Hello all,

1978 Caprice: Stock 2004 5.3 avalanceh with 4L60E

I'm keeping the stock AC system in the 78 caprice (new parallel flow condensor, but I'm keeping the basic setup stock), but I'm keeping the original compressor from the 2004 5.3. I've read several posts and videos about swapping these, but most people are using the later pancake style compressor.

How to I get the PCM to control the compressor as it should and work with my original system? This system has already been converted to 134a. Many of the threads I'm reading quickly leave me confused and I'm afraid what may work for their specific setup won't work for mine. I believe I can use some of the original AC wires from the PCM to turn the new compressor on and off, but I'm not sure how to integrate the new wiring harness into the wires that are still in the car from the original system. Any help would be appreciated because I feel like I keep going in circles and get lost with this.

thanks,
Brandon

 

long answer:
one of the main things it depends on is what type of ecm you will use (roughly grouped 2000-2002 with red/blue connectors vs 2003-2006 with green/blue connectors, there are exceptions to this) and what operating system the tune uses (truck vs f-body for example)
i want to say the red/blue ecms for the most part use an analog request signal (+12v off/on) while the green/blue use a serial signal from the hvac module
they also use different pressure sensor arrangements
i believe the red/blue ecms using either a truck or f-body operating system can be integrated to work with your stock controls but doing this would require you to run a cable throttle body vs drive by wire tb (again, for the most part, exception would be the express vans using blue/green ecm with dbc tb)

i'm sure someone can correct me if i'm wrong or fill in some blanks but that should at least help you get things started
ls1nova71 knows alot about this from what i recall

short answer: i dunno

 

Thanks, I'll shoot him a PM.

Just to make sure I'm right here, the high pressure switch disengages the compressor if the pressures get too high and the low pressure switch does the same, which allows the compressor to cycle, correct?

 

What about not running the A/C thru the computer, and just running it as it did originally? Basically, make the pump think it's still on a small block running as it always did. Then use a Sanden or later R4 on a Holley relocation bracket on the upper passenger side.

 

I already have the 5.3 compressor and it fits in the car so i'd like to keep it.

 

If it's a fixed displacement compressor, it can be run the old-school way via the controls already in the car. That would keep it fairly simple.

 

Vetteman, I got your PM and figured I would post the response here in case others have a similar question. There is no real easy way to get a PCM controlled A/C set up on a 2003 or newer engine due to GM switching from a 12v A/C request to a serial data request. There are PCM's that can be made to work if you have some tuning software, but not all of them have the ability. The ones that can will have IAC drivers in them since they were mainly used in vans from 2003 to around 2006 or so when vans were still for the most part drive by cable. Vans didn't use the serial data A/C request, they still used the older 12v request. That being said, I would almost guarantee you have one that wont support the change. What I would do is just wire it exactly like it was in the '78, which would be, 12v coming out of the switch on the dash, then to the low pressure switch mounted on the accumulator (the big aluminum can looking thing on the box under the hood), then out of the low pressure switch to a high pressure cut off switch then to the compressor clutch. Basically you want the two switches, the high and low in series so that either can cut the compressor off when needed. Another thing to note is the compressor itself. I have heard that if the vehicle had the serial data request, which uses the 3 wire pressure sensor, it will have a variable displacement compressor. I have used them before in an older system and I have also used a fixed displacement compressor and the fixed seemed to work better. The fixed displacement would have been on a van and can be identified by the high pressure cut off switch mounted in the rear of the compressor. I am going to assume you don't have a high pressure switch in any of the lines because I don't believe the older systems used them, they just had a blow off valve that would let pressure out if need be, along with some R12! So, it may be easier to just get a compressor from a van application that has one already installed in it.

Hope this helps a bit and hasn't confused you further. On a side note, since it wont be PCM controlled, you will just have to remember to shut off the A/C if you want to race the guy next to you, just like in the good old days before the car decided to do everything for you!

 

Consider contacting classic auto air. They do update/conversions for older cars and can supply everything you need.
Also, I'm running classic auto and vintage air on 2 swaps without computer control, no issues.

