FastKat

iTrader: (10)

I am trying to sort out my A/C wiring for my hotrod. My engine is controlled by a 2006 Silverado 2500 HD PCM and wiring harness. I have F-body accessories (including the A/C compressor) and Corvette DBW.

Question 1:
I would like to have the PCM control the A/C compressor. My car (the hotrod chassis) has a 12v A/C compressor request wire that is turned on by the climate control when it wants the compressor turned on. Unfortunately, the PCM is looking to the serial data system for the request to turn the A/C compressor. So you can see the problem... my chassis sends a 12v signal when it wants the A/C turned on, but the PCM wants the request sent through the serial data lines, which is not a simple 12v on/off switch! This is not a new problem, and has been discussed before.

I was looking through some wiring diagrams, and it looks like the 2005 Express 2500 (cargo van) has a simpler setup. Here, the van's climate control sends a 12v+ (I think) signal to the PCM, and the PCM turns the A/C compressor on. This is exactly what I want! Is there any way to transfer this part of the van's tune over to my Silverado-based PCM, so that I could have the simple single-wire A/C request?

Question 2:
Let's say there is no easy way to make the PCM control the A/C compressor. If I bypass the PCM and wire up the compressor clutch to turn on manually when requested by my car's climate control, is there a way to make the PCM turn on the fans when I turn on the A/C? I was thinking maybe if I left the A/C high-pressure switch wired to the PCM, the PCM may turn on the fans when it sees high pressure. (My harness has a 3-wire high pressure switch.)

garys 68

iTrader: (1)

I ran into the same problem with an 02 LQ9 with Vintage Air, and now an L59 with Classic Air.
Not having an AC on signal to the ecu has not made much difference, so I'm not worried about that.
I use the AC system to control the compressor. AC request, thermostat, binary switch, to the compressor.
On fans, I use the ecu for temp only on one fan. The other fan is controlled by a separate temp switch AND an AC trinary switch.
Simple and effective.
BTW, I'm using a positive displacement Sanden compressor. I know others have used the f body variable displacement, but the pressures for each system is different and less efficient.

ls1nova71

iTrader: (4)

I tried the Express van idea, and could never get it to work right. Well, I didn't, but my tuner guy did. He tried everything from taking segments of different tunes, and copying them into other tunes, must have tried ten times. In the end I just bypassed the PCM for the A/C control. As for leaving the pressure sensor in to turn on the fans, some say they have done it, and it works, but I have no experience with it. But I'm probably going to try that very idea on my 2006 5.3 that I'm working on currently.

Earl Filter

iTrader: (1)

FastKat, you're asking all the right questions...for me! Keep 'em coming!

arthropraxis

I am running into a similar problem and the solution someone else used and recommeded to me was to use a F-body PCM then you can run the 12v A/C request directly to the PCM. My motor was out of a GTO that used the BMC for AC request and one of the fan controls.

FastKat

iTrader: (10)

Thanks for the info guys. I think I am in a little too deep to start swapping PCMs, but the thought has been looming in the back of my mind.

I talked to Jesse Bubb of Wait4Me Performance (he setup my PCM to run my 5.7L LS1 and 4L80E) and he said that the segment swap for the van A/C controls was not possible. It might be somehow, but that right there is enough to make me not want to mess with it.

If I could find out for sure that the PCM will control the fans using only the inputs from the A/C Pressure Sensor (and not requiring an A/C request signal on the serial data lines) then I would be happy. That means less work for me, and I would feel a lot better with the PCM controlling the fans!

So I guess this is the big question: Does anyone using a late truck PCM have the PCM controlling the fans using only the A/C pressure sensor?

niceguy678us

The serial data system is basically just one wire connecting all the modules in the vehicle. (usually the grey wire I think)

If one of the guru's can figure out what the voltage value is for the "A/C request signal" from the 05-06 A/C control switch in the dash to the serial data wire system, then we can rig up the right resistor that would give the right value into the serial data wire system.

I might be way off, but that's just because there is so much convoluted information on here and nobody has really put in any effort to figure out the serial data system. That's all that needs to happen from what I have found in my research. The language is based on different voltage values and that's how each component knows which one is talking on the serial data wire.

Once we find out that value, then we can go to radioshack or other electrical supply and get the right resistor and solder that in line to the serial data wire.

