Although I've been towing for years, and have always had a transmission cooler on every tow vehicle, I can't remember whether the fluid should come from the trans thru the cooler then to the rad and out back to the transmission, or vice versa? In other words before or after the the radiator?Recently purchased an '04 GMC Envoy to tow our 4000 TT, has tow package but no cooler. On the Trailvoy forum, there didn't seem to be a consensus as to which way it should be. Since those using their Trailblazer or Envoy for towing was a minority, I'm thinking us RV'ers here would know best.The climate you live in seems to be the reason for placing the cooler before or after?

I tow with an Envoy and installed a cooler AFTER the radiator.

Trailvoy is not really used anymore. The active members with all the knowledge have moved to http://gmtnation.com/forums/. When I had my Trailblazer I routed the aux cooler after the radiator. Not sure I really needed it though, and I didn't put it in until I bought the current trailer (Jayco X20E). That said keep up with your trans fluid changes, every 50k miles. And use Dex VI, not Dex III. Dex VI is a blended synthetic that replaces Dex III. It runs cooler and is backward compatible in all Dex III applications. I had 160k on my '03 Trailblazer when I sold it and the trans was a strong as ever. Is yours the I6 or 5.3 V8? If you have the I6 and don't at least have 3:73 gears I can tell you from experience that you are not going to like towing that trailer with it. It's OK on flat terrain for short runs, but in the end we got the PU with a 5.3. The high mileage on the TB made the decision easier. My Sierra tows our trailer like a dream.

Another of these never ending opinions...Most all OEM setups has the ATF go to the external aux first, then into the main radiatorATF needs to operate in a range...can be too cold or too hot (worse)It needs to see a min temp in order to get rid of over nite condensation, so has to see the main radiator coolant temps before going back to the trannyAlmost semantics...whether before or after the main radiator in 'most' conditions...but in very cold areas...it may not get warm enough to vaporized that moisture....which has a high change of turning acidic to rot out parts of the tranny innardsI always plumb my external ATF aux radiators BEFORE the main radiator

BenK wrote:Another of these never ending opinions...Most all OEM setups has the ATF go to the external aux first, then into the main radiatorATF needs to operate in a range...can be too cold or too hot (worse)It needs to see a min temp in order to get rid of over nite condensation, so has to see the main radiator coolant temps before going back to the trannyAlmost semantics...whether before or after the main radiator in 'most' conditions...but in very cold areas...it may not get warm enough to vaporized that moisture....which has a high change of turning acidic to rot out parts of the tranny innardsI always plumb my external ATF aux radiators BEFORE the main radiatorX2Aux needs to be before the unit in the radiator for the reason above.

Trans cooler fliud route | Good Sam Community

Campfire_Time
Explorer
Dec 29, 2015
Trailvoy is not really used anymore. The active members with all the knowledge have moved to http://gmtnation.com/forums/.

When I had my Trailblazer I routed the aux cooler after the radiator. Not sure I really needed it though, and I didn't put it in until I bought the current trailer (Jayco X20E). That said keep up with your trans fluid changes, every 50k miles. And use Dex VI, not Dex III. Dex VI is a blended synthetic that replaces Dex III. It runs cooler and is backward compatible in all Dex III applications. I had 160k on my '03 Trailblazer when I sold it and the trans was a strong as ever.

Is yours the I6 or 5.3 V8? If you have the I6 and don't at least have 3:73 gears I can tell you from experience that you are not going to like towing that trailer with it. It's OK on flat terrain for short runs, but in the end we got the PU with a 5.3. The high mileage on the TB made the decision easier. My Sierra tows our trailer like a dream.
jfkmk
Explorer
Dec 29, 2015
I tow with an Envoy and installed a cooler AFTER the radiator.

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