No A/C, driving me crazy.

[clandr1]

If you say so.

It’s the truth

 

[mikez71]

Yes, but if the PCM is getting a reading it doesn’t like re: high pressure, TPS, engine coolant temp, ambient temp,Or engine idle, jumping the low pressure switch connector won’t make the clutch engage.

Right, it looks like the PCM controls +B to the compressor.

Have you tried a new A/C relay yet?
Since it sounds like the shop confirmed PCM is sending the right signal, and jumping the relay also works.
Perhaps when the shop commanded the clutch on with a scanner and it worked, it was a short period of time before the relay could act up?

 

[TJ Baker]

If you say so.

And so do I. The low pressure cycle switch is nothing more than an input to the HVAC control module. The HVAC control module sees that as one of several inputs to be used in the decision making process.

 

[TJ Baker]

Additionally, I should note that the PID values I can read with my inexpensive OBD II reader are all in spec

Exactly what PIDS are you reading and monitoring in real time?

Ambient temperature??

About the only values I can't read are cabin air temp, low pressure status, and high pressure voltage

What app are you using? Using an OBDII adapter you can see high pressure reading. It is available as PID 1144 from the PCM. The equation is where it becomes either the raw voltage reading or the pressure in psi, kPa or whatever.

Cabin air temp and low pressure switch status are a bit tricky to get since the HVAC control module does not support mode $22.

 

[nonickatall]

I wrote in another forum:

Here's a short list of how you can check your air conditioning system yourself...

First, you'll need a multimeter.

The most important thing is that the air conditioning system is properly filled.

If it is, the low-pressure and high-pressure sensors must be delivering the correct readings. You can measure this with a multimeter. The low-pressure sensor is located at the back of the engine bay at the dryer cartridge in the low-pressure line and must have continuity when the system is sufficiently filled.

This is the most important thing, because air conditioning systems usually contain too little refrigerant, and then this switch shuts it off.

The high-pressure switch is a bit more difficult to measure, but it's not rocket science either. It shuts off when there is too much refrigerant in the system, which is rarely the case.

The high-pressure sensor is a pressure sensor and delivers different voltages to the control unit. With the engine off and the system properly filled, it should deliver between 1.2 and 1.7 volts. You need to measure the black or blue wire against ground.

If these two conditions are met, then your air conditioning system should be able to be engaged via the climate control unit.

If that's the case, then power is reaching the base of your magnetic clutch relay. You can easily measure this with a multimeter. If that's the case, then power must be reaching the connector at the bottom of the air conditioning compressor. If that's the case, then the magnetic clutch must be engaging.

You can check the magnetic clutch by measuring the resistance. Resistance must be 4 Ohm.

That's how I would proceed logically in your situation. If the system is properly filled and the pressure switches aren't defective, then the system should be enabled, and if your climate control unit isn't defective, then there should be power reaching the magnetic clutch relay.

The diagnosis is not so difficult...

 

[clandr1]

Right, it looks like the PCM controls +B to the compressor.

Have you tried a new A/C relay yet?
Since it sounds like the shop confirmed PCM is sending the right signal, and jumping the relay also works.
Perhaps when the shop commanded the clutch on with a scanner and it worked, it was a short period of time before the relay could act up?

View attachment 490036

I forgot to mention that, yes, I did use a new relay to no effect. When jumping the relay, the A/C clutch engaged, however the efans didn't turn on since that is apparently controlled by the PCM. So, the PCM is seeing something it's not happy with... I just don't have a scan tool to know exactly what.

 

[clandr1]

Exactly what PIDS are you reading and monitoring in real time?

Ambient temperature??


What app are you using? Using an OBDII adapter you can see high pressure reading. It is available as PID 1144 from the PCM. The equation is where it becomes either the raw voltage reading or the pressure in psi, kPa or whatever.

Cabin air temp and low pressure switch status are a bit tricky to get since the HVAC control module does not support mode $22.

