Vintage Air Binary to Trinary and..?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

epfatboy · 07-16-2025, 04:07 PM

I have a '74 C10 running a typical standalone harness to a P59.
Went to pull a wire to the harness and discovered I have a binary pressure switch... which is great until you remove the fixed fan and replace it with cool electric fans.

Now I need a Trinary to run the fan relays..

I have been running the truck this way for a little bit not realizing this, the driving mostly cruising with little stop and go.

Gonna order the parts now, should I put her up until this is repaired?

Also.. the original idea was to run 12v when the compressor came on. Which means I still need to pull from that side of the pressure switch ... right?

I believe the fan side works on a different pressure so the fans don't cycle with the compressor which isn't the feed the PCM is looking for.

Is that accurate?

**A1971Blazer** · 07-17-2025, 10:15 AM

Not sure if this is relevant to your application but I have two fans.
Fan 1 is activated by the trinary switch as wired according to the VA instructions.
Since the fan trigger is just a ground to the relay, initially I just wired the trinary ground and the PCM ground for fan 1 together so that either/or would turn the fan on.
This worked but if the fan was already triggered by the trinary switch and the PCM wanted to turn the fan on when the ECT reached the set temperature,
the PCM didn't like it, and would set a fan relay code MIL.
I ended up adding an additional relay to separate the trinary fan trigger from the PCM trigger. This fixed the fan relay MIL.
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epfatboy · 07-17-2025, 02:10 PM

What’s one more relay…

And while I’m add it let’s have a new dryer as well..

But I still maintain nothing like a hot rod with ac.. so it’s worth it.

87chevy.com · 12-24-2025, 06:58 PM

do you have hp tuners, maybe the p59 ecu have fan control table ? and you can wire it like OE and be able to tune the fan operation to your liking

epfatboy · 12-25-2025, 12:14 PM

So I do have HPT.. and a standalone harness.

I’d need to pull from the compressor side of the switch to …. Gonna need figure out if I have an “AC IS ON” wire (pretty sure I do) already pinned out. If not I’ll need to add it which sorta sucks balls but it’s not like I haven’t done it before… pin out a new wire that is.

Pretty sure this is why I asked the Q in the first place to see if there was a different way, which there appears to be, but I do like the HPT approach better.

07-16-2025, 04:07 PM	#1
epfatboy Registered User Join Date: Mar 2018 Location: Barrington nh Posts: 137	Vintage Air Binary to Trinary and..?? I have a '74 C10 running a typical standalone harness to a P59. Went to pull a wire to the harness and discovered I have a binary pressure switch... which is great until you remove the fixed fan and replace it with cool electric fans. Now I need a Trinary to run the fan relays.. I have been running the truck this way for a little bit not realizing this, the driving mostly cruising with little stop and go. Gonna order the parts now, should I put her up until this is repaired? Also.. the original idea was to run 12v when the compressor came on. Which means I still need to pull from that side of the pressure switch ... right? I believe the fan side works on a different pressure so the fans don't cycle with the compressor which isn't the feed the PCM is looking for. Is that accurate?

07-17-2025, 10:15 AM	#2
A1971Blazer Senior Member Join Date: Nov 2013 Location: East Tennessee Posts: 2,407	Re: Vintage Air Binary to Trinary and..?? Not sure if this is relevant to your application but I have two fans. Fan 1 is activated by the trinary switch as wired according to the VA instructions. Since the fan trigger is just a ground to the relay, initially I just wired the trinary ground and the PCM ground for fan 1 together so that either/or would turn the fan on. This worked but if the fan was already triggered by the trinary switch and the PCM wanted to turn the fan on when the ECT reached the set temperature, the PCM didn't like it, and would set a fan relay code MIL. I ended up adding an additional relay to separate the trinary fan trigger from the PCM trigger. This fixed the fan relay MIL. __________________ 1967 C10 Step side 1968 C10 Step side 1970 Chevrolet K/5 Blazer ............. Last edited by A1971Blazer; 07-17-2025 at 10:48 AM.

07-17-2025, 02:10 PM	#3
epfatboy Registered User Join Date: Mar 2018 Location: Barrington nh Posts: 137	Re: Vintage Air Binary to Trinary and..?? What’s one more relay… And while I’m add it let’s have a new dryer as well.. But I still maintain nothing like a hot rod with ac.. so it’s worth it.

12-24-2025, 06:58 PM	#4
87chevy.com BMW & ASE Master Certified Join Date: Feb 2004 Location: Houston, Texas Posts: 1,274	Re: Vintage Air Binary to Trinary and..?? do you have hp tuners, maybe the p59 ecu have fan control table ? and you can wire it like OE and be able to tune the fan operation to your liking __________________ 1987 Silverado - L83 5.3 w/ 6L80e ,A/C,P/W,P/L,TILT, HID projectors, Wilwood C-10 Pro Spindles w/ 2018 silverado front brakes & C-5 Corvette Rear Disc Brakes 1999 BMW 528i - 5.7 LS1 w/ 4l60e, Ford 8.8 IRS w31 spline posi & FX-r projector retrofit New project: 2006 BMW x5 6cyl AWD to L83 6l80e 4wd to 2WD / RWD A poor man buy's it twice finally got my domain name back, 87chevy.com.... site rework in progress

12-25-2025, 12:14 PM	#5
epfatboy Registered User Join Date: Mar 2018 Location: Barrington nh Posts: 137	Re: Vintage Air Binary to Trinary and..?? So I do have HPT.. and a standalone harness. I’d need to pull from the compressor side of the switch to …. Gonna need figure out if I have an “AC IS ON” wire (pretty sure I do) already pinned out. If not I’ll need to add it which sorta sucks balls but it’s not like I haven’t done it before… pin out a new wire that is. Pretty sure this is why I asked the Q in the first place to see if there was a different way, which there appears to be, but I do like the HPT approach better.



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