I'm no expert, I'm just trying to keep this going so we can finish our conversions properly.

arthropraxis

I was told the express van or another table would work. But sounds like it has been tried and failed. If you have the system turned on manually how about piggy backing that ON signal to the A/C clutch control at the ECM?

I copied this from another site regarding the A/C and fan control

The electric cooling fans are controlled by the PCM. The PCM controls the ground path for the three cooling fan relays. The relays are used to control the high current flow to power the cooling fan motors. Both fans operate together. When minimum cooling is required, the PCM energizes cooling fan relay #1 and both fans operate at low speed, since the fans are connected in series through cooling fan relay #3. Cooling fan relay #2 is open (not energized) and is not used for low speed operation of the fans. When maximum cooling is required, the PCM energizes all three cooling fan relays. The left fan is still powered through cooling fan relay #1, but is now grounded through cooling fan relay #3. The right fan is now powered directly through cooling fan relay #2 and both fans operated at high speed.

The PCM will also enable the cooling fans after the ignition is turned off. When the engine coolant temperature exceeds 113°C (235°F) and the ignition is turned off, the cooling fans will be enabled on low speed for 150 seconds.

Low Speed Cooling Fans
The low speed cooling fans are controlled by the PCM based on the following inputs:

The A/C system.
The Engine Coolant Temperature (ECT) sensor.
The Vehicle Speed Sensor (VSS).
The PCM will turn the cooling fans on low speed when any of the following conditions exist at idle:

Certain PCM Diagnostic Trouble Codes (DTCs) are set.
The ECT above 108°C (226°F).
The A/C high side pressure above 215 psi.
Once the low speed fans are turned ON by Engine Coolant Temperature, the PCM will turn the fans OFF when that temperature has dropped about 4°C (7°F). If the low speed cooling fans are turned ON by high A/C head pressure, the PCM will turn the fans OFF when the pressure has dropped to 190 psi. The minimum ON time for the low speed cooling fans is 50 seconds.

The cooling fans are turned OFF at certain vehicle speeds. Adequate airflow through the A/C condenser and radiator occurs with vehicle movement to properly cool the A/C refrigerant and engine coolant without the assistance of the cooling fans.

High Speed Cooling Fans
The High Speed Cooling Fans are controlled by the PCM based on the following inputs:

The A/C system.
The Engine Coolant Temperature (ECT) sensor.
The Vehicle Speed Sensor (VSS).
The PCM will enable both fans on high speed when any of the following conditions exist:

Certain PCM Diagnostic Trouble Codes (DTCs) set.
The ECT above 112°C (234°F).
The A/C high side pressure above 248 psi.
Once the high speed cooling fans are turned ON by the Engine coolant temperature, the PCM will turn the fans OFF when that temperature has dropped about 4°C (7°F). If the high speed cooling fans are turned ON by high A/C head pressure, the PCM will turn the fans OFF when the pressure has dropped to 208 psi. The minimum ON time for the high speed cooling fans is 30 seconds. "

Docked Wage

Awesome explanation!

Here's a schematic diagram of the 3 relay system too.

FastKat

iTrader: (10)

I feel like if it was that easy, one of the aftermarket harness companies would have figured it out and either 1) incorporated it into their harness, or 2) sold some sort of resistor/voltage/whatever box.

As far as I know, not even Speartech has a solution for this, and those guys are cutting edge. Plus, this issue has been out at least since the 2006 model year. That leads me to believe that it's more than just voltage or resistance.

pdsq98gt

It's more like "1100011110101" in the data stream that the PCM decodes as "turn on the A/C". So a resistor would just take down the whole data line and really screw it up. Why not just have the hvac controller turn on the A/C compressor and fan relays. I don't think the current draw of two relays would be too much for the internal switch in an hvac controller to handle.

ls1nova71

iTrader: (4)

The reason is packaging. It's hard to mount the whole A/C control module because it's the whole dash switch. I thought of trying to hide mine under the dash and then running my factory controls to it, but it was going to be way more work than it was worth in the end.

