Installing AC High Pressure Sensor on 99-02 Silverado

[53Bowtie]

Hi All,

I have completed my e-fan swap on my 02 Silverado and have a couple questions regarding AC control. I have been researching this for the better part of a month and it would seem that the preferred way to control the AC would be through a high side pressure sensor. This would allow me to utilize the Fan 1 and Fan 2 AC pressure settings in the tune in the same manner the Fan 1 and Fan 2 ECT settings are used. So here are my questions:

1) Can I follow an 05 Silverado pin out to install the pressure sensor on my 02? It would appear that the 5v pin is the same but the other two pins are not used on my 02 so not sure if they would be the correct ones to use or not.
2) Once the sensor is installed will my 02 OS automatically read it and use the pressure values in the tune to operate the fans or will I have to go to an 05 OS?

Additionally, if anyone has done this please point me in that direction. I searched and could not bring anything up. Most aftermarket harnesses I have seen simply trigger high speed on fan 2 for the AC but it would seem a "both fan low" "both fan high" approach, same as for ECT, would be best.

Thanks for any and all help!

Thomas

[Markmx6]

Yes, that would be the proper way to do it, and yes the pins should be fine.

I am looking to do something similar, but instead of trying to find a place to install a high pressure switch I am just making a simple circuit that will show 4 volts to that pin on the PCM when the A/C is engaged. That will provide the same effect with out having to add a new port to the A/C system.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Is what I will be using to drop the voltage from 12V to 4V. For the source I was going to use Pin C2 39 for positive and Pin C2 43 for the ground, then for the positive output of 4V to Pin C2 14.

I haven't done it yet, so if anyone sees something wrong with this method feel free to speak up.

[53Bowtie]

Yes, that would be the proper way to do it, and yes the pins should be fine.

I am looking to do something similar, but instead of trying to find a place to install a high pressure switch I am just making a simple circuit that will show 4 volts to that pin on the PCM when the A/C is engaged. That will provide the same effect with out having to add a new port to the A/C system.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Is what I will be using to drop the voltage from 12V to 4V. For the source I was going to use Pin C2 39 for positive and Pin C2 43 for the ground, then for the positive output of 4V to Pin C2 14.

I haven't done it yet, so if anyone sees something wrong with this method feel free to speak up.

Thanks for the reply! The 05 Silverado pin out is C2-14 for signal, C1-45 for 5v ref and C2-80 for low ref. so sounds like you are good to go. My 02 pin out C2-14 is not used, C1-45 is 5v ref and C2-80 is not used so wasn't sure if those were the correct pins or not but sounds like C2-14 is the ticket. I just hope my 02 OS will read it and control accordingly....

Just out of curiosity, do you know what the voltage to psi correlation is with the sensor? In other words, what psi does 4v equate to?

[Jason B]

You can't just swap an '02 pcm for an '05. You know this right?

[Markmx6]

Who is talking about swapping PCM's? We are talking about using pin locations on a newer PCM that should work for an older one. It does work for controlling electric fans.

[Jason B]

2) Once the sensor is installed will my 02 OS automatically read it and use the pressure values in the tune to operate the fans or will I have to go to an 05 OS?

Yes, that would be the proper way to do it, and yes the pins should be fine.

OS, PCM, whatever. You can't swap between '02 and '05.

[Jason B]

Yes, that would be the proper way to do it, and yes the pins should be fine.

I am looking to do something similar, but instead of trying to find a place to install a high pressure switch I am just making a simple circuit that will show 4 volts to that pin on the PCM when the A/C is engaged. That will provide the same effect with out having to add a new port to the A/C system.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Is what I will be using to drop the voltage from 12V to 4V. For the source I was going to use Pin C2 39 for positive and Pin C2 43 for the ground, then for the positive output of 4V to Pin C2 14.

I haven't done it yet, so if anyone sees something wrong with this method feel free to speak up.

So your high speed fan is going to stay on when the a/c is on.
4 volts is way up there on the pressure scale. Why not just run a relay so the fans come on with the a/c? It's going to operate the exact same way as what you plan to do.

[53Bowtie]

OS, PCM, whatever. You can't swap between '02 and '05.

