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I'm in the middle of an electric fan swap on my 2002 Silverado and want to use the PCM to turn the fans on/off as needed without any sort of external switch nonsense. I read that there's an option to use 3 relays to run the fans on both low and high depending on the signal sent from PCM. Unfortunately I can't read a wiring diagram to save my life. I did find this diagram online, is it correct? I don't really get why you need Ignition Hot, and the PCM inputs don't make sense (Blue 42 provides the input from the PCM to the relays to turn on the fans, but what about the signal to turn them high?)
Ignition hot is to provide power to the high side of the relay coil since the PCM signal provides a ground to energize the relay.
I haven't dug through the picture other than the explain your ignition hot, but usually low/high is simply a matter of whether the fans are wired in series or in parallel. When in series 12V is dropped across both fans so they split the voltage and have the same current flowing through them. This would be low. When high, they would be in parallel and each fan receives its own 12V from the battery and has a separate current path which can drive the fans harder.
Ok, that seems to make more sense - so the above diagram is okay to follow? I'm tempted to just buy a premade harness but they're close to $200, I'd rather solder my own.
Ok, that seems to make more sense - so the above diagram is okay to follow? I'm tempted to just buy a premade harness but they're close to $200, I'd rather solder my own.
Waytek wire sells all kinds of nifty relay holders and whatnots if you want to go this route. This thing holds 3 ISO280 relays. They can be had up to 30amps each.
Just don't use cheap, prebuilt relay holders with pigtails. Get some good wire and a few crimping tools and make a killer harness yourself.
Waytek and Mouser are my new crack cocaine...
Personally, I would be looking at a standalone PWM fan controller.
Andrew, appreciate the input on the high quality stuff, but I'm not looking to go nuts on this one. Here's what I picked up, I ran some far less robust relays (spade connectors jammed onto the relay terminals) on my BMW with no issues when I knew even less than I know now.
I actually looked at the picture and it looks like turning on relay 1 is your low, or series config. Turning on all 3 relays puts the fans in parallel/high configuration. As long as you program whatever you're using to control the relays to match this it should work.
Andrew, you're not using those fancy GT 280s again are you???? hehe.
I did read that you don't like wiring diagrams but this may be of some help, perhaps to someone else reading the thread even. Attached is a 2006 Tahoe fan wiring diagram and below is how I wired mine.
Only some of us are OCD, others... well they can't be cured. Haha!
I save the nice stuff for my nice cars and the sloppy stuff for my slop projects.
Some may recall that last winter I put together an L92/LS3 swap into my C5 - I did that one "right", but as a result it took far longer than I wanted it to. It's really refreshing to turn the OCD off once in a while.
Many cooling fans ,,, really need 50A relays,,, especially for the HIGH speed.. (Ford Taurus fans) YMMV,, also Volvo fan controllers are kinda sweet. And Cheap.
Just a follow up - using the diagram in Post #1 the setup works perfectly after enabling the fans in the tune. I used the alternator bracket as a ground, be sure to grind off any paint on the black steel bracket that bolts to it to get a good connection.
I have a question, when using the three relay set up to control two fans with signals from a gen 4 ( E38 I believe) ECM. I'm also running Vintage Air and from what I understand the ECM doesn't have the ability to receive a "AC on" signal, correct?
So, can I just feed the ground signal from the trinary switch the the ground signal from the ECM fed to the #2-3 relays. I believe this would automatically activate the high speed fan sequence. My concern is that the ground signal from the trinary switch does not cause issue's with the #2 fan signal from the ECM.
ETA, My Speartech harness has individual "key on" sources for each fan signal, will it be ok to use one for relay #1 and the other for relay's #2-3
I'm thinking like this. Again my concern is that the ECM will actually see a chassis ground when the ECM and the AC send signals at the same time. Maybe a Diode on the AC input?
ETA. I spoke with Speartech today and they confirmed this setup would work. There is no issue with the ground signal from the AC causing problems with the ECM.
11-28-2018, 07:30 AM
Explain 3 Relay, Dual Fan wiring like I'm 5 
