Disclaimer
The community here at 355nation.net urges you to please use caution and seek professional assistance when performing modifications to your vehicle. Before attempting any modification it is advised that you refer to your Colorado or Canyon service manual or contact a certified mechanic as not all GMT355 trucks are the same. The staff and the associated members are in no way responsible for any damages, injuries or other harm inflicted to your vehicle or yourself which may result in attempting these modifications. The posts and content presented on this site reflect in no way the views of 355nation.net or it’s ownership.
![Image]()
A 355nation How To presented by
Metal_EMT (username changed to HK_Titan)
Project Name
How To Install a Transmission Cooler
IMAGE URL Replacement In Progress - Temp Fix
Looks like the forum no longer allows image URLs that are http and require https links. Until I find time to move my images to my https webserver, here's a link to the images for this how-to (copy/paste): dancoco.com/GMT355/howto/transcool_index.html
Project Description
This guide will demonstrate the steps needed to install an auxiliary transmission cooler. This will work in addition to the cooler inside the stock radiator to both improve transmission temperatures, but will also help cool the engine since the radiator is dealing with cooler transmission fluid after passing through the auxiliary cooler.
NOTE: All images can be clicked on to view in a larger size if needed.
Skill Level
Medium
Project Vehicle
Make: Chevrolet
Model: Colorado
Year: 2004
Engine: 3.5L
Transmission: 4L60-E Automatic
Power windows: Yes
Sun Roof: No
Tools Needed
Flathead screwdriver
Wire Cutters
Knife / Scissors / Shears (something to cut the transmission cooler hose)
Small flat-head or dentists pick type tool
Channel locks / Pliers
Rags or Paper Towels
Drip Pan
Funnel
Wrench set would be handy
Sockets (If you have a skid plate to remove)
Dremel or Die Grinder (Optional - See step 9)
Parts Needed
Transmission Cooler Kit
I used a Hayden (Imperial) High Performance Transmission Oil Cooler Part number 240514. Link to Advance Auto Parts.
Extra length of transmission line
Hayden 397 Fitting
You can get this from Summit Racing for about 9 bucks. Direct Link
Convoluted Tubing
Zip Ties
Quart of ATF (can be a partial bottle, just enough to allow you to top it off if needed when done.)
NOTE: In Advanced Auto (and I'm sure elsewhere,) they sell a fitting that claims to be for all GM after 1996 or 1998 or some year around there. This DOES NOT work on our radiators where we plan to attach the hayden 397 fitting above. Don't get this:
Project Time
1-2 hours depending on experience.
Project Cost
$60-80 plus Effort
Steps
1. Let truck sit long enough to allow transmission to cool. Check the transmission cooler kit to see that nothings missing. (should have the cooler, one section of hose, radiator clamps (qty 4), hose clamps (qty 4), and fittings (qty 2) to attach to cooler.
2. Lift front of truck and remove grill. If you're unsure how to remove the grill, click here.
3. If you have a skid plate (stock or otherwise) remove it now. For the superskidz, only the front plate should need to be removed.)
4. The kit should come with a set of fittings like this:
Screw these onto the ends of the cooler. Wrench tight is good enough, don't over tighten.
5. Attach one end of the hose that came in the kit to one fitting. Attach the second hose you bought to the other fitting. Using a little ATF to lubricate the fittings can help here. (twisting back and forth with a set of channel locks / pliers helps also) attach the hose clamps and tighten these until the rubber of the hose becomes flush with the grooves in the clamp. Leave these hoses long for now, We'll cut them in a minute.
6. Now we need to find out where to mount the cooler to the front of the radiator. Some people have made brackets and have found different places to put the cooler. I opted to just strap this cooler directly in front of the factory radiator. There are a few ways to orient this. The most effective cooling area of the radiator with a stock fan is the top half because the bumper isn't blocking airflow and the fan is up higher. If your engine runs hot, it might be better to not block this with the trans cooler. I had no issues with the location I chose.
NOTE: If you have a hard time getting the cooler behind the center support here, removing the stock horn may be needed until it's mounted.
Vertical mounting:
Horizontal Bottom mounting:
Horizontal Top Mounting: (This is what I chose)
Whichever position you choose, make sure there is a clear path for the hoses to go beneath the radiator to connect to the transmission lines. make sure there is room for them to turn without kinking. A few inch bend should work here.
7. Mounting the cooler. Get those radiator clamps out. (long sticks with one flat end.) My kit came with foam blocks. Get these also.