Hi Jason. I had read that folks with 99-00 OS had to switch to a Camaro or 01-02 OS (like I have) in order to have electric fan controls in the tuning software. I have never done this but thought maybe my 02 OS would then not have the capability to read the high pressure sensor that would be installed in later years....maybe it does though. In any case, for my knowledge only, why would I be able to swap to a Camaro OS but not a later model Silverado OS? Thanks for your reply!

[Markmx6]

You don't have to change the OS, the older OS will still control the fans, and I have not verified, but should interpret the High pressure value. I currently have a 2002 Silverado running the factory OS controlling my fans just fine. I also have a 98 suburban running an 0411 running electric fans with a completely different OS.

The advantage of using the PCM to control the fans via the pressure switch vs the A/C relay is that the PCM turns the fans off when you reach a designated speed. There is no reason to have the fans running when the vehicle is traveling at 45+ mph. I personally believe it would actually cause a restriction instead of allowing the fans to free spin at those speeds.

Jason, keep in mind I'm not trying to argue with you, I'm just explaining my thought process and experiences. I did miss your comment when you replied to the need to run an 05 OS, which I agree you most cannot run a 05 OS on a 02 PCM. I am pretty sure you can go the other way around, but that isn't the point right now...

53Bowtie, the reason you can't use the later OS is because it needs the 1mb PCM which is the blue and green vs the 512kb PCM which is the blue and red that you have.

[Jason B]

Hi Jason. I had read that folks with 99-00 OS had to switch to a Camaro or 01-02 OS (like I have) in order to have electric fan controls in the tuning software. I have never done this but thought maybe my 02 OS would then not have the capability to read the high pressure sensor that would be installed in later years....maybe it does though. In any case, for my knowledge only, why would I be able to swap to a Camaro OS but not a later model Silverado OS? Thanks for your reply!

I just happen to have two '02 LS1 camaros and I have an S10 with a 2002 silverado pcm and a/c.
The only thing I was pointing out, like Mark said, you can't swap an '02 and '05 OS thats all.

Either silverado or camaro OS should work to control the a/c. If you already have the 2002 silverado C2-14 will read the sensor signal although I have seen nothing on the internet about it, I just connected a AAA battery to it and watched it on my hptuners scanner read that batteries voltage. C1-45 is the 5 volt signal, and low reference is any ground. This is for a 2002 silverado pinout.

I think I just repeated everything Mark already said so you're good to go. I would pull up a 2001 or 2002 camaro file and copy all the pressure related stuff into your silverado tune and it should work normally, fans and a/c.
And use the sensor out of a camaro and I think s10 truck installed in the liquid line.

[53Bowtie]

And use the sensor out of a camaro and I think s10 truck installed in the liquid line.

Thanks Jason! Regarding the above statement....I was planning to use the 05 truck sensor and high pressure line between the compressor and condenser. Should I use the Camaro sensor instead? and it goes in the liquid line instead of the discharge line? Thanks!

[Jason B]

Thanks Jason! Regarding the above statement....I was planning to use the 05 truck sensor and high pressure line between the compressor and condenser. Should I use the Camaro sensor instead? and it goes in the liquid line instead of the discharge line? Thanks!

It just has to be a pressure transducer, I'm not familiar with the '05 tahoe. I am very familiar with a/c and I think a transducer mounted in the discharge line and liquid line would have very different outputs.
Me personally, I would use one that goes in the liquid line. But again I have a camaro and since reading this thread I have connected the scanner to it and looked at how the a/c voltage rises and how it turns the high and low fans on. I can't do that with a tahoe.

[Markmx6]

From a purely AC standpoint the pressures at the discharge of the compressor and the liquid line between the condenser to the expansion device are going to be very similar. The condenser does have a small pressure drop, but it is negligible for this purpose. So you you should be good to go with a sensor in either location.

[Jason B]

I've been doing a/c and refrigeration for 25 years. I'd put it in the liquid line, it has nothing to do with the condenser.
The factory put high pressure cut outs in the discharge lines, not transducers, not that I'm aware of. Seems to me the fan on/off times would be erratic with the transducer in the discharge line. The factory did put mufflers in discharge lines, and theres a reason.
Not trying to be a know it all and I don't like being that guy that throws 'I've been doing this for xx years' All I'm saying is I would put it in the liquid line.

