Coast Resorts Open Roads Forum: Towing: automatic transmission shifting procedure
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Timeking

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Posted: 04/16/14 04:12pm Link  |  Quote  |  Print  |  Notify Moderator

Just checking my methodology to see if "all" agrees. I am towing a 5000 lb TT with a 2006 Chevy Trailblazer with the 5.3 vortex v8. I tow in 3rd. When going up a steep hill, I wait until the computer decides to go into 2nd, then pull the lever down to 2nd so it stays there until I get to the top of the hill. Max speed in 2nd is 45 mph, ~3500 rpm. On a few occasions (like getting up onto Skyline Drive from Orange VA) I've had to go all the way down into 1st and crawl to the top, keeping an eye on tranny temps as I go. So what is the best way to go into 1st? I've got a Scanguage and can see the engine "load", and it worries me when the engine is pumping out 95% of available power, and then downshifting to first with that much torque might break something. I've had both my motor mounts replaced, though I don't know whether the guy that owned this vehicle previously did that, or whether it was me. I've towed this trailer some 40,000 miles. I am thinking that I should let the vehicle slow down to 30 mph, and then jam it into 1st, rather than let the computer make that decision. The risk is that if I let the rpm's get too low in first, then I could slowly slow down to a stop ... and that would suck, not only for me, but for everyone that is behind me!!! Do I trust the computer, or trust me brain?

I am heading to the Arches/Glen Canyon NP area next week.....

bguy

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Posted: 04/16/14 04:24pm Link  |  Quote  |  Print  |  Notify Moderator

I often wonder what happens to the rear axle as tranny slams into lower gears.


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K Charles

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Posted: 04/16/14 05:38pm Link  |  Quote  |  Print  |  Notify Moderator

I shifted when I had a standard but with an automatic I let it do it's thing. The computer knows what is going to happen and removes some torque from the transmission when it shifts.





Teamfour

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Posted: 04/16/14 05:47pm Link  |  Quote  |  Print  |  Notify Moderator

The way to fix that is to get a tow vehicle suitable for your towing needs. If the tranny is having to shift to first to get up a hill you are severely under powered.


Lee and Anne


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Halmfamily

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Posted: 04/16/14 06:13pm Link  |  Quote  |  Print  |  Notify Moderator

I just let our automatics do there job. If you have tow/haul mode make sure its on and let the truck do what its programmed to do. I pulled a 7000 lb load with my 07 Sierra 1500, with the 5.3 up hills and never had to manually shift the tranny.


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Lynnmor

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Posted: 04/16/14 06:37pm Link  |  Quote  |  Print  |  Notify Moderator

I would only put in a lower gear if the transmission is "hunting," that is changing back and forth often.





Trackrig

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Posted: 04/16/14 07:09pm Link  |  Quote  |  Print  |  Notify Moderator

Except for using the "tow/haul mode" if your transmission has one, the rest is "automatic". As Teamfour said, if you have to use first gear, then something is wrong.

Bill


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carringb

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Posted: 04/16/14 09:25pm Link  |  Quote  |  Print  |  Notify Moderator

Regardless of how you shift, your transmission will not lockup the torque converter in 1st and 2nd, which means you are on borrowed time with your transmission.

Not to mention your concern about coming to a stop if you get slowed down too much... That wounds like an impending road hazard! What if there's traffic (or deer) on one of those 1st gear climbs?


Bryan

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kaydeejay

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Posted: 04/16/14 09:32pm Link  |  Quote  |  Print  |  Notify Moderator

That 5.3 V8 is found in Suburbans, Tahoes etc and a LOT of pickup trucks. Most are heavier vehicles than the T'Blazer.
IMHO if your Trailblazer is struggling with a 5000# trailer, you have a much bigger problem that transmission shifting.
You need to get it checked out.
I pulled a 3500#+ boat with a Trailblazer with the 4.2L L6 (which has NO torque at the bottom end!) and while it wouldn't win any races, I had no problems approaching what you are describing.


Keith J.
Sold the fiver and looking for a DP, but not in any hurry right now.

PUCampin

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Posted: 04/16/14 09:49pm Link  |  Quote  |  Print  |  Notify Moderator

OP states "max speed in 2nd is 45mph at 3500rpm"

This is part of the problem. Modern small block Overhead Cam V8s DO NOT have a ton of low end torque, most torque and power is made by spinning the motor. This is the opposite of the way V8s used to be and is disconcerting for those who have driven older big block V8s. Peak torque for the 5.3 is around 4400rpm, and peak power around 5300! At 3500 rpm you are falling off the torque curve and only making around 220hp.

DO NOT be afraid to put your foot in it and let the engine run, it may sound to you like it is going to blow up, it is not, and will happily spin at peak torque for a long time.

If the transmission is waiting too long to down shift into second, like 2500rpm, then it is letting you fall too far off the power curve and shifting down won't get you far enough back on it. I saw this all the time with my dad's old chevy truck Pre-empt it. When the tach hits 3200rpm manually drop it into second so you will hit second around 4200rpm and stay in the power band.

And no, something is not necessarily wrong if you have to use 1st. Depending on the grade, elevation, and axle ratio you could have to use it. 1st on the 4sp is not particularly low at 3.01, and you could likely have 3.39 axle gears. If you are talking 6% at 200ft, then yea, you should not need 1st, but somewhat mountainous driving with some elevation, yea. Going from Palmdale CA to table mountain was over a 5000ft elevation gain, even with my 6spd, I definitely had some 1st gear work, but it let the engine stay at 4000-4500rpm at peak torque.

* This post was edited 04/16/14 10:10pm by PUCampin *


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