Electric Fan Conversion on our 2002 GMC 1500HD

We wanted to take an opportunity to document the electric fan conversion on our '02 1500HD since we often see questions come up regarding what is involved in properly completing the conversion on the 99-04 GM Trucks.  This write up will cover the basic of the install on the 99-02 platforms, but is relatively similar to the 03/04 installations as well (the only difference being in A/C trigger wiring).

There are several benefits to the electric fan conversion.  Most people are looking for a few extra horsepower, but in addition to 10-20 hp gains, electric fans will provide better low speed and idle cooling as well as improved A/C performance at idle.  Keep in mind that your truck will require a tune in order to enable the computer to control the electric fans!

We chose to complete the swap using 2005+ style dual fan assemblies, specifically the nine blade HD cooling variants found on the 2010+ trucks.  Using this style fan is absolutely what we recommend if you truck is equipped with the larger 34" radiator.  These fans bolt directly in place, as the 99-04 trucks already have all of the mounting provisions necessary for the later model fans.  Even if your truck is equipped with the 28" radiator, it may be worth considering upgrading to the 34" for the additional cooling capacity.  Another commonly selected option for the 28" radiator trucks are the 98-02 F-Body fans.  However, those fans are considerably smaller than the 05+ truck fans and will require some modifications for installation such as OE fan mount trimming and custom mounting brackets.

There are a number of companies out there that offer the plug and play conversion harnesses, but we chose to use the harness from Nelson Performance.

2005+ GM Truck FANs and Conversion harness

The first step is, of course, gaining access to the OEM clutch fan for removal.  The only things that need to come out of the way are the air inlet tube and the upper fan shroud.  The upper fan shroud is held in place with two bolts to the core support and 4 push-clips connect it to the lower portion of the shroud.  Once those are out of the way, you have access to the OEM clutch fan for removal.  There are specialty tools out there for removing the clutch fan, but if you have access to an air compressor and and air hammer, nipping the side of the fan clutch nut with the air hammer is often the easiest way to break it loose from the water pump pulley.  The fan clutch on the GM trucks has a standard thread orientation, and turning it to the left will loosen it.

With the OE fan out of the way, the lower fan shroud can then be removed as it simply slips into place.  With everything out of the way, you can begin to prep the electric fan assembly for install.  Remember to move all of the original retaining clips from the OE shroud over to the new electric fan assembly.  In our case, we had one clip that retained the upper radiator hose and another that retained the oil cooler hard lines.  Slipping the new fan assembly into place can be a bit of a geometric challenge, but with enough finesse and some careful prying, it will drop into place without removal of any other components.  Remember to be sure that the tabs on the edge of the fan slip into the clips that are molded into the radiator end tanks.  There are also several slots in the bottom of the fan assembly that are designed to rest on the lip of the lower radiator core support.

With the new fan bolted in place, we're able to now move onto wiring.  On the 99-02 trucks the electric fan conversion harness has 6 total connections to make: one connection to each fan, constant power to the relays, ground, blue trigger wire to pin 42 on the ECM connector, and green trigger wire to the A/C clutch signal wire.  You will need to determine which wire triggers the A/C compressor clutch.  This wire runs in the OE wiring loom directly to the left of the red junction box near the alternator on the driver's side.  Carefully remove a section of those wires from the loom and look for the green trigger wire.  NOTE: there will likely be more than one green wire in this bundle.  You will need to test for 12v power to the wire when the A/C clutch in engaged.  Be careful here, as one wire will receive power under the same condition, but it will not be a full 12v.  You will want to be sure that the wire you're testing sees a full 12v (or 13v/14v when engine is running) when pinned during A/C compressor clutch engagement.  Once that wire is positively identified, use the included wire tap to tap into the OE A/C clutch signal wire.

After identifying and tapping into the A/C clutch signal wire, we moved onto installing the blue trigger wire to pin 42 on the ECM.  Remove the ECM dust cover and then the connector closest to the engine.  The connector uses a 7mm socket to unscrew the connection.  Once the connection is loose, remove the wire cover from the outside of the connector.  This cover uses several barb style connections to retain it, simply use a small screw driver or other device to carefully work the barbs loose.  With the cover out of the way, find the pin labeled "42".  This is usually on the lower side of the connector closest to the front of the truck and should be labeled with the number 42.  The fan conversion harnesses come with a pin already crimped on that is designed to be inserted through this connector and into place in position 42.  Lubricate the pin, and insert it through the back of the connector.  Carefully work it into position until it becomes fully seated.  This can sometimes be difficult to complete, so please take care when attempting to work the pin/wire through the connector.  If the wire folds over and becomes kinked, it will be much harder to get the pin fully seated.  Once fully inserted, the pin will snap into place - verify this by gently pulling back on the wire - it should remain in place.

Finally, we connected the relay power and ground connections for the harness.  The two ring terminals connected to the red wire and red wire/white wire on the harness are connected to the power stud in the red junction box near the alternator.  The bottom of the box will require some minor trimming to allow the new wires to exit without becoming crimped.  The ring terminal connected to the black ground wires are connected to one of the bolts that secure the junction box mounting bracket to the alternator bracket.  The power stud in the junction block takes a 17mm wrench or socket and the grounding bolt will take a 10mm.

After that, it's just a matter of connecting the fan harness to the electric fans and mounting the relays to the outside of the ECM cover using the supplied screws.  Remember, the truck will need to be tuned in order to enable electric fan control by the factory PCM.  There are a number of companies that offer discounted tuning services to enable fan control only, but it is certainly worth the extra money to go ahead and have a full tune done to the truck.  Even if the vehicle is stock, like ours, there are a number of gains to be had for both performance and fuel economy even when using regular 87 or 89 octane fuel.

Now that the fans are in, go ahead and load your new tune if needed and enjoy the extra HP and cooler A/C! 

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