This foam block goes between the cooler and the factory radiator. push the radiator clamp pin through the fins of the cooler, push it through the foam pad and push through the radiator.
There's another "washer" type piece that clips onto these radiator pins from the back side. We need to move the cooling fan to be able to get in here unless you can wiggle your hands in between the fan blades.
There are 3 or 4 places this attaches that we need to free up.
There is a metal hose for the A/C attached to the fan shroud. unclip this for the moment and move it aside. The fan shroud is in two pieces. Top and bottom. there is a metal clip below the A/C line which holds the 2 halves together. take this off. Then you'll see where the upper shroud is attached to the radiator. unclip this.
Now you should be able to pull this back enough to work. It's still a tight fit but we'll manage. Push those "washers" over the ends of the radiator pins and push through from the front so they're snug. once secure, take your wire cutters and snip the excess off. Reverse the above steps to re-attach the fan shroud. (When I took the metal clip off mine it sprang off into oblivion, I'm not having any issues without it)
8. Now that you have gotten the cooler mounted, Lets disconnect the transmission fluid line from the factory radiator. Get the drip pan and the rag ready to catch any ATF that comes out. There shouldn't be much, we're not draining the pan or anything.
Find this area under the truck:
If you look at the two lines entering the radiator, we will be disconnecting the short one that goes straight into the radiator.
Now you'll see the part I'm holding screwed into the radiator. Pull the plastic cover back and unscrew this. (don't worry about twisting the hose, this fitting spins freely.) Use the rag and drip pan to catch any ATF that comes out. Now we're going to take this fitting off of the transmission line. Get your small flat head or dentists pick. See that clip behind my hand? that is what we need to take out of the part I'm holding. (which should still be attached to the transmission line.)
Once this clip is removed, pull the fitting off the transmission line. (pulls straight out, no threads here) Get the Hayden 397 fitting out. You should have these parts:
The hayden 397 fitting attaches to this part we removed from the radiator. in the above picture the right side of that hayden fitting sits in the fitting from the radiator. replace the clip. screw this back into the radiator, then replace the plastic cover. (Don't overtighten) You should now see this.
Phew... done with step 8.
9. Now we can determine how long to cut the lines we attached to the cooler.
Now grab that convoluted tubing and put it over the hoses, then feed the hoses between the bumper and the radiator. This will add protection to the hoses where they pass the bumper brackets. Use zip ties to secure the ends of the convoluted tubing. If you feel the need, you can also take a grinder to the edges of the brackets here to dull them.
Now line these hoses up with the two disconnected transmission lines. If you mounted the cooler horizontally, ideally, the top hose should go to the transmission line and the bottom one should go to the radiator.
cut these hoses so that you have enough to cover the end coming out of the radiator and about the same length going up the transmission line.
slip the last 2 hose clamps onto these hoses. shove em up and out of the way for now.
To make this next part easier, get some ATF and lube up the tips of the hoses and the fittings. slip these hoses on. Again, twisting back and forth with a set of channel locks helps a lot.
You'll see in the above picture that the hose isn't fully seated on the radiator. make sure this snugs all the way up against the plastic cover. Go ahead and bring the hose clamps down and tighten these like the others, just until the rubber is flush in the grooves. If needed, use zip ties to secure the hoses out of the way.
10. Wipe down the hoses and fittings. check the transmission fluid level. You'll likely need to add some since there is now more area for the fluid to travel through. stick a funnel in the dipstick tube and add some in. Start the truck, put on the emergency brake and run through the gears a few times. Shut it off and check for leaks. Check the fluid level again in case you need to add any.
Go ahead and replace your skid plate and put the grill back on.
Post-Mod Observations:
Before performing this mod, using my scangauge, my engine temps were averaging 190-205 degrees (about halfway on the gauge cluster) and the transmission would be around 200-215 degrees depending on if i'm on the highway or off-roading. (I attribute this to my larger diameter tires) If going over those PA mountain hills, hauling dirt/gravel offroad, or otherwise putting the transmission to some heavy-duty use, I had seen the temp get to almost 230 which is the main reason for doing this mod. After doing this mod, I haven't seen my transmission temps above 175 degrees (it averages 120-155 degrees with normal driving) My engine temp now gets to about 175-180 (little past a quarter on the gauge cluster) degrees once warmed up.
The post mod temps were obtained with the average outside temperature being in the 60-75 degree range. I'll update after the summertime if I notice anything different.
Special Thanks:
Thanks to HawkZ28 for the original how-to and for the mods to unlock the original how-to so I could make the improvements needed.