I was able to read TPS %, ECT, and RPM using my ANCEL AD310. It also had IAT but not certain that's what the PCM is using for ambient temp as I think that is another PID.

Ambient temp <40 F would cause the PCM to disengage the clutch, or so I've read on the internet...

I bought a OBDLink MX+ specifically for this purpose, and it was unable to read the A/C pressures despite me finding a "widget" to add to my dashboard. Neither populated any information, unfortunately.

 

[clandr1]

I wrote in another forum:

Here's a short list of how you can check your air conditioning system yourself...

First, you'll need a multimeter.

The most important thing is that the air conditioning system is properly filled.

If it is, the low-pressure and high-pressure sensors must be delivering the correct readings. You can measure this with a multimeter. The low-pressure sensor is located at the back of the engine bay at the dryer cartridge in the low-pressure line and must have continuity when the system is sufficiently filled.

This is the most important thing, because air conditioning systems usually contain too little refrigerant, and then this switch shuts it off.

The high-pressure switch is a bit more difficult to measure, but it's not rocket science either. It shuts off when there is too much refrigerant in the system, which is rarely the case.

The high-pressure sensor is a pressure sensor and delivers different voltages to the control unit. With the engine off and the system properly filled, it should deliver between 1.2 and 1.7 volts. You need to measure the black or blue wire against ground.

If these two conditions are met, then your air conditioning system should be able to be engaged via the climate control unit.

If that's the case, then power is reaching the base of your magnetic clutch relay. You can easily measure this with a multimeter. If that's the case, then power must be reaching the connector at the bottom of the air conditioning compressor. If that's the case, then the magnetic clutch must be engaging.

You can check the magnetic clutch by measuring the resistance. Resistance must be 4 Ohm.

That's how I would proceed logically in your situation. If the system is properly filled and the pressure switches aren't defective, then the system should be enabled, and if your climate control unit isn't defective, then there should be power reaching the magnetic clutch relay.

The diagnosis is not so difficult...

I need to invest in a good multimeter and a back probe kit - with my current MM setup I'm not able to read the voltages due to how small the high pressure sensor connectors are. I have but a lowly harbor freight multimeter, and admittedly need to invest in a better one.

 

[TJ Baker]

was able to read TPS %, ECT, and RPM using my ANCEL AD310. It also had IAT but not certain that's what the PCM is using for ambient temp as I think that is another PID.

All of those are not related to the HVAC system decision making process. And the PCM doesn't read ambient temperature at all.

The ambient temperature sensor is wired to the HVAC controller, not the PCM.

Ambient temp <40 F would cause the PCM to disengage the clutch, or so I've read on the internet

That is almost correct, though the cutout temperature varies among models and such. The other somewhat inaccurate part is that while it is true that the PCM disengages the clutch it does so at the direction of the HVAC control which monitors the ambient temperature. If the ambient temperature sensor gets disconnected by a break in the wiring the HVAC controls generally recognize this type of fault and substitute a default value of 50° F.

bought a OBDLink MX+ specifically for this purpose, and it was unable to read the A/C pressures despite me finding a "widget" to add to my dashboard

The OBDLink can likely read the A/C high pressure but it needs an app to be properly provisioned to do that. What app are you using? Torque Pro? OBD Fusion? Car Scanner ELM OBD??

 

[clandr1]

All of those are not related to the HVAC system decision making process. And the PCM doesn't read ambient temperature at all.

The ambient temperature sensor is wired to the HVAC controller, not the PCM.




That is almost correct, though the cutout temperature varies among models and such. The other somewhat inaccurate part is that while it is true that the PCM disengages the clutch it does so at the direction of the HVAC control which monitors the ambient temperature. If the ambient temperature sensor gets disconnected by a break in the wiring the HVAC controls generally recognize this type of fault and substitute a default value of 50° F.






The OBDLink can likely read the A/C high pressure but it needs an app to be properly provisioned to do that. What app are you using? Torque Pro? OBD Fusion? Car Scanner ELM OBD??

They are all related based on my research.