Wiring 2 sources (engine and AC) to one cooling fan

[kabinet]

I'm trying to wire the AC to the engine cooling fan. Power from the engine is coming from ECM. It is fused and relayed. The power for the AC is coming from a relay grounded by a trinary switch. Here is a diagram of what I have going on. Can I just plug both wires into the fan or is that bad? If one system is powered and the electricity will flow back to the other system and that one also turns on will it do something crazy? I basically want to know if I can take the "?" from my diagram and connect it to the purple. Thank you!
https://static1.pt-content.com/images/pt/2017/08/6ECEToz-1.jpg

[sccacuda]

Run the 12 volt power from the ECM you have shown to the 87A leg on the A/C relay.

[Richard454]

Run the 12 volt power from the ECM you have shown to the 87A leg on the A/C relay. Not as his connections indicate...

30 to the fan 87a power from ECM relay and 87 to +12 volt See below-



143291

[dhutton]

If you connect the red from the ECM relay to the purple supplying power to the fan the two relays will be in parallel and if you lose the AC relay the fan will still operate under ECM control. A lit bit of redundancy for slightly improved reliability.

I've done it this way and it works fine.

Don

[kabinet]

Not as his connections indicate...

30 to the fan 87a power from ECM relay and 87 to +12 volt See below-



143291
I didn't even think about flipping the relay around. That is really smart! I'm not sure if the relays have diodes or something in them that only allow electrical flow in one direction but if not that would work great. Don is right about the peace of mind in the redundancy so I'll put the ECM relay directly to the fan as well. Good thinking outside the box though! I'm kinda mad I didn't think of that when I was trying to figure out ways to wire it.

[ccmc]

Is your ecm sending 12v or is it sending ground?

[Richard454]

I didn't even think about flipping the relay around. That is really smart! I'm not sure if the relays have diodes or something in them that only allow electrical flow in one direction but if not that would work great. Don is right about the peace of mind in the redundancy so I'll put the ECM relay directly to the fan as well. Good thinking outside the box though! I'm kinda mad I didn't think of that when I was trying to figure out ways to wire it.

THANKS- as a kid- instead of putting the square peg in the round hole- I found it was a lot easier to cut the square peg in ½ and put it in the triangle...

Most relays don't have diodes-and if they do- it's usually to minimize voltage spikes and have better control of the relay.

Don's way will work- not knowing which ECU and length of wires...could change things... If you really want a redundant back up- look below. Adds another relay- BUT will also help the life of the ECM relay- as less current will flow though it.


Is your ecm sending 12v or is it sending ground?

Hopefully a +12Volt RELAYED output- if it was a positive OR negative trigger you would NOT want to wire it like I posted- UNLESS you like smoke...

Redundant fan wiring-

143321

[ccmc]

null

Yes a 12 v relayed output but what is the trigger the ecm is sending, ground or positive? I mention this because the ecm is never identified and my LS1 ecm sends a ground trigger to activate the fans so wiring the trigger to the relay is going to be different depending on what the trigger is.

In your original diagram 86 is a ground triggered by the trinary switch. I connected my ecm trigger wire here as mine gives off a ground not a positive. I also connected a switch to ground for manual override too. All three just went to 86.

However, if it is a 12v trigger then go with the above diagrams.

I just wanted to make sure you verified what the ecm sends to trigger the ac

Rick

[H2Ogbodies]

All modern EFI PCMs use ground trigger outputs to trigger relays. Too much hassle to route power through an ECM board.