Towing - When to shift?

[TurboDiverArt]

Hi Guys,

I'll be towing for the first time with my 99 Suburban K2500 (3.73:1 gears) from New Jersey to Maryland. I'm only towing about 5500 lbs so it's not as much as others have towed. I have gauges so I'll make sure I let off when the EGT's start to approach 900 (post turbo).

It should be all highway flat towing. I plan to tow in OD on the highway at about 65 MPH. When should you down shift into manual drive (D)? I assume slower than 55 is a no brainer. Or should I just play it by ear when the transmission starts to hunt between D and OD?

Also, from a transmission temp standpoint. With the converter unlocked, is it better to be in a lower gear (higher RPM's)? My question is, if the trans is running hot at say 40-45, is it better to downshift into second or keep it in drive? I would think that the higher RPM's would allow better fluid coupling so the converter would be tighter but I just don't know if the higher RPM's would create more heat.

I read the posted articles in the towing section from February but it didn't answer these questions.

Thanks!
Art.

[CareyWeber]

Originally posted by TurboDiverArt:
Hi Guys,

I'll be towing for the first time with my 99 Suburban K2500 (3.73:1 gears) from New Jersey to Maryland. I'm only towing about 5500 lbs so it's not as much as others have towed. I have gauges so I'll make sure I let off when the EGT's start to approach 900 (post turbo).

It should be all highway flat towing. I plan to tow in OD on the highway at about 65 MPH. When should you down shift into manual drive (D)? I assume slower than 55 is a no brainer. Or should I just play it by ear when the transmission starts to hunt between D and OD?

Also, from a transmission temp standpoint. With the converter unlocked, is it better to be in a lower gear (higher RPM's)? My question is, if the trans is running hot at say 40-45, is it better to downshift into second or keep it in drive? I would think that the higher RPM's would allow better fluid coupling so the converter would be tighter but I just don't know if the higher RPM's would create more heat.

I read the posted articles in the towing section from February but it didn't answer these questions.

Thanks!
Art. Art,

The only times I ever shift out of OD while towing is if the trany starts to hunt and back and forth between gears (Almost never happens), and going down hill.

The one other time is maybe on a big mountain pass if I want it to stay in a lower gear.

I really find that the 4L80E does a great job while towing I have +179,000 miles on mine (nothing but filter & fluid changes and cleaning the speed sensor).