The community here at 355nation.net urges you to please use caution and seek professional assistance when performing modifications to your vehicle. Before attempting any modification it is advised that you refer to your Colorado or Canyon service manual or contact a certified mechanic as not all GMT355 trucks are the same. The staff and the associated members are in no way responsible for any damages, injuries or other harm inflicted to your vehicle or yourself which may result in attempting these modifications. The posts and content presented on this site reflect in no way the views of 355nation.net or it’s ownership.
A 355nation How To presented by
Metal_EMT (username changed to HK_Titan)
Project Name
How To Install a Transmission Cooler
IMAGE URL Replacement In Progress - Temp Fix
Looks like the forum no longer allows image URLs that are http and require https links. Until I find time to move my images to my https webserver, here's a link to the images for this how-to (copy/paste): dancoco.com/GMT355/howto/transcool_index.html
Project Description
This guide will demonstrate the steps needed to install an auxiliary transmission cooler. This will work in addition to the cooler inside the stock radiator to both improve transmission temperatures, but will also help cool the engine since the radiator is dealing with cooler transmission fluid after passing through the auxiliary cooler.
NOTE: All images can be clicked on to view in a larger size if needed.
Skill Level
Medium
Project Vehicle
Make: Chevrolet
Model: Colorado
Year: 2004
Engine: 3.5L
Transmission: 4L60-E Automatic
Power windows: Yes
Sun Roof: No
Tools Needed
Flathead screwdriver
Wire Cutters
Knife / Scissors / Shears (something to cut the transmission cooler hose)
Small flat-head or dentists pick type tool
Channel locks / Pliers
Rags or Paper Towels
Drip Pan
Funnel
Wrench set would be handy
Sockets (If you have a skid plate to remove)
Dremel or Die Grinder (Optional - See step 9)
Parts Needed
Transmission Cooler Kit
I used a Hayden (Imperial) High Performance Transmission Oil Cooler Part number 240514. Link to Advance Auto Parts.
Extra length of transmission line
Hayden 397 Fitting
You can get this from Summit Racing for about 9 bucks. Direct Link
Convoluted Tubing
Zip Ties
Quart of ATF (can be a partial bottle, just enough to allow you to top it off if needed when done.)
NOTE: In Advanced Auto (and I'm sure elsewhere,) they sell a fitting that claims to be for all GM after 1996 or 1998 or some year around there. This DOES NOT work on our radiators where we plan to attach the hayden 397 fitting above. Don't get this:
Project Time
1-2 hours depending on experience.
Project Cost
$60-80 plus Effort
Steps
1. Let truck sit long enough to allow transmission to cool. Check the transmission cooler kit to see that nothings missing. (should have the cooler, one section of hose, radiator clamps (qty 4), hose clamps (qty 4), and fittings (qty 2) to attach to cooler.
2. Lift front of truck and remove grill. If you're unsure how to remove the grill, click here.
3. If you have a skid plate (stock or otherwise) remove it now. For the superskidz, only the front plate should need to be removed.)
4. The kit should come with a set of fittings like this:
Screw these onto the ends of the cooler. Wrench tight is good enough, don't over tighten.
5. Attach one end of the hose that came in the kit to one fitting. Attach the second hose you bought to the other fitting. Using a little ATF to lubricate the fittings can help here. (twisting back and forth with a set of channel locks / pliers helps also) attach the hose clamps and tighten these until the rubber of the hose becomes flush with the grooves in the clamp. Leave these hoses long for now, We'll cut them in a minute.
6. Now we need to find out where to mount the cooler to the front of the radiator. Some people have made brackets and have found different places to put the cooler. I opted to just strap this cooler directly in front of the factory radiator. There are a few ways to orient this. The most effective cooling area of the radiator with a stock fan is the top half because the bumper isn't blocking airflow and the fan is up higher. If your engine runs hot, it might be better to not block this with the trans cooler. I had no issues with the location I chose.
NOTE: If you have a hard time getting the cooler behind the center support here, removing the stock horn may be needed until it's mounted.
Vertical mounting:
Horizontal Bottom mounting:
Horizontal Top Mounting: (This is what I chose)
Whichever position you choose, make sure there is a clear path for the hoses to go beneath the radiator to connect to the transmission lines. make sure there is room for them to turn without kinking. A few inch bend should work here.
7. Mounting the cooler. Get those radiator clamps out. (long sticks with one flat end.) My kit came with foam blocks. Get these also.