 

There and still there is a lot of misinformation about this. Any 98-07 OS can be made to work with an a/c request. This is regardless of the type of compressor (fixed or variable) and control type orifice tube or thermal valve like the aftermarket units. Most GM vehicles use a orifice tube instead of a thermal valve with a temp sensor. Here's the basics:
1. You need to have the IAC drivers for this to work. All red/blue pcms have them and a number of blue/green pcms do to but you will need to check your service number. If it lists the van as one of the vehicles that the pcm replaces then you are good to go. This is the easiest way to check for the IAC drivers it has nothing to do with if you are DBW or DBC a lot of these pcms came on a number of vehicles.
2. All variable and fixed compressors with electric fans come with a high pressure sensor. This is the mechanism for the fans (think low/high) and replaces the high pressure switch. If you are using a variable compressor it is also used for low pressure cutout meaning there is not enough R134 in the system to turn on.
3. Fixed compressors with orifice tubes all use a low pressure switch. This is to turn the compressor off and on. Note variables do not have these because they are made to run continuously.
4. The only time a high pressure switch is used is when you have a fixed compressor and no electric fans.
5. This may be plug n play but you probably need to have your pcm programmed correctly and some wiring for the ac request. If you are using the system exactly as it came from the vehicle using the exact same wiring along with the ac request it will work. If not say you go to electric fans and your original OS didn't have them then you will have to program the OS for this.

There is a lot more to the routines for the ac request than people realize. I personally never have and never will run one that was not hooked up to the pcm. Most guys up the idle to take care of this. As stated this works for any system even aftermarket systems. Variable compressors also work on any system and are every bit as good as a fixed. They have to be filled properly to work right and they have been used by GM on all types of systems. PCM's with the proper programming setup can be sourced for $100 or less but you need to find somebody that knows what they are doing.

 

Thanks guys for the responses. I'm going to try to work through this.

What are the benefits of having the computer control the a/c?

I'm using a 2004 5.3 from a 2004 avalanche as well as the PCM and harness from the same truck. This vehicle did not have electric fans but did have drive by wire. I am not using the original fan on the motor and I am changing to dual electric fans and I am retaining the drive by wire.

I don't know what system my 1978 Caprice uses. I have a manual for the car, but it's a chilton's and contains almost no helpful information on this specific year. (I have a manual for a '71 pontiac and it gives a very detailed explanation of the ac system, but the caprice's does not). What should I look for to determine what kind of setup the original system is running as far as orifice tubes, and what and how many switches (high/low) it had? I do know that the original a6 compressor would cycle and was not fixed. Will I need to put in the high pressure sensor since I'm going to be using electric fans?

I have a picture below of my compressor. I am unable to determine which type of compressor this is. Are you able to tell by looking at what I have here?




According to this website: https://www.mrk-motorsports.com/gm-iac-pcms/ the IAC drivers are not supported on the PCM that I have (# 12586242), but I also read that this number was used on express vans, so that means I should be able to have the drivers installed on the PCM when it is reprogrammed.

 

 

You're trying to mix components that use different refrigerants, etc. Check with classic auto air for what you'll need for the R12 to R134, also condenser, drier, hoses.
For high low pressure compressor control and fan control without the ecu you might want to consider a trinary switch. I've used the ecu to control one fan and the other by the trinary and/or a temp sender.

 

I'm going to have to put this part of the project on hold for a little while. I've had a couple of setbacks in different areas and I'm not able to keep all of this new information straight in my head at once. I'll come back to this once I've gotten the other issues resolved and I can dedicate more mental capacity to this, which is all pretty confusing from my perspective.

 

OK guys, I'm digging back into this A/C wiring hurdle. After a long story, I've ended up having to use a Sanden 508 which is mounted up top on the passenger side. I also would prefer to have the PCM control the system and fans. I saw on the LT1 tech website where the PCMs can have a wire added for fan 1 and a wire added for fan 2.

My system includes: Big aftermarket parallel flow condenser, big Griffin radiator with combo dual electric fans with two relays, Sanden 508, aftermarket dryer, PCM from 2004 Avalance and everything else is stock 1978 GM B-body (Caprice).

Where should I start in trying to understand how to wire this up? I tried to find if my PCM was a replacement for a van, therefore making it compatible to use as a controller for the ac system, but I wasn't able to find the info for sure.

Thanks,
Brandon