Yeah the recirc signal would cause the PCM to automatically close the fresh air door when you had a mechanical fan if pressures got too high (AKA the AC system stopped pumping out as cold of air). It happened to me a few times when I had a mechanical fan, but NEVER happened with the standalone e-fans. Thus, I presume it's definitely not needed with the higher CFM factory style e-fans.

Hopefully those pins are unused on my PCM, too. Perhaps 5V and 0V pins there are free pins for and the "signal" pins for programming things are what are completely used?

Anyone have a schematic that shows what C1/pin8 and C1/pin49 are programmed to do? I'd be interested just for learning's sake.

 

The PCM pinouts vary some depending on the vehicle model. GM's eSI system pretty much says those pins are unused for all trucks, suv's, and vans from 99 through 2007 (classic). I've seen other things online that say the C1/8 is a 5v reference for TPM. I did try calling BP Auto at one point and their tech pretty much would not tell me anything or give me any advise since I only bought the sensor kit. He said too much liability and if I was having them install it that would be different.

I can only guess that maybe those pins are used on some vehicle model and are technically active in all of the PCM's and just not used on some.

 

I suppose all I need to do is hook them into those pins and if everything works, we're good. If I really wanted to, I could check voltage to the connector and make sure the grey wire reads 5V and the black wire reads 0V. They're just reference signals, so the source shouldn't really matter - it'd just be nice to NOT have to splice into preexisting wiring if I don't need to (by utilizing open pins).

I'll dig into it this weekend. Probably won't wrap up the entire swap, but I'll at least get the ECM disconnected and take a look at the current pin usage.

 

I got these hooked up over the weekend and wrapped up tonight. Fortunately I didn't fully redo the harness into the ECM before I tried things out.

The good news is that the fans do come on when the truck warms up. So far, I can only get them to kick into low speed since the truck is stationary and the weather is cool, but I'm assuming it'll kick into high speed once the truck gets hot enough.

The bad news is that the fans don't come on with the AC. My tuner is going to dig into things further, as it may require more than just a segment swap.

The AC Sensor is definitely reading between 0-5V depending on the AC system pressure (appears to read 0.8V when the truck is off and key is on, read 2.8v and was still climbing when I kicked the AC on, but I didn't wanna hurt the compressor since air wasn't flowing across the condenser. One strange thing is that the AC Signal (C2/17) only read 9.6V when the AC is on (and 0.0V when the AC is off). Not sure if this is related because I never measured it before the swap. The AC definitely is kicking on though.

 

I installed my fans this weekend also. Everything pinned to ECM and using the BP Automotive A/C harness too. I have to start datalogging and test to see if the fans go to high with A/C still.

I will note that the 5/7 blade 500w fans are fairly quiet. Definitely not to noticeable in cab. I had the 5/5 blade 700w fans to test for a few days, and those things absolutely roared at high. It was actually a bit annoying in the cab, but man they moved tons of air.

Will report back on more later as I test everything. Glad to get these installed though, it means faster warm ups for winter. They probably will hardly run for the next few months haha.

 

If you look at the schematic "2002SilveradoAutoACControlSchematic.pdf" I posted you will see that there is a "A/C High Pressure Switch" which interrupts the "A/C Request Signal" input to the PCM coming from the HVAC module. This "A/C High Pressure Switch" is located on the back of the A/C compressor and if your high side pressure gets high enough it will cut off the compressor before anything is damaged. Or at least that is the design intent. What I'm getting at is that you shouldn't have an issue in testing with letting the A/C run without the fans since if the pressure on the high side gets high enough the compressor will just cut out and then start cycling with a really high high side pressure.

 

Agreed. Compared to my 28" Flex-A-Lite fans, they're basically silent I didn't even realize when the fans came on in the cab and they're barely noticeable with the hood shut when they're running on low.


Don't get your hopes up too much. When I went from mechanical fan to the Flex-A-Lite fans it wasn't too noticeable.


Good to know. Before I realized what was happening, I let them go for quite a while. The air in the cab was no longer blowing cold, but the compressor was still running and the condenser was WAY too hot to touch. The fans definitely aren't coming on when they're supposed to. I wish I had a way to diagnose though... downsides of a tuner a state or two away.

 

Still having issues getting the ECM to control the fans based on AC pressure. I'm not giving up, but it's really in the tuner's hands at this point.

As a backup, I'm looking into trinary switches and how I'd wire them effectively. First, I need to figure out what pressure range switch to use. Based on my multimeter readings and the scale listed in an earlier post (here), I see the low pressure switch kicks the compressor on when the AC pressure sensor reads 1.61v (132.5psi) and the high pressure switch disables the fan clutch at around 4.7v (423.2psi). A strange aside... with the key on and truck off, the sensor reads 0.8v (56.4psi) which is ~4x atmospheric pressure (14.7psi). Not sure if this is correct - could be that the sensor simply clips due to its range.

Anyway... does anyone know what pressure the fans *should* kick on? Further, if I were able to find a switch that could control both low and high speed fan engagement, what pressure should the fans come on at low speed and what pressure should the fans come on at high speed?

Thanks!

 

So a coworker of mine has EFI live and we were able to take a look at my current calibration.

Looks like the AC pressure sensor is read by the ECM, so that's good. It also looks like (not pictured) the fan tables include AC pressure on and off settings for both fan signal #1 (low speed) and fan signal #2 (high speed). It does, however, look like my tuner forgot to adjust the C2/33 pin to allow for fan control instead of recirculation, but I can't imagine that alone prevents the low speed fan triggering. Any thoughts from the PTnet forumgoers? Is there a setting we're missing? Or are more segments needed to be swapped from the Camaro tune to make this work?

 

What year is your truck DMAN?

Low speed should come on if you can get it up to temp regardless of any other settings I would think. Fan 2 should be the 2nd trigger or high speed.