[kingtal0n]

I've been doing a/c and refrigeration for 25 years. I'd put it in the liquid line, it has nothing to do with the condenser.
The factory put high pressure cut outs in the discharge lines, not transducers, not that I'm aware of. Seems to me the fan on/off times would be erratic with the transducer in the discharge line. The factory did put mufflers in discharge lines, and theres a reason.
Not trying to be a know it all and I don't like being that guy that throws 'I've been doing this for xx years' All I'm saying is I would put it in the liquid line.

Hello I would LOVE to learn more about this. I need a HIGH pressure sensor to safely switch off the A/C compressor I think. Is that the only important sensor? Can I damage the system with a low pressure as well as high?

Can you also please explain which line is the liquid line, how to tell, high from low etc... I believe the high goes into the condenser and by being such high pressure on the way, that identifies the liquid line for surely it is more likely to be a liquid at high pressure than a gas. IS this correct thinking? and then where does it go.. dryer or something? I don't know my A/C at all and curious how to set it up safely.

[Jason B]

Hello I would LOVE to learn more about this. I need a HIGH pressure sensor to safely switch off the A/C compressor I think. Is that the only important sensor? Can I damage the system with a low pressure as well as high?

Can you also please explain which line is the liquid line, how to tell, high from low etc... I believe the high goes into the condenser and by being such high pressure on the way, that identifies the liquid line for surely it is more likely to be a liquid at high pressure than a gas. IS this correct thinking? and then where does it go.. dryer or something? I don't know my A/C at all and curious how to set it up safely.

I'm not going to teach you, sorry. These are very basic questions that can be found on the internet. The discharge line comes off the compressor and takes high pressure gas to the condenser, the liquid line comes out of the condenser and goes to the evaporator, theres a metering device in the liquid line, the suction line comes out of the evaporator and goes to the compressor carrying low pressure gas. There are receiver/driers, accumulator/driers, thermal expansion valves and orifice tubes, variable displacement compressors and fixed displacement, compressors that cycle and those that don't some rely on a low pressure switch. There are also mufflers, people often mistake these for filters but they are a hollow chamber that absorbs pulses from the compressor, these are always in the discharge line.

You didn't say what vehicle you have so I can't say what components you're dealing with.

[53Bowtie]

Hi Jason, thanks again for the dialogue. I agree the pressure in the liquid line would definitely be more stable for the sensor to sense......that is the whole point of sub-cooling for the TXV after all right? Its more stable to meter liquid than gas....

In any case I ended up taking the easy way out and just wiring up the AC to bring both fans on low. I thought I had an additional port in the liquid line already which is why I started down this path and come to find out it is a shrader valve but has the threads on the inside not the outside of the port like what would be necessary for a sensor to screw on. Maybe eventually I will pull charge and replace the port and add a sensor but I have a feeling the way I have it wired will work "well enough" and I probably wont ever "get around to it". Thanks again though!

[Markmx6]

I've been doing a/c and refrigeration for 25 years. I'd put it in the liquid line, it has nothing to do with the condenser.
The factory put high pressure cut outs in the discharge lines, not transducers, not that I'm aware of. Seems to me the fan on/off times would be erratic with the transducer in the discharge line. The factory did put mufflers in discharge lines, and theres a reason.
Not trying to be a know it all and I don't like being that guy that throws 'I've been doing this for xx years' All I'm saying is I would put it in the liquid line.

No worries, I'm also an A/C tech just quite a bit newer to the field.

I was only referring to the minimal pressure drop that occurs in the condenser not being a factor in this instance. I agree the liquid line would be better though. The bigger issue would be to find out which of the two is easiest to get a pressure transducer on. Especially since we are only talking about a fan activation purpose.

[53Bowtie]

No worries, I'm also an A/C tech just quite a bit newer to the field.

I was only referring to the minimal pressure drop that occurs in the condenser not being a factor in this instance. I agree the liquid line would be better though. The bigger issue would be to find out which of the two is easiest to get a pressure transducer on. Especially since we are only talking about a fan activation purpose.

Definitely the liquid line is much easier to get to than the high side line on my 2002 Silverado and it already has two fittings welded in. Unfortunately neither have the correct threads to support screwing on a sensor so I was not able to use them.....yet. I may remove charge and change on over to the correct fitting sometime in the future but for now I am good just turning the fans on low with the AC on I think.

[Markmx6]

That makes sense. Other than I personally prefer to have the fans on high for the A/C. Here in AZ it is kind of mandatory.