I have a 99 lm7 with reworked harness and pcm.
PCM has 1 positive and negative wire for cooling fan and 1 speed set to on at 205. I have 1 puller fan.
Im wanting the fan to come on automatically when the AC is turned on. I have a separate relay wired from the binary switch as a trigger, power from the battery, grounded of course and the power wire to fan split into the power wire from the computer. Any issues I’m not thinking about?

 

As long as the power from relay to fan doesn't backfeed to the computer it should be OK

 

Wait... your PCM has a NEGATIVE (12V-) wire AND a positive trigger wire for fans? I don't think I've ever seen that. LT1SWAP.com has the pin-outs--here; https://www.lt1swap.com/99-02_vortec_pcm.htm

I think the wires involved are on RED connector #33, and BLUE 42, are the two "trigger" wires used to control both the high, and low speed fan. BOTH of those wires to my knowledge are NEGATIVE wires designed to energize the coil in the relay. Am I wrong about that?

Anyways, how I have my system wired up is like this.... I am running the Ford Thunderchicken fan from a 1997 4.6L. It is a TWO speed fan that uses a single motor. That motor has 3 wires coming out of it. 1 for ground, another for low speed power operation (12V+), and the last for HIGH speed operation. My PCM is set to control ONLY the low speed (as it moves enough air that the vehicle will never over-heat just utilizing the low speed). Then, I have the HIGH speed side set to trigger when the A/C is on. I used the A/C compressor control wire coming from the PCM to trigger TWO relays.. 1 relay for the a/c compressor clutch, and the 2nd relay for the HIGH speed fan control. I have this system wired up with a 3rd relay so that when the HIGH speed fan is triggered by the A/C request, the low speed fan cannot be energized. This prevents BOTH the high and low wire from getting power at the same time. I understand that GM electric fans use 2 fan motors, with 3 relays. If you are running the GM set up, I'm sure you can figure out how to do this the same way. OR... plumb in a trinary switch into the A/C lines like GM did on the F-bodys and then the HIGH speed fans will be triggered off of PRESSURE in the A/C lines. If you would like a diagram for how I have mine set up, I can provide it. I draw them up and keep them in the car in case I ever need to trouble shoot the system, or if/when I sell the vehicle the new owner can easily figure out what is happening with the system.

When I wire up my fans I use 8 gauge wiring on ALL wires including the ground, and ALWAYS use a 12V 50A circuit break that automatically resets if/when tripped, and I use a 12V 75A relay on BOTH the high and low speed sides such as this;
This might seem like over-kill but I have learned the hard way that the cheapie 12V 30A relays like this; tend to melt the wires, and or the contacts, or the plastic housing that the wires clip in to. Don't waste your time with the 12V 30A relays is my point. Spend a little more and you will be saving yourself some major headaches. I also solder all connections, heat style shrink wrap, and then the plastic conduit over ALL power providing wiring with zip ties holding the conduit on the wiring in multiple places along the wire.

 

When the AC is on and the fan is working, the fan on wire going to the pcm fan relay will be hot but shouldn’t go any further correct?
And if the pcm triggers the fan to come on while the ac is on it can’t get to the pcm trigger correct?
Its just a hot circuit???
Seems like this is logical but I’m no expert.
Thanks for the reply.

 

Kawabuggy
I read that the positive wire is hot and the pcm grounds to make the fans operate. But w switch on there’s no power on the hot wire yet it’s grounded. ​​​​​​

 

I will need you to be much more specific with what you have typed. When you say the "Fan on wire going to the PCM fan relay will be hot"... You are losing me there... The wires coming out of the PCM to control the FAN, or A/C compressor are all NEGATIVE signal wires... When you say they are "hot", in my mind I'm thinking they are producing voltage like 12V+ which is NOT the case. They are GROUND, or GROUNDED, wires when the PCM turns them on. Your relay for your fan, or a/c compressor has a coil inside of it.. That coil will, or should, have 12V+ going to it when the key is in the "RUN" position. Then, when the PCM wants to energize that circuit, it provides the GROUND for the relay coil, which then allows the current to pass through the relay and then on to your fan, or compressor clutch, or what have you.

When you say "it's just a hot circuit"... Well.. That depends.. It's a "hot circuit" when the GROUND signal is provided by the PCM, which then closes the circuit in the relay coil, which then allows current to pass through the relay.

If you are worried about the signal from one fan acting on, or interfering with, the signal for the other fan... When wired up correctly using 2 separate relays-there is no possibility of the two fan signal wires interfering with each other.... I hope that answers your questions. I will admit I'm not 100% clear on your post.

 

One other thing here to consider.... if you are testing either of the two trigger wires coming out of the PCM with the key ON, or in the RUN position, there should be 12V+ on those two wires (unless they are currently being turned on or GROUNDED by the PCM). The reason they will show 12V+ with the key on is because the 12V+ being provided to the coil inside the relay is passing that voltage through the coil, and back feeding to your PCM. Some relays have a diode in them so you will not get the 12V+ reading at those wires.. but some do not, and you would then see the 12V+ on those wires.

 

Thanks for all the thoughts and replies. I have. Howell harness and they include a wire just for this situation.
Guess I should’ve called them first. Nice people.

 

I assume your ECU grounds the relay coil to turn it on? (Your 85 or 86 terminal on the relay.) and the other side of the coil is to pink/switched ignition?

If so, then you can add a relay in the line from the ECU to the fan relay

Terminal 30 to the relay
Terminal 87a (normally closed) to the ECU
Terminal 87 to ground

So, if this relay is off, the ECU is controlling the fan relay
if this relay is energized, it grounds the line, turning on the fan relay.

This is all just grounds getting switched, so no chance to backfeed anything.

 

It is a good idea to use relays with a"flyback" diode inside the relay. This will help to reduce the possibility of an inductance spike harming the pcm. Here is a good article about it