G-Body

iTrader: (4)

I have a guy working on this, and I will keep everyone updated. The problem is in the programming of the PCM. The hardware is there to support either request type, because both use the same physical PCM. As it is now, there is no way in any of the available tuning software to switch between the request type. Just as a year ago there was no way to switch the air recirc circuit over to a 2nd fan circuit in HPTuners, or the way that right now there is no way to play with the system options matrix / platform type unless you have a beta version of HPT.

I`m just not computer savy enough to decode the hex code (at least I think its in hex) and see which line controls the request type.

Up until about a year ago there really wasn`t much demand to be able to switch the request type, however in the last 3 months I have seen a lot of people asking. I think that one or all of the tuning software venders will figure it out within the next year.....I just hope its a lot sooner.

pdsq98gt

I'm guessing you're referring to an older hvac control head system that was never intended to have A/C. Why not just add some type of switch to turn on the A/C similar to the little snowflake button on many modern controllers? I would think that would be easier than mounting an entire control head.

FastKat

iTrader: (10)

Geez, I really hope the AC system pressure sensor function will control the fans independently of the PCM AC request line. I just buried my PCM AC request line in my wiring harness under the radiator, and I REALLY don't want to pull that all apart to find it!

ls1nova71

iTrader: (4)

On my application, I was referring to the A/C module out of the 2003 Silverado that the engine was out of. You can't take out just the A/C switch because it's a module, the switch won't work without it. I took it apart and looked inside, and there's alot going on in one of them. Believe me, I tried everything I could think of to get the A/C to work through the PCM.

ls1nova71

iTrader: (4)

I've been waiting for over a year for somebody to do this and report back with a definite, yes it does work, or no it does not work. So far no one has.

What has me concerned is that the PCM will comand the fans on when certain DTCs are present, but it doesn't say which ones. I would hate to wire it up and have the fans on all the time because the PCM is seeing A/C pressure without seeing an A/C request. Don't know if that would even happen though.

If I wasn't always in a hurry to finish my swaps, I would just try it myself, but I don't have that kind of patience!

MrDude_1

iTrader: (4)

the fans for the AC is an easy fix..
when the AC clutch is engaged, it gets 12+ volt to the clutch wire.
by tapping off this wire, you can trigger a relay.

this relay must switch GROUND (NOT POWER, GROUND!!) to the fan relays, on the same wire as the PCM.
pulling that wire to ground outside the PCM will not harm the PCM in anyway, and will command the fan relays to turn on.
you must use a double throw relay if you wish to turn on two fan relays.. a single pole relay will work if you only want one fan to come on.


doing this will mean whenever the AC clutch is engaged, the fans come on. it does not bypass any other PCM commands for the fans to come on. the fans will not be on when the AC compressor cycles off (unless commanded by the PCM)

FastKat

iTrader: (10)

There are a couple ways to do it, and this is one. However, if I can have the PCM control my fans based on high-side AC line pressure, I would prefer that.

I see we have some experienced guys posting on this thread, so I'll throw this out here:
Any ideas for making a test rig? I am guessing that the AC pressure sensor varies the voltage as the AC line pressure changes. I could weld a metric nut onto an air fitting to make a test rig, but my air compressor will only go up to about 130psi, which is not high enough to turn on the fans. SO... maybe there is another pressure or temp sensor out there which has an output on the same scale as the AC pressure sensor??? For example, maybe an LS1 oil pressure sensor? I am pretty sure that has a variable-voltage output. Or maybe an ECT sensor?

For example (hypothetical): Let's say that the AC pressure sensor is (0v at 0psi and 5v at 400psi) and the LS1 oil pressure sensor is (0v at 0psi and 5v at 100psi). If that's the case, I can make a rig for the oil pressure sensor and take the pressure up to 100 psi using an air line. Of course the oil pressure sensor would be temporarily hooked up to the A/C pressure sensor wiring, which should be easy enough... they're only metri-pack connectors.

To do this, I need to know what other sensors have a similar output to the AC pressure sensor.

It's about time we had a definitive answer to this question!

G-Body

iTrader: (4)

A/C sensor
Pressure (psi) Voltage
0 0.1
76 1.0
400 4.4
450 4.9

I can`t seem to find the voltage output for the oil pressure sender, but if you hooked it up instead of the a/c pressure sender and used a pressure regulator on the compressor to slowly increase the pressure from 0 to 100 psi you would hit the entire voltage range of the a/c pressure sensor.