ftc

Howzit,
I have a 2002 4.8 nbs silverado that had LS1 efans installed for quite a few years, I don't drive the truck very much in the last 5 years since it hasn't been my dd. They recently stopped working so I decided to change the fans and wiring instead of doing diag to save time, because I was never really satisfied with the wiring harness anyway, I believe it was undersized for the application.

Anyway I changed over to a 34" Mishimoto rad recently so that gave me the option to use NNBS fans. So i got the 5/7 blade NNBS TYC fans and installed it with a Racetronix HD 3 relay series/parallel harness. ATM I only have the low speed trigger wired into pin 42 and set the turn on temp real low (150 deg) for the time being so they at least run, after hooking up the high speed trigger I will turn the setpoint up to a normal setting.

From what I can tell the harness is wired the same way LS1 Camaros are where "relay 1 and 2" output go to the fans positive and "relay 3" switches between series and parallel based on triggering.

What I was thinking of doing that is different is using the compressor clutch signal like a vast majority of harnesses do, but take another relay to change over the comp clutch positive to a negative trigger as thats how the racetronix harness is configured (hi/lo on negative triggers). I'll just call this "relay 4" for my case. Wiring its output to the high speed trigger ONLY, if I'm understanding the schematic correctly this will control ONLY fan 2 in parallel (based on A/C clutch status), since "relay 3" would not be energized. This would in essence add a 3rd speed/mode to the harness, since to get both fans in parallel aka "high speed" both triggers need to be activated. Wanted to get another opinion if this would be feasible or not? I believe this is might have been how the harness I took out was wired but it used a positive trigger for the A/c clutch thus not needing the 4th relay so the pinning was probably different than the current racetronix harness.

What I'm envisioning the control strategy would be is as follows:
Low speed (Both fans in series) - low speed wire triggered by pcm temp setpoint
"Mid speed" (Fan 2 only in parallel) - high speed wire ONLY triggered by A/C clutch
High speed (Both fans in parallel) - Triggered if setpoint temp is reached AND A/C clutch is engaged

Anything wrong with my logic or something I'm overlooking?? I know there is the BP update harness to control it off of A/C pressure but I really don't drive the truck enough to warrant it, and I feel like the cooling system at this point is robust enough... I drove the truck for 10 mile drive with no fans at all and it never went over 200 deg. Before with the LS1 fans I don't think they ever reached the temp to kick into high speed.

Thanks,
TY

ftc

This is how the Racetronix harness is wired for reference.

Also one other thing I forgot to ask is, racetronix support suggested to use a PNP transistor instead of a relay to change the polarity of the a/c clutch, has anyone done it this way before? I’m not very familiar with transistors, what would a diagram look like to achieve this? Seems like packaging would be a little neater with physical size.

thanks