This foam block goes between the cooler and the factory radiator. push the radiator clamp pin through the fins of the cooler, push it through the foam pad and push through the radiator.
There's another "washer" type piece that clips onto these radiator pins from the back side. We need to move the cooling fan to be able to get in here unless you can wiggle your hands in between the fan blades.
There are 3 or 4 places this attaches that we need to free up.
There is a metal hose for the A/C attached to the fan shroud. unclip this for the moment and move it aside. The fan shroud is in two pieces. Top and bottom. there is a metal clip below the A/C line which holds the 2 halves together. take this off. Then you'll see where the upper shroud is attached to the radiator. unclip this.
Now you should be able to pull this back enough to work. It's still a tight fit but we'll manage. Push those "washers" over the ends of the radiator pins and push through from the front so they're snug. once secure, take your wire cutters and snip the excess off. Reverse the above steps to re-attach the fan shroud. (When I took the metal clip off mine it sprang off into oblivion, I'm not having any issues without it)
8. Now that you have gotten the cooler mounted, Lets disconnect the transmission fluid line from the factory radiator. Get the drip pan and the rag ready to catch any ATF that comes out. There shouldn't be much, we're not draining the pan or anything.
Find this area under the truck:
If you look at the two lines entering the radiator, we will be disconnecting the short one that goes straight into the radiator.
Now you'll see the part I'm holding screwed into the radiator. Pull the plastic cover back and unscrew this. (don't worry about twisting the hose, this fitting spins freely.) Use the rag and drip pan to catch any ATF that comes out. Now we're going to take this fitting off of the transmission line. Get your small flat head or dentists pick. See that clip behind my hand? that is what we need to take out of the part I'm holding. (which should still be attached to the transmission line.)
Once this clip is removed, pull the fitting off the transmission line. (pulls straight out, no threads here) Get the Hayden 397 fitting out. You should have these parts:
The hayden 397 fitting attaches to this part we removed from the radiator. in the above picture the right side of that hayden fitting sits in the fitting from the radiator. replace the clip. screw this back into the radiator, then replace the plastic cover. (Don't overtighten) You should now see this.
Phew... done with step 8.
9. Now we can determine how long to cut the lines we attached to the cooler.
Now grab that convoluted tubing and put it over the hoses, then feed the hoses between the bumper and the radiator. This will add protection to the hoses where they pass the bumper brackets. Use zip ties to secure the ends of the convoluted tubing. If you feel the need, you can also take a grinder to the edges of the brackets here to dull them.
Now line these hoses up with the two disconnected transmission lines. If you mounted the cooler horizontally, ideally, the top hose should go to the transmission line and the bottom one should go to the radiator.
cut these hoses so that you have enough to cover the end coming out of the radiator and about the same length going up the transmission line.
slip the last 2 hose clamps onto these hoses. shove em up and out of the way for now.
To make this next part easier, get some ATF and lube up the tips of the hoses and the fittings. slip these hoses on. Again, twisting back and forth with a set of channel locks helps a lot.
You'll see in the above picture that the hose isn't fully seated on the radiator. make sure this snugs all the way up against the plastic cover. Go ahead and bring the hose clamps down and tighten these like the others, just until the rubber is flush in the grooves. If needed, use zip ties to secure the hoses out of the way.
10. Wipe down the hoses and fittings. check the transmission fluid level. You'll likely need to add some since there is now more area for the fluid to travel through. stick a funnel in the dipstick tube and add some in. Start the truck, put on the emergency brake and run through the gears a few times. Shut it off and check for leaks. Check the fluid level again in case you need to add any.
Go ahead and replace your skid plate and put the grill back on.
Post-Mod Observations:
Before performing this mod, using my scangauge, my engine temps were averaging 190-205 degrees (about halfway on the gauge cluster) and the transmission would be around 200-215 degrees depending on if i'm on the highway or off-roading. (I attribute this to my larger diameter tires) If going over those PA mountain hills, hauling dirt/gravel offroad, or otherwise putting the transmission to some heavy-duty use, I had seen the temp get to almost 230 which is the main reason for doing this mod. After doing this mod, I haven't seen my transmission temps above 175 degrees (it averages 120-155 degrees with normal driving) My engine temp now gets to about 175-180 (little past a quarter on the gauge cluster) degrees once warmed up.
The post mod temps were obtained with the average outside temperature being in the 60-75 degree range. I'll update after the summertime if I notice anything different.
Special Thanks:
Thanks to HawkZ28 for the original how-to and for the mods to unlock the original how-to so I could make the improvements needed.
