Could use some help on a 2004 Silverado fan question

[bronkx1]

I have a 2004 Silverado and replaced the 28" Radiator with a 34" unit and added the OEM electric fans
from a 2005+ Silverado. I then paid for a tune from an unnamed source that would activate them via
a Nelson 05+ OEM fan harness and the 2 pins on the PCM. The fans do not come on and the high
speed trigger on the PCM seems to not be turned on as I get no ground from it when it should be
triggering. I get a single low speed fan based on coolant temp but nothing with the AC.

Can any one tell me what it is specifically one must do to get the fans running with the coolant temp
and the AC pressure? I am just about to the point of getting a Flash Scan and Tune and fixing it
myself. I am far from a tuner but I have a backup PCM and am willing to give it a shot.

[Taz]

GM changed the A/C and E-Fan control strategy in truck PCMs in 2003. E-Fans didn't arrive in trucks until the 2005 - but the calibration is setup for this, and needs very little changed (in 2003 and 2004 tunes) to get the E-Fans working correctly.

How did you wire the E-Fans and relays you purchased ?

If wired correctly, I would suggest contacting the tuner you used, and have them check the settings - these are extremely straight forward.

[bronkx1]

I used the Nelson harness for the 05+ oem fans. It is a 3 relay setup that has you add a wire to each of the
PCM connectors. I asked the tuner and he said its an issue with the operating system in my PCM and it had
no fields to enter AC pressure settings. I opened a stock 04 tune I found on the web and with EFI Live it
seems simple to turn them on. Set the fan type then populate the rest of the listed settings for the on and
off details. Then Blue 42 and Green 33 from the PCM provide the switched ground to run the relays.

Spending $900 just to clean up a tune is nuts. But this is the second tuner I have used on this
truck and I dont want to have to go to a 3rd. I dont think my current tuner runs EFI Live but
rather TunerCat and I am not sure if it can correctly do the fans.

[Gregs]

If he uses tunercat its possible he isn't able to see those settings. If you don't want to buy efilive outright I would recommend finding a good tuner that sells AutoCals so you can update your truck as is needed.

[Chevy366]

That Nelson fan harness setup is crap, I can say for sure I had one and this isn't a Nelson forum so I am safe, I think? :music_whistling_1: Would cause huge drain and lights would dim when fans kicked on. It is the only one for older PCMs without the fan parameters.
I went through this with my 05 HD, which has a clutch fan from factory (and it does not over heat when pulling, I have pulled several times with since E-fan install), Nelson uses the A/C clutch to trigger the fans on A/C running not the PCM, Pin Blue 42 for temp, if I remember right, I have a rewiring diagram for the harness still somewhere, there is my original on the net somewhere too. Pin 42 and 33 are for the 05 PCM fans factory harness.
Like Taz said my 05 had the correct tables and it was just a matter of enabling them.
So the Nelson harness does not need the high side A/C switch, it uses the A/C clutch (dark green/white wire, ground) to activate high speed fan when A/C is on. Straight from Nelson install diagram -- 2. When the A /C compressor clutch circuit grounds, Relay 2 and Relay 3 are energized and Fan2 will run at full power.


Hope this helps.

If you need a copy of the Nelson fan install diagram let me know.

[RonC]

I have the same Nelson harness & factory fan setup on my 05 1500 HD that I put on when it was new, using both ECM pins & the a/c wire as Chevy366 said. The only problem I've had with it in 9 years is the HS relay. I've replaced 3 of them & learned to keep a spare in the truck.

[Gregs]

That Nelson fan harness setup is crap, I can say for sure I had one and this isn't a Nelson forum so I am safe, I think? :music_whistling_1: Would cause huge drain and lights would dim when fans kicked on. It is the only one for older PCMs without the fan parameters.
I went through this with my 05 HD, which has a clutch fan from factory (and it does not over heat when pulling, I have pulled several times with since E-fan install), Nelson uses the A/C clutch to trigger the fans on A/C running not the PCM, Pin Blue 42 for temp, if I remember right, I have a rewiring diagram for the harness still somewhere, there is my original on the net somewhere too. Pin 42 and 33 are for the 05 PCM fans factory harness.
Like Taz said my 05 had the correct tables and it was just a matter of enabling them.
So the Nelson harness does not need the high side A/C switch, it uses the A/C clutch (dark green/white wire, ground) to activate high speed fan when A/C is on. Straight from Nelson install diagram -- 2. When the A /C compressor clutch circuit grounds, Relay 2 and Relay 3 are energized and Fan2 will run at full power.


Hope this helps.

If you need a copy of the Nelson fan install diagram let me know.

Just an FYI for the OP, I used the harness from Blackbear on my 03 and it worked perfectly

[Chevy366]

Just an FYI for the OP, I used the harness from Blackbear on my 03 and it worked perfectly
Bro, did this way before Blackbear was a tuner. In fact he may have used my design who knows.
The factory harness I found by using a 05 1/2 ton VIN search on gmpartsdirect long before Blackbear even existed and posted it all over the web, for all to use.

[Chevy366]

I have the same Nelson harness & factory fan setup on my 05 1500 HD that I put on when it was new, using both ECM pins & the a/c wire as Chevy366 said. The only problem I've had with it in 9 years is the HS relay. I've replaced 3 of them & learned to keep a spare in the truck.

Back years ago I posted the part number for the factory harness, all you had to do was strip out the running lights, it came in a front light assembly harness, was about $108 back in latter of 05. Not one problem ever, just like a factory 05 install.
I understand the part numbers got revised and the part got more expensive, must have caught on what we were doing.

[Taz]

... was about $108 back in latter of 05 ...

GM list price now is $540 ... lowest published price (for an original GM part) is $340.

[Chevy366]

GM list price now is $540 ... lowest published price (for an original GM part) is $340.

Holly crap! I knew they went up but geez.

[bronkx1]

I have both PCM connectors 33 and 42 in place so my harness is not like you describe with the green ac trigger. I have
enabled both fans and it seems to work just fine now. I was asking for tune help as this is a tune forum. I needed
to know what all needed turned on. I used a stock 2006 tune and just copied all the fan settings over and it
was good to go. I am not crazy about the Nelson harness either but new OEM is a lot of cash so until I can pull
the harness and fuse/relay box in a junk yard it will have to do.

[Chevy366]

Use the stock 06 settings, they work fine.
On the Nelson harness pin 42 gives fan 1,2 both in series at half power, and then with the other ground it runs fan 2 at full power, I wanted fan 1 at full power temp and A/C trigger and fan 2 at full power with fan 1 at full power when temp warranted it.
Thus I had to shift some wires around and to make that happen, ran the modified Nelson for a while until I got the stock harness. There is not but a few wires that need to be changed to make it act like the factory harness of fan 1 full power and fan 1 and 2 at full power when needed, the fan 1 and 2 half power are throwbacks to the old small fans used in cars not the larger higher output truck fans.