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2006 chevy trailblazer_lt
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Discussion Starter · #1 ·
Hi,

I recently bought a small travel trailer (about 3,000 lbs) to tow with my '06 Trailblazer and have found this forum to be a great resource. Thanks for all the great info!

I've read a lot of threads about transmission coolers and it looks like the key to keeping the transmission cool to begin with is to keep the torque converter locked.

I have been observing how the transmission shifts and it seems to me that the torque converter will only lock in the highest selected gear. For example, if I select third gear, the converter will lock in third gear. If I select overdrive, the converter will not lock until the transmission shifts to overdrive.

So if I select overdrive, but the transmission spends most of the time in third gear, then torque converter spends most of the time unlocked.

I usually select third gear when towing. But if I encounter some flat terrain, I can shift to overdrive and easily hold overdrive most of the time. But if the transmission downshifts to third, I need to select third in order to get the converter to lock in third.

Are these observations correct? All I'm really doing here is watching the tach, counting the shift points and assuming the last "shift point" is the converter locking up. Perhaps it's obvious by now that I don't know much about transmissions. :p

Basically, I want to select whatever gear will allow the converter to remain locked most of the time. True?

Thanks any replies.
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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3,700 Posts
I know the 4T60 and the 4L60 (700R4) would go into lockup in third, then shift to OD while locked up... I'm pretty sure that if I hang out around 40mph, and it doesn't shift into OD (even though OD is selected), it'll be locked up in third.

Mike
 

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2006 chevy trailblazer_lt
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Discussion Starter · #3 ·
When I accelerate from a dead stop, I sense the following pattern...

Shift to 2nd - Shift to 3rd - Shift to OD - Lock

You're saying that the pattern may alternately be...

Shift to 2nd - Shift to 3rd - Lock - Shift to OD

Okay, I can see that. So how does one tell if the torque converter is locked or not? Or am I just over analyzing this?


Well maybe I should just explain how I tow when in overdrive. I prevent the transmission from hunting by modulating my speed. Let's say I'm cursing along at 60mph in OD. When I come to a hill, I'll keep gentle pressure on the gas and let myself drop to around 55mph. Then I'll apply more pressure, which causes the transmission to downshift to 3rd. I maintain an increased pressure on the gas to keep the transmission in 3rd and accelerate at least back up to 60mph before I let up and allow the transmission to shift back to OD. I try to decelerate and accelerate between 55 and 60 as slowly and as smoothly as possible so as to minimize the number of times the transmission is shifting.

I guess I'm not sure if the converter unlocks when downshifting from OD to 3rd. But I am pretty sure it does unlock when up shifting from 3rd to OD.

Wait, let's make this simple. My bottom line question is this...

If I select OD, should I try to minimize the amount of time the transmission is in third?
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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When I accelerate from a dead stop, I sense the following pattern...

Shift to 2nd - Shift to 3rd - Shift to OD - Lock

You're saying that the pattern may alternately be...

Shift to 2nd - Shift to 3rd - Lock - Shift to OD

Okay, I can see that. So how does one tell if the torque converter is locked or not? Or am I just over analyzing this?


Well maybe I should just explain how I tow when in overdrive. I prevent the transmission from hunting by modulating my speed. Let's say I'm cursing along at 60mph in OD. When I come to a hill, I'll keep gentle pressure on the gas and let myself drop to around 55mph. Then I'll apply more pressure, which causes the transmission to downshift to 3rd. I maintain an increased pressure on the gas to keep the transmission in 3rd and accelerate at least back up to 60mph before I let up and allow the transmission to shift back to OD. I try to decelerate and accelerate between 55 and 60 as slowly and as smoothly as possible so as to minimize the number of times the transmission is shifting.

I guess I'm not sure if the converter unlocks when downshifting from OD to 3rd. But I am pretty sure it does unlock when up shifting from 3rd to OD.

Wait, let's make this simple. My bottom line question is this...

If I select OD, should I try to minimize the amount of time the transmission is in third?

Yes, you should...

At least on the hydraulic controls, it always used to be

Shift to 2nd - Shift to 3rd - Lock - Shift to OD

When you are in OD, and it needs more power, it drops lockup first, THEN drops to third...

Though... when I was towing a moderately heavy trailer (4500#) on the interstate, I would let it downshift by itself, as I went up hills, then while it was downshifted, I'd put the selector in third so it didn't shift cycle going up hills... I guess when I was doing that, I didn't notice it locking back up once I pulled the selector down, so it may depend on load, and how long you've been in OD... By that I mean: when you just get up to speed, and move the throttle around, you can see it lock and unlock the TC clutch, but after you've been at speed for a mile or so, you can get much farther into the gas without it dropping lockup...

The way to tell is have a scangauge hooked up to it while you're driving..

Mike
 

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2005 chevy trailblazer_ls
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Thanks for the replies.

I've read alot about the scangauge. I'll probably get one to play with.
I used my Scan Gauge to keep track of the trans temp on this last trip down to Southern Utah. Going over some pretty tall passes I downshifted to third because the trans would eventually do it anyway and I saw trans temps get up to around 220. After the second tall hill, I got a P0741 code and the TC wouldn't lock up, even in 4th gear. I used the Scan Gauge to clear the code and it was fine after that. A long stretch of level ground let the temp come back down to around 200.

After seeing the temps get that high and how the transmission operated, I decided I need to get a cooler. I am even researching upgrading the TC to a more heavy duty unit. Keep the same stall, just a stronger TC. I can't afford to burn up a transmission, especially out in the middle of the Utah desert. :worried:
 

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Basic Vendor- Skid Plates
2007 chevy trailblazer_ls
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3,700 Posts
220, while not ideal, is something you'd see in traffic in a lot of vehicles (probably even the TV), and especially while towing... You want to keep Dex 3 below 260F, and Dex 6 below 300F, to stay away from oxidation... Those temps are sump temps, though... The real temperature you're worried about is the output of the TC, which is more dependant on load than cooling capacity... you want to keep the output of the TC below 300F for Dex 3... I don't know what it is for Dex 6...

Mike
 

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2005 chevy trailblazer_ls
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220, while not ideal, is something you'd see in traffic in a lot of vehicles (probably even the TV), and especially while towing... You want to keep Dex 3 below 260F, and Dex 6 below 300F, to stay away from oxidation... Those temps are sump temps, though... The real temperature you're worried about is the output of the TC, which is more dependant on load than cooling capacity... you want to keep the output of the TC below 300F for Dex 3... I don't know what it is for Dex 6...

Mike
I don't really do a lot of driving around town, just to work, then the gym, then home. No more than 15-20 minutes at a time, so my trans temp doesn't even get close to the ~190 in the radiator. Its the long trips that makes the temp climb, and there are a lot of hills so it seems like there isn't time for the fluid to cool back down before the next one.
 
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