Cooling Fan relay setup

[57FleetsideC4]

Originally Posted by Hcb3200

Going to call overthinking on this one.

Yes the PCM has a two fan option.

With the three relays it controls speed

It does this by putting the fans in series on low speed call.

As in

Positive wire ----- fan 1, wire out of fan one to fan two then wire out of fan two to ground. (in reality i think it does the grounds but either way works)
aka sharing power therefore half speed each.

When it calls for high
The relays flip and

Fan one positive direct fan one and wire out to ground

Fan two with a separate positive and wire to ground.
Both fans get full amps and run wide open.

Since you have one fan just wire the fan to the low side of fan one. Low side is always on and hot when pcm calls any fan to run.

Set your PCM to be on when you want temp wise since you are going to remove all the vats and other items anyway. and be done. It will run full speed as long as the pcm calls it to run.

Assuming you do not have a E78 ecm. it runs a mechanical fan and the pcm does not call the fan relays. even though its in the computer and the relays are in the fuse box. Ask me how I know. E38 which is the most standard is relay and pcm fan triggering worthy all day.


I doubt in a old school truck you will ever need two fans even with AC.
It needs the higher speed in modern trucks with a crammed full engine bay not letting out heat. mountains of towing power in modern trucks running full bore AC to multiple systems in some cases. This will tax engine heat etc.

Originally Posted by Hcb3200

This really spells it out.

Thank You for the information. Steve