neal64ss

iTrader: (1)

Putting a 5.3l into a 64 chevelle. Had the PCM programmed for 2 fans using pin 42 blue and pin 33 green.

If I understand correctly the PCM supplies ground to the these pins when it wants to turn on the fan(s). Therefore I need to wire the releys in a "ground to turn on" circuit, not a "power to turn on" circuit.

The reason I am confused is that the conventional wisdom seems to be use three relays to do this, but looks to me like two will work just fine. I suspect I am missing something and want to find out now rather than later.

Also I am not running A/C at this time, and the two fans are each one speed, PCM set to turn on 2nd fan at a slightly higher temp than the first fan.

ls1nova71

iTrader: (4)

The factory uses three relays to run the both fans in series for low speed, then changes to parallel for high speed. If you just want one fan to come on at low speed, then the second fan come on at high, then two relays would work.

Pocket

iTrader: (22)

Correct

This is how Ive done all of my dual fan setups. Single fans use the low pin only

LS1MCSS

iTrader: (18)

Here is the three relay schematic, in case you want to do it this way. Somewhere on here is a schematic with the relay terminal number on it.

neal64ss

iTrader: (1)

So if I understand correctly the three relay setup turns on both fans in series for a "low speed" operation using pin blue 42 and then turns on both fans in parallel for "high speed" using pin green 33.

Forgive my asking twice but wiring is the bane of my attempt at hot rodding.

the_merv

iTrader: (88)

Yup..

neal64ss

iTrader: (1)

OK, I think I finally understand it. Thanks for the help.

clm69z28

Herer is a pic of the three relay system I built from the relay schematic above. I used the dual fan setup off of a Camaro for my Chevelle and it is quiet and does a better job of maintaining a constant temp. I highly recommend it. Sorry about the size of the pic- you can search for fan threads and find my post with this pic and other ones of the radiator and fans setup.