GND: Shows 10.7v then drops to 0.5v (got it to read 8.3v steadily one time)
• Actuator Power Supply 2 - (should be 7v)
Driver Side (B283) 19 to Chassis GND: Shows 10.7v then drops to 0.5v
Passenger Side (B283) 18 to Chassis GND: Shows 10.7v then drops to 0.5v
• Continuity to be tested... Driver's Side Actuator is buried under the dashboard (can continuity test passenger side if needed)
• Position Switch Signal -
Driver Side (B282) 12 to 15 : Good
Passenger Side (B282) 4 to 15: Good

Interior Temp Sensor - Seems Operational
• 1.75kV at 75°f
• Input Signal - (i55) 2 to 1: 3.5v (should be 5v)
• AC Control Module Output Signal - (B282) 5 to 15: 3.5v (should be 5v)
• Continuity - (i55) 2 to (B282) 5 & (i55) 1 to (B282) 15: 0.5 ohms each

The resistance of the interior temp sensor itself is nearly identical (a bit higher) than the resistance of temperature sensor wiring harness (i55) 1 to 2. Not sure if that is helpful.

When I previously tested voltage of the ECM AC Relay Control (B135) 33 to Chassis GND (with engine on an ac set to max cool) it showed 14.2v (should be 0v). Which is why I was considering it might be the ECM.

I'm going to try a known functioning A/C Control Module from a local mechanic and see if that does anything.

This is probably a bit more than you bargained for, Steve. I appreciate your time and knowledge.

More data makes it easier in most cases. The signal wire staying at battery voltage means the ECM either isn't getting the signal to turn on the clutch, is being told not to, or it cannot internally. That is the question now. Which of the three is it. If you were to jumper that pin to ground the AC should function, but without any control. By testing the aspirator and ambient temperature you eliminated those, you already know the compressor will turn on and cool as well. That leaves the ECM and the control head. Both which communicate on the CAN to transfer that data. This would be when a scan tool would help, if you turn the A/C on and the scan tool shows that the ECM received that data, but the relay stays off, it's a bad driver in the ECM. If you turn it on but the ECM doesn't show it, it's likely the control head.
Now if you had a 94 or even 2004 version of this it would be much easier, they didn't stick everything in a module and make it hard to test.

Evaporator sensor is reading 1.2k ohms @ 78f

If I unplug the evaporator sensor connector, and short (B282) 13 to 15, would that effectively bypass the sensor? Or is the sensor sending more complicated data vs an open/close signal?

Separately, should (B283) 6 (A/C Power On to ECM) have continuity to GND once power is applied? Currently cannot get continuity to chassis ground for (B283) 6. If that is the case would it point to an actuator or sensor input to the A/C control module being the culprit?

That Evap sensor tells the control head if the core freezes up. The 1.2K sounds about right for the resistance.
Pin 6 at the ECM is a simple voltage trigger key on it should be around ground with the A/C on, battery voltage with the A/C off. Here are the tests for the ECM with the 2 for A/C control highlighted. The info I have doesn't do this for the A/C module.

Thanks, Steve. I'll test the ECM.
I've attached the terminal assignments for the AC Control Module

A few items I'm noticing after previous tests:

1. Interior Temp Sensor power supply shows 3.5v (should be approx. 5v) - (B282) 5 to 15
2. Evap Sensor power supply shows 3.5v (should be approx. 5v) - (B282) 13 to 15

*Could this be an issue?
**Is it worth looking at the (i3/B38) 11 connector for a short?

Versus:
3. Mode Door and Air Mix actuators showing 5v (B282) 8 to 15

4. Mode Door actuator power supply cuts out: 10v down to 0.5v
(B283) 7 to Chassis GND & (B283) 17 to Chassis GND
5. Air Mix actuators power supply cuts out: 10v down to 0.5v
(B283) 8 to Chassis GND & (B283) 18 to Chassis GND - Passenger Side
(B283) 9 to Chassis GND & (B283) 19 to Chassis GND - Driver's Side
*Is this normal operation?

Think it's figured out. I should have started with these tests...but lesson learned.

ECM:
A/C On Switch [from the AC Control Module] (B137) 17 - normal operation
A/C Relay Control (B135) 33 - normal operation

*A/C Middle Pressure Switch (B137) 30 - battery power when A/C is on, and off (should cut to 0v).

The actuators are somewhat a unique item. They use a pulsed signal back to the control head. Basically, an optical encoder that recalibrates at every start. So, 3.5 on the power to them is okay. They also won't stop the system from working. The 12 volts is a polarity reversal drive, it also will change voltage somewhat depending on actuator position and as soon as an actuator hits a travel stop the current spike causes the control head to cut power, until either the key on reset or you request a change in the door position.

The pressure switch only changes if the compressor activates. Basically, key on engine off with a charge in the system the low side turns on, then you turn on the A/C and the compressor starts, and the middle switch cycles the compressor on and off based on the system pressures, if the system pressure gets too high the high-pressure switch cuts power to the compressor. In yours those switches are all one transducer. The thing is that way back you said that if you jumped the relay the system would start and cool. That would mean that at least the low side is working. Based on all your current testing the issue is either in the control head or the ECM. The signal to the relay just isn't getting there.

One Correction from previous post on ECM.

ECM Terminals:
A/C On connector terminal (B137) 17 [from the A/C Control Module (B283) 6] - normal operation
*A/C Relay Control (B135) 33 - Battery voltage is present when A/C Power Switch is off (good), and when it is on (bad)
A/C Middle Pressure Switch (B137) 30 - Battery voltage is present when A/C Power Switch is off (good), and when it is on (bad)

When the A/C is switched on from the Control Panel/Head Unit, via the A/C Control Module, both ECM terminals [(B135) 33 and (B137) 30] should cut to 0v, they do not.

If the A/C power signal coming from the Control Panel to the A/C Control Module (B283) 6, to ECM (B137) 17 [battery voltage when ac is on, zero voltage when it is turned off] is functioning normally, would that mean the issue is not in the AC Control Module (or anything preceding the signal from (B283) 6? Isolating the problem to the ECM?

That would be the control side of the relay which should get pulled to ground to activate the A/C, as you tested earlier it doesn't switch. Your other testing shows that the remainder of the system seems to function. So, an easy test, connect a meter to the medium pressure switch and jump pin 33 to ground. Does the A/C work? Does the pressure signals change as it builds pressure? If the pressure signals operate properly then you know that it's the driver in the ECM that is bad.

Please correct me where I'm misinterpreting:

Step 1: Back probe (B135) 33 and jump to Chassis GND.
Step 2: Unplug pressure sensor connector and use the multimeter to test continuity across (B10) 3 and 4?
Step 3: Plug in the Pressure Sensor and test the A/C operation with a manifold gauge?

Step one- correct. Step 2 would be to simply observe the system, if you can back probe the pins on the pressure switch you can verify its operation as well. If it functions and the A/C is cold you already eliminated other areas and have narrowed it into the ECM. You could test with a gauge but simply measuring the temperature drop at the center vent on high would work.

Much appreciated, Steve.

Even though it's a brand-new compressor in the car, I decided to run the Lock Sensor diagnostic flow today. Got some puzzling (to me) results in Step 7 (attached pics). Anything to dig into here?

I found two loose terminal blades (F37) 16 and (B143) 14 in my Main Fuse Box (they appear to be physically connected/one unit) - pushing one blade will directly move the other (the blades are inline across two connectors).

In Step 7 of the attached A/C Lock Sensor diagnostic flow I measured the resistance between two sets of terminal blades as well as the corresponding connector terminals on the fuse box and kept getting O.L. for everything.

1. Are the loose terminal blades a factor in not getting a continuity reading?
2. If yes, are they affecting the signal to/from the A/C Lock Sensor?
3. Is the O.L reading in Step 7 related to the ECM?
4. In the attached A/Csystem wiring diagram the fuse box terminal [from the Lock Sensor] (B143) 14 connects to the ECM via (B135) 23 which looks to be the Camshaft Position Sensor GND - anything here? - I'm guessing that the lock sensor is syncing with the camshaft position sensor.
5. Could the Camshaft Position Sensor(s) be affecting the A/C system via the Lock Sensor?

Thank you!

It is not uncommon to find mated contacts. If you look at the various wiring diagrams you can see many instances where they use terminals like that simply as connection points without them connecting to power or ground inside the block, they are in. That is because they tend to reuse things like fuse blocks and harnesses, plus when your car was built the harness will have all the wires in it for all options that were available. Saves money by having just a single harness and saves errors on the line.
As long as there is continuity between the adjacent terminals it is okay.
I wouldn't worry much about the lock sensor though; its only job is to look at the compressor speed and the engine speed and make sure they match when the compressor engages. Yours doesn't engage and you are not seeing a code for an rpm mismatch so it's not an issue. It uses the RPM signal from the ignition module.
No on the cam sensor causing the A/C to not work. The lock sensor does nothing until the compressor engages.

Thank you for all the teaching on this.

With ECM AC Relay Control (B135) 33 jumped to Chassis GND, the AC Compressor fires up and blows cold.

Should I order an ECM and get it programmed?

*After some conversations with a local mechanic and another friend, it appears that if you added an iPod 1/8in jack kit to the stereo (which I did back in the day - Jazzy Engineering), it can affect the ECM negatively. I also added a cheap Bluetooth extender to the hack. Maybe this caused the ECM issue?

I've seen multiple issues caused by add-ons, to the point that if a vehicle shows up with say a remote start on it, I've been known to remove the entire thing and then start the repairs (in the event that the removal wasn't the repair!)
I would probably grab a used one and program it. That would be after I stuck a scan tool on it and verified that I couldn't control the AC through the system.

Thanks again, Steve.

I swapped in and tested a functioning head unit I pulled from the savage yard and two other AC Control Modules all to no avail.

I'm going all in on broken ECM.

Keep us informed please.

Will do...

If I were to put a scan tool on the car (before swapping the ECM), what would I be looking for exactly?

Well, with it you can watch as the A/C is turned on and off as to what the ECM and control head is doing. It should also show the switch changes as to the low/mid/high pressure line switch, clutch activation and more depending on the scan tool. So, say you turn the AC on, the control head data shows the AC commanded on and what pressures are shown, plus if the ECM is receiving the on signal and its response to that command. So, if it shows that all of the system is on and the ECM shows ON but there is no control signal to the relay, the ECM is the problem. Or if you turn it on and there is no status change from the control head, it is the likely problem.
However, you need to be careful in terms of the make of tool due to the coverage.
This is a neat video on some of them. https://www.youtube.com/watch?v=SHctRaX68Pw Note the prices!!! Topdon also has a wide range of tools at different prices.

Hi Steven,

Bought a new ECM from Subaru.
Got it flashed today, put it in at the dealer, and AC worked as I began to drive home.
Big smiles.
Then AC quit 10 mins into the drive.
Got home a bit frustrated.
Ended up noticing a bunch of corrosion on the battery ground lead and terminal.
Cleaned both thoroughly.
AC now works.

Was it the corroded ground lead the entire time...? Or just a needed reset (battery disconnect) on the new ECM?

I would say it was both. The corroded ground would have made the ECU search for other places to ground and would have increased the draw through the various driver transistors. One thermally overloaded from the draw, and you went and found the bad ground. That would restore the proper ground current capacity and reset the thermal shutoff. So now things work.

It's been an education.

Thank you for your time and attention.

No problem, Return anytime with your questions. Thank you for being someone who actually tests and gives us info. So much nicer than the "My car makes this noise, what is it?" with no description or video of the issue. You seem like you might be good at this stuff, if you want to learn a lot more about testing and diag I'd suggest scannerdanner and pine hollow auto both on you tube. Paul Danners book is also a great tool to have.