GMC Truck Forum banner

1 - 10 of 10 Posts

·
Registered
Joined
·
156 Posts
Discussion Starter · #1 ·
I have a 1995 Suburban that the manaul recommends towing in third gear. What is the rationale behind this?

I am in the process of modifying my shift points after a gear/tire change and want to get everything as correct as possible.

Is the lock-up for the torque converter supposed to come on and stay engaged during third gear while towing to decrease the possible constant shifts from OD to OD/lock-up and back, if you attempted to tow in OD? (ie. fried transmission!)

Any opinions appreciated.

Thanks, Glenn
 

·
Registered
Joined
·
47 Posts
That was 12 years ago and I have trouble remembering yesterday, but I'll give it a shot.

GM did not recommend towing in OD because it put too much strain on the transmission. The OD system was not designed to handle the extra strain. I also remember discussion about the trans slipping in and out of OD when towing, which caused extra wear plus aggravation on the part of the driver.
 

·
Im walking on sunshine!
Joined
·
5,651 Posts
That was 12 years ago and I have trouble remembering yesterday, but I'll give it a shot.

GM did not recommend towing in OD because it put too much strain on the transmission. The OD system was not designed to handle the extra strain. I also remember discussion about the trans slipping in and out of OD when towing, which caused extra wear plus aggravation on the part of the driver.
:read: Only tow in drive Do not tow in overdrive.
 

·
Registered
Joined
·
343 Posts
They say Tow in drive for the reasons tstr mentioned. You can get away with od on a flat surface no hills if your truck isnt shifting in and out of od mabye, depending on the weight you are towing. If your truck is constantly shifting in and outta od its gonna build up alot of heat. Heat and transmissions aren't the best of friends so you get it hot enough, enough times and your tranny is toast.
 

·
Registered
Joined
·
343 Posts
Yeah especially with your setup, towing something of decent size in OD would probably be hell on your tranny.
:word: yeah, 3.73's and 33's isnt exactly a fantastic towing combo. It will work though, I towed some decently heavy stuff with my 04 Z71 with 5.3 and 3.73's. (5-6K) My 06 tows like a champ though :bowtie:
 

·
HAPPY NEW YEAR
Joined
·
3,599 Posts
i have the same year truck and I tow 6+ yards of mulch on a 16ft. trailer with 35's and 4.56's...I ALWAYS put it in 3rd from 0-45, after im cruisin good on flat surface ill bump it into OD.
 

·
New Member
Joined
·
4,725 Posts
I tow in 3rd essentially to minimize heat which is the big killer on automatics from the shifting in and out while in OD.

I also run a temp gauge just in case.
 

·
HOT AIR BALLOON OWNER
Joined
·
14,932 Posts
Trans is in a mechanical DISadvantage in overdrive. Where the lower gears have a mechanical advantage, and 3rd (in the four speed units) is direct 1:1.

Keeps shifting in and out down. Heat build up down. Etc.
 

·
Registered
Joined
·
156 Posts
Discussion Starter · #9 ·
Trans is in a mechanical DISadvantage in overdrive. Where the lower gears have a mechanical advantage, and 3rd (in the four speed units) is direct 1:1.

Keeps shifting in and out down. Heat build up down. Etc.
I don't think it would be shifting in and out of OD (ie. from 3rd to OD and back), but it would be engaging and dis-engaging the lock-up in OD.

My question would be that if I was able to maintain lock-up in OD, would that be any more harmful than driving locked-up in third gear?

You are correct: third gear is 1:1 ratio and OD is .70:1 ratio. Lock-up is available in both third and OD.

I was considering having the lock-up engage at 68 mph and at less than 35% TPS in OD and lock-up to occur at 54 mph and less than 35% TPS in third gear.

Any Thoughts?
 

·
Registered
Joined
·
156 Posts
Discussion Starter · #10 ·
I don't see the issue. How fast do you need to be going while pulling?
The issue as per my original posting is some opinions on how to set up my shift points and lock-up to allow for towing. If the TC is to be locked for towing and I should only be towing in third gear, then I will need the lock-up to come on in third gear at some speed after the initial acceleration has taken place allowing for the torque multiplication the TC and shifts give. I presently do not have a TC lock-up occurring in third gear. I would also want the TC in lock-up at highway cruising speeds in OD. Should I attempt to allow for the possibility of light duty towing (ie. utility trailer, snowmobiles, etc) in OD at highway cruising speeds if the truck can maintain a TC lock engagement because of higher rpm/speed? I don't know. Or should I just plan on the third gear/TC lock for any towing scenario?

I thought that some owners of trucks with the "tow haul mode" option and an understanding of the mechanism behind it may have chimed in to give me some ideas from the newer transmission operation.

Maybe I will attempt to take a tow-haul equipped truck for a drive and see what it seems to do to the shifts.
 
1 - 10 of 10 Posts
Top