Having an issue and searching doesn't seem to turn up how to address it
Car is a 2003 Honda Accord EX. 4 door. 4 cyl engine. Has dual zone automatic HVAC/audio panel in dash.

When I bought car it had a hole from a rock or something in the condenser so I found a junkyard condenser, swapped it out, pulled a vacuum on it and attempted to charge the AC with a fresh can of R134. Turned on Auto, set both drivers and passenger side ***** to "LO" and recirc on. If I press A/C button twice HVAC display says "AC ON". Found out that even with pressure on low side the AC compressor would not engage, checked relevant 7.5A fuses on left kick panel and under hood which were fine, have +12V to both coil and contact side of AC compressor relay, and if I jump the contact side A/C compressor will run. Appears that coil side is not getting the 0V/ground signal from the ECU. Jumped relay out so I could finish putting the can of R134 in and not waste it. I know these cars have a reputation for having the AC relays go bad but if the coil isn't getting +12v and ground I know its not going to pull in.

Checked pressure switch on condenser - have +12v coming in one side when I pull the connector off and continuity across the switch. If I put a jumper between the terminals AC compressor still doesn't come on nor do I get the ground signal from ECU to the A/C compressor relay

Previous owner installed an aftermarket radio below the audio/HVAC panel and stole the radio plug so display wasn't turning on (even though it would respond to button presses), I pulled the dash up and swapped the radio plug back and HVAC display works now. Will probably split out wires at some point in the future so the audio/hvac panel has power in addition to the aftermarket stereo. Did same tests above after swapping radio plug over, no change.

I have a Bosch/Vetronix Mastertech scan tool with a Honda PGM card but it won't connect to the ECU using the Honda specific card, I can connect using generic OBD-II but unfortunately generic OBD-II live data won't tell me if the AC has been requested or if its sending out the compressor coil signal. Have a different scan tool card on order from eBay but hasn't got here yet.

I found these threads: https://www.hondaaccordforum.com/for...-code-s-63594/ https://www.hondaaccordforum.com/for...ou-help-59108/ and attempted the procedure to get diagnostic codes off the hvac panel, it didn't flash any diagnostic codes after holding "auto" and "off" down for a minute which I think means a clean bill of health?

Is A/C compressor turn on via ECU requested from the HVAC system via a signal wire or is it done over a communication bus?

A friend who works in a shop with Mitchell got me a wiring diagram which shows ECU pins A15 and A26 as "com sig lo" and "com sig hi" but these appear to be coming off the "gauge control module" behind the instrument cluster and this diagram shows the only connection between the HVAC control and the "gauge control module" as being wire A5 "AC press switch" on the hvac module, which routes thru the underhood relay box, then to the pressure switch on the condenser, then back to the underhood relay box thru a generically named "control module", thru the left kick panel box and then to the gauge control module as brown/yellow wire pin 25
Nothing I have been able to find by searching tells me what these signal wires should read when A/C is requested or how exactly the system is supposed to work that would help me narrow down where my problem is

Thanks for any help you can offer

Edit: looking at a different diagram appears that A15 and A26 off the ECM are CAN bus hi and lo lines. So if I understand this correctly the HVAC panel sends power grounds line A5, which goes thru the underhood box, then to pressure switch, back thru the underhood box, to module behind left kick panel which sends a serial signal on to pin 25 brown/yellow on the gauge control module, and the gauge control module sends signals down the CAN bus to the ECU telling it to turn on the AC compressor. So tomorrow I'm going to see if I can borrow a buddy's scan tool and see if it will read live data from this ECU and find out if AC is being requested, if not I can trace the wiring from the pressure switch back to the gauge cluster and see if I can find the problem AC is not being requested according to the scan tool. With key on engine off I can manually request AC clutch with scan tool and hear relay and clutch click, can also request rad fan and that works as well. With AC off I have +12V to ground at pin 5 (blue wire) on 22 pin A connector at backside of HVAC controls. With AC turned on I have 0V.

 

Found a portion in the manual I bought last night that shows which connector "H" is in the underhood box. Pulled bottom off and backprobed it, with AC requested from the HVAC system I get <1 volt at the blue/red wire H2, with AC off or HVAC control turned off I get 12V. No change in the status of the AC switch request on the scan tool.

Neither diagram nor the manual I found seems to explain the relationship between underhood box wire I7 which goes to D11 on the underdash box, but I'm guessing that the "control block" has some sort of serial connection to the underdash box which connects to the gauge control module.

So I'm guessing some component of the underhood "control block" is not functioning. Going to hit a junkyard and see if replacing the underhood fuse/relay box fixes my AC problem.

 

Well, poop.
Went to pick-a-part yard today and found a 2003 Accord that had been hit on the front and got the underhood relay/fuse box out of it. Verified same code/part numbers and fuse/relay locations, checked that wire colors matched up, installed it and AC requested still isn't registering.
I had thought the underhood relay/fuse box was a likely possibility as when I went to buy the car the previous owner had apparently hooked up the jumper cables backwards and burnt the main fuse, but the car started once the main fuse was replaced with a new one and since the A/C condenser had a gaping hole and wasn't charged I wouldn't have been able to test it then anyways. I can't claim that the new relay/fuse box is a known good, but the car I got it from had been hit hard in the front passenger side and looked like it had been maintained decently other than the accident damage.

I know I'm getting communication with the underhood relay module (both the new and the old one) because I can command the A/C clutch and radiator fans from the scan tool
I'm just at a loss as to why the underhood relay module won't tell the ECU that AC is being commanded when H2 gets grounded by the HVAC system

 

I'm looking at a the 03-06 shop manual you can get from automanualsource.com as a pdf.

You need to figure out which side of the circuit (power side vs ground) is causing a problem. The easiest place to split the circuit is the a/c pressure switch.

For the 4-cylinder with climate control, the a/c pressure switch blu/red wire should get 12V from the H2 pin on the relay control module. I'd unplug the a/c pressure switch and use a test light connected to ground to verify you have enough current to light a test light.

At the a/c pressure switch, you can also jump the blue/red wire to ground. The a/c compressor should turn on. It looks like the blue wire from the a/c pressure switch is grounded by the climate control unit.

Let us know what you find.

 

If I probe blu/red wire at H2 on underhood relay control module I get +12V when A/C is not requested from the HVAC panel and <1V when A/C is requested in either auto or manual mode
I have continuity across the A/C pressure switch.
With A/C pressure switch connector pulled off and A/C requested I get +12V on the blu/red wire and 0V on the blue wire
Jumpering out the A/C pressure switch doesn't turn compressor on and neither does tying the blu/red wire to ground

My problem seems to be that even when I'm showing <1V on the blu/red wire at H2 live data on my scan tool is not showing the AC request being made at the ECM and as such, A/C clutch relay output is not being commanded either.
Key on engine off I can do a functional test to command the A/C clutch relay and if I do that I can hear the relay and clutch clicking in. I can also request cooling fans and those turn on as well
So it seems like I'm getting communication out from the ECU to turn things on in the underhood relay box, but I'm not getting the signal back from the underhood relay box to the ECM that A/C has been requested
It seems much more direct coming from the ECU as there are outputs direct to relay coils for the cooling fans and A/C clutch, whereas inputs to the underhood relay box go to the body serial bus, thru the gauge control module, to the CAN bus lines and then to the ECU

 

If you can command the compressor clutch to engage with key on/engine off, then go through the different inputs with your volt meter to narrow down what is missing vs when the engine is running.

 

Scan tool won't let me command compressor clutch or radiator fan tests with engine running. If I try it says "test aborted - engine needs to be off"
H2 on the underhood relay box is the furthest I can get up the chain testing with a volt meter. Everything from underhood relay box to underdash relay box to gauge control module to ECM is all serial communication, as I stated above.

 

What I was suggesting is that you can test at H2 with your meter using your scan tool with key on/engine off to get the correct reading. You can start your engine and test H2 using your climate control system instead of the scanner then measure voltage at H2. This is just to see if you have a difference.

It looks like pin 5 on the climate control is grounded through a transistor in the climate control system based on the wiring diagram. I am not sure what conditions must be met for this to work. Being able to scan for HVAC codes will help, although you already tried by the non-scanner route.

I am also not sure how the lt blu wire from between the gauge control module and relay module affects the relay control module. You almost have to have a working scan tool to see what the climate control unit is seeing. Otherwise, you need to look through the various hvac codes diagnostics to understand the correct inputs into the system.

The CAN systems make diagnostics difficult if you can't use a scanner to see what the climate control is seeing.

 

I have already tested the output on blue wire A5 from the HVAC system, it's at +12V when A/C is not being commanded, and 0V when AC is being commanded, engine running or not. From what I can tell the HVAC module doesn't seem to have any error codes and is doing its part by grounding A5 when I ask it to turn the A/C on.

Blue wire A5 from the HVAC module connects to one side of the A/C pressure switch at the condenser, the other side is blu/red wire H2 that goes into the underhood relay box. It basically makes a big loop from HVAC panel thru the A/C pressure switch to the underhood relay module. From what I can tell the signal isn't getting from the underhood relay control module that H2 plugs into back to the ECM, or the ECM live data would reflect the A/C request coming on and respond accordingly by turning on the output to the A/C compressor relay and cooling fans.

I was able to get my Mastertech to talk to the ECM and give me live data on A/C request and output but it won't talk directly to other modules on the 1-wire bus, I guess my next step is see if I can rent or borrow a scan tool with more capability than the one I've got.

 

Looking through the shop manual, the B-CAN troubleshooting is much easier with a scanner that works with your car. You may want to look for B-CAN trouble codes. There is a way without a scanner in the shop manual that may give you a direction. Some scanners can look for modules that are awake and on the CAN network. Maybe one is missing?