How To: ’99-’04 GMT800 OEM E-fan upgrade
This is a step by step tutorial on how to upgrade any 99-02 GMT800 truck or SUV to the 05-06 GMT800 electric cooling fans. This same setup can be used on 03-04 trucks as well, however the only difference is the wiring that connects to the PCM. A custom tune is also needed, because the pins to control the fans in the PCM need to be turned on with tuning software.
First of all, you need to measure your radiator to see what size it is. Some trucks came with a smaller radiator (28 inches wide) and some have the larger radiator, which is 34 inches wide. The 05-06 Tahoe/Silverado electric fans can only be used if you have the larger radiator. So if you have the smaller 28 inch one, you need to upgrade your radiator to the 34 inch one first. If you have the 34 inch radiator, the fans will bolt right up as if they were made for your truck. The holes for the fan to bolt on are already in the radiator support. It uses the same bolts as the radiator.
If you have an 03-04 truck, then you will wire it exactly like this diagram shows. But on a 99-02, the PCM only has pin 42 available to use. The other trigger wire has to be connected to the AC compressor clutch relay wire. That wire is positive, so by me using an OEM harness, I had to add a relay to that wire to change it to a negative in order to trigger the fan relay pack, which is set up to be triggered by negative signals from the PCM. First you want to remove your under hood fuse panel cover. Not just the lid, the entire cover. You will have to unbolt the corner fender brace in order to get it up and off. Now lift up the fuse junction block to get access to the underside. You will need to splice into the dark green AC clutch relay wire. This is the wire you need.
This is where you will tie in your relay.
The relay needs to be wired as follows:
86-Green AC wire
30-Blue trigger wire on OEM fan relay pack. This is how your relay should look.
Now the low speed fan trigger needs to be connected to the PCM. Remove the PCM from its mounting bracket and unplug the wiring harness, which i
s held in with one screw. Unscrew it and slowly wiggle it side to side until it comes loose. You will need to insert a pin into an empty slot, pin 42 on the side that says “BLUE”. Carefully remove the blue plastic cap on the side of the harness that plugs into the PCM. Here is Pin 42 this is where you need to insert your green trigger wire pin. Now take your pin, and carefully push it thru the pin 42 hole until it is fully seated just like the rest of the pins.
Now you need to mount your relays. The OEM harness mounts right alongside the fuse block right where it sits on a 05-06 truck. It just clips right in.
The main power feed for the relay needs to be connected to the battery, and the ground needs to be secured to a factory bolt on the frame or body. I secured mine to the horn mounting bracket, because the OEM harness grounds in that general area on a 06 truck.
You can just zip tied my harness alongside another factory wire loom that runs alongside the bottom of the radiator.
Now that the wiring is squared away, on to the actual fan swap. First, you need to remove your air intake tube. Next, you want to remove the upper half of your fan shroud, which is held on with two 10mm bolts on the top, and two plastic push rivets on each side. Notice the empty bolt hole just to the left of the 10mm bolt that secures the fan shroud. This is where the electric fan shroud will bolt up to. (I find that using a door panel removal tool is perfect for getting the plastic rivets out. If you don’t have one, a pair of wire cutters will work too.) Now once you get the fan off, the bottom half of the fan shroud will simply lift right out.
Now just drop your new electric fans down where the fan shroud was. You will have to do a little wiggling to get it around the radiator hoses. The bottom of the shroud rests on the radiator support just like the original fan shroud did. It also rests in the “hooks” that are molded into the radiator tank. Now look at the top and the two mounting holes should line up perfectly with the two holes in the radiator support. Now just bolt it in. Now climb underneath and finished securing all the wiring.
Now just bolt your air intake tube back on and you are done with the physical install.
All that is left is to have a tuner properly calibrate your PCM to control your new fans.
If your truck has the 105 amp alternator, I strongly suggest that you upgrade to the 130 amp or 145 amp version. You may notice your voltage gauge drop from 14v down to 12v when both fans are on and you are idling.