Nelson LS1 Fan Harness Problem

[Chevy366]

The only thing I don't like about this design is it cycles fans with A/C compressor on and off . Good for the little tiny fans from a LS1 but not for the big fans of a 05 .
There is no reason a PCM can't be set up to run the pins 33 and 42 correctly and use the correct harness design .

[farmtruc]

There is no reason a PCM can't be set up to run the pins 33 and 42 correctly and use the correct harness design .

Yes there is, (with this harness), on some truck pinouts that pin is already taken, I can't remember exactly what its connected to, but you can't tap into it or replace it.
On some trucks you can use the pcm for the a/c, however, the pcm needs to support that, like on an fbody or like the '05+ trucks are setup.

I'm sure a harness can be designed to work with anything, but not with this setup/harness (what he has). So, that's what we have to fix.

Oh, I just looked it, pin33 "recirculation door control", can't use it!

[GasGuzzler]

Recirc door control on what? No truck. PCM control of the recirc door? Wow...not sure the application.

Anyway,,,,my giant single Flex-a-lite on the Blazer cycles with the compressor as well and is fine.....and loud.

[farmtruc]

Recirc door control on what? No truck. PCM control of the recirc door? Wow...not sure the application.

Anyway,,,,my giant single Flex-a-lite on the Blazer cycles with the compressor as well and is fine.....and loud.

I have no idea either, but I tried this pin one time and the fan barely moves, kind of an on and off motion. Like the relay is triggering, then off, then on.

I'd be interested as well in a new fan harness for the pre 04 trucks, but the fans have to come on with the compressor.

[RedHeartbeat]

My harness does that but it takes 4 relays and you have to tie into the power lead to the A/C clutch. It has all the controls the factory setup has with the dual PCM control leads.

[farmtruc]

My harness does that but it takes 4 relays and you have to tie into the power lead to the A/C clutch. It has all the controls the factory setup has with the dual PCM control leads.

Ya, the fbody pcm is much nicer and actually has values you can change in the pcm.

[Chevy366]

Yes there is, (with this harness), on some truck pinouts that pin is already taken, I can't remember exactly what its connected to, but you can't tap into it or replace it.
On some trucks you can use the pcm for the a/c, however, the pcm needs to support that, like on an fbody or like the '05+ trucks are setup.

I'm sure a harness can be designed to work with anything, but not with this setup/harness (what he has). So, that's what we have to fix.

Oh, I just looked it, pin33 "recirculation door control", can't use it!

As Red said 4 relays can do it .
Perhaps I look at things differently than that , I think 2002 up can be wired to use 33 and 42 , some use pin 16 for fans (red connector , green 33) . Can you look at pin 16 and see if it will work ?
Funny that pin 33 will not work in some , cause all you need is PCM to ground the wire , can a segment swap be done with just the needed fan parameters ?
I agree that fans need to be on when A/C pressure reaches critical pressure , keeps it from going boom .

[GasGuzzler]

2003 Blue connector 33 is TCC Brake Switch/Cruise Control Release Switch Signal. Green 33 is not used in 2003 but there are some 20-25 unused pins.

I looked back to 2000 and found Green 33 to be as Allen said, "HVAC Recirculation Door Control"....

Looking forward I see that it's like that to 2002. I would guess it's because the 2003+ have a Class2 HVAC control head that controls the recirc itself.

Does that mean a 2003-2004 can use the empty Green 33? Not sure. You can't just make any empty pin live can you?

So 1999-2002 Green 33 is taken but 2003-2004 Green 33 is empty. 2005-2006 Green 33 is used for fan operation.

Now I see why people are mentioning Green 33. It's the fact the 05-06 uses this for high speed operation. Instead of high right side with compressor on (like the Nelson harness), the PCM can command high fans (both) when AC high side pressure crests 265 psi.

In the end it's six of one, half a dozen of the other. With the LH fan on low and the RH fan on high (like with the Nelson harness), I doubt the high side AC pressure would ever get too high. The blow off valve in the compressor opens between 425-440 psi.

If it would help, I could list all the empty pins for 2003-2004....but once again, I'm not sure if just any pin can be made to carry out certain functions.

What is the internal design difference between the 03-04 PCM and the 05-06?

[Chevy366]

If a fan that ran is a ran fan then a ran fan is a fan !

[txarkitekt]

With the PCMs, the PIN slot receives a signal from the sensor it is wired to, and then carries out that required function. Like with the fans, all the PCM does is ground the fan's circuit. Nothing else. It is already hard wired to receive power. The PCM tells its self (from the water temp sensor) how warm the motor is, and grounds the fans to complete the electrical circuit. So, with that said, I doubt you would be able to use an empty PIN location to operate a required function.

In order to get the PIN to operate correctly, one would have to have PCM editing software that could program what the exact PIN location needs to do, e.g. what it needs to rely on by other sensors to make it's function happen. For example, to get the fans to come on when the A/C is engaged, the PCM would have to receive a signal from the A/C fan clutch--then know to ground the circuit for the fans to come on at 100%. Or once the climate control button has been pressed to engage the A/C, the A/C clutch is activated---and that electrical circuit is completed...and the wiring for the fans is bridged to it, so that is how they come on?? I really don't know how GM decided how to wire everything up. I bet they just thought that they needed (lets just say for grins) 50 PIN locations. You just don't go out and make a PCM to accomodate 50 pins...you make one with like 60 slots--to make sure you CYA if you ever have to go back and alter the wiring. I could be wrong on all of this, but I would imagine that our PCM's tuning software (HPT, LS1EDIT, EFILive) can only modify/edit what PCM functions are available--not program empty PIN slots...