PCM fan outputs
Thread Starter
On The Tree
Joined: May 2004
Posts: 161
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From: London, UK
PCM fan outputs I'm still a little confused as to what outputs the PCM gives the fans.
On the PCM there is a high speed output (RED 33) and a low speed output (BLU 42).
When the temperature of the engine is low, I assume the low speed output is switched (to earth?).
When the engine temperature gets high, I assume the high speed output is then switched, but what happens to the low speed? Does it stay on or turn off?
I am not going to be using variable speed fans, I am devising a circuit to drive just one fan (low temp.) or both (high temp.). I just need to know what happens to the low speed output - does it remain on or turn off?
On the PCM there is a high speed output (RED 33) and a low speed output (BLU 42).
When the temperature of the engine is low, I assume the low speed output is switched (to earth?).
When the engine temperature gets high, I assume the high speed output is then switched, but what happens to the low speed? Does it stay on or turn off?
I am not going to be using variable speed fans, I am devising a circuit to drive just one fan (low temp.) or both (high temp.). I just need to know what happens to the low speed output - does it remain on or turn off?
I am only using the "low" curcuit to drive my twin Flex-a-lite 220 fans. I have them set to come on at 200*F and they stay on until the temperature comes down to under 195. They don't turn off when the "high" fan (which nothing is hooked up to) is supposed to come on. The stock turn on temps are very high (~229*F for the low temp fan), so you will probably want to edit them with something (HP Tuners, LS1Edit, etc).
Hope that helps.
-Matt
Hope that helps.
-Matt
Originally Posted by robin427
Thanks, guys.
Slow, I've still got that grommet for you if you need it...
Slow, I've still got that grommet for you if you need it...
Ryan
Originally Posted by Scooter70
I am only using the "low" curcuit to drive my twin Flex-a-lite 220 fans. I have them set to come on at 200*F and they stay on until the temperature comes down to under 195. They don't turn off when the "high" fan (which nothing is hooked up to) is supposed to come on. The stock turn on temps are very high (~229*F for the low temp fan), so you will probably want to edit them with something (HP Tuners, LS1Edit, etc).
Hope that helps.
-Matt
Hope that helps.
-Matt
Steve
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Originally Posted by 55chevy
Matt, Are you using one or two relays for the dual fans? I look at your cooling page but I don't think you were through with the wiring. Thanks,
Steve
Steve
-Matt
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The factory uses 3 relays for the two fans. When cold, both fans are wired in series and run in slow speed. When hot, the fans are wired in parallel and run in fast speed.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
I did mine the opposite of Scooter's idea, two relays (one for each fan) and both being switched by the PCM's high-speed controller.
I was worried about using the PCM's low-speed controller, as it appears to cycle the relays on/off at some predetermined frequency to make the fans spin slower. In my case I'm using older mechanical relays from a '88 Camaro parts pile, not the spiffy newer relays the LS1 fuse/relay panel had.
I wasn't sure the older relays would work reliably being cycled that way, so I used the high-speed controller which just holds the relays closed until the off temp is reached.
I was worried about using the PCM's low-speed controller, as it appears to cycle the relays on/off at some predetermined frequency to make the fans spin slower. In my case I'm using older mechanical relays from a '88 Camaro parts pile, not the spiffy newer relays the LS1 fuse/relay panel had.
I wasn't sure the older relays would work reliably being cycled that way, so I used the high-speed controller which just holds the relays closed until the off temp is reached.
Originally Posted by crainholio
I was worried about using the PCM's low-speed controller, as it appears to cycle the relays on/off at some predetermined frequency to make the fans spin slower.
When the PCM high speed output goes low it grounds the fast speed relays which wires the fans in parallel giving 12V to each fan making them run faster.
Originally Posted by 70 LS1
The relays do not cycle to spin the fans slower. The PCM low speed output will go low and ground the slow speed relay. Like I said above, this wires the fans in series giving only 6V to each fan making them run slower.
When the PCM high speed output goes low it grounds the fast speed relays which wires the fans in parallel giving 12V to each fan making them run faster.
When the PCM high speed output goes low it grounds the fast speed relays which wires the fans in parallel giving 12V to each fan making them run faster.
I pulled the '01 Helms down and looked at the schematic, and I'm glad I didn't try to reproduce it for my swap either way.
It does, however, look like a good future mod for me would be to wire the low-speed fan control to one fan relay, and have the high-speed control manage the other fan relay...similar to the way my old '90 TPI ECM did.
Set the low-speed fan on/off temps to 195/190 and the high-speed fan on/off to 205/195 and I'm back in the primary + aux cooling fan business instead of spinning up both simultaneously.
Thread Starter
On The Tree
Joined: May 2004
Posts: 161
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From: London, UK
I'm going to wire one fan to each relay but wire in an override switch that will turn both fans on regardless of temperature. I need to wire a diode into the circuit for this, but that shouldn't be a problem.
I have wiring diagrams if anyone is interested...
I have wiring diagrams if anyone is interested...
Originally Posted by 70 LS1
The factory uses 3 relays for the two fans. When cold, both fans are wired in series and run in slow speed. When hot, the fans are wired in parallel and run in fast speed.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
Thread Starter
On The Tree
Joined: May 2004
Posts: 161
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From: London, UK
Originally Posted by snakeoiler
Yes, I would be very interested in this schematic. Do you hook into the A/C as well?
Here is my entire schematic. It's made with a program called ExpressPCB (www.expresspcb.com, I think). It should be self explanatory how the fans work. If not, I can do a separate version for you...
Here is the Schematic of the way GM does it. one relay energizes running the fans in slow (series). Then all three energize to run the fans in fast (parallel). The optional wire running to a toggle switch then to ground will allow you to turn on the fans in slow from the dash.
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The factory uses 3 relays for the two fans. When cold, both fans are wired in series and run in slow speed. When hot, the fans are wired in parallel and run in fast speed.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
I wired up mine per the factory manual and have had no problems. If anyone is interested in a schematic of the 3 relay, 2 speed fan setup, let me know.
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From: Central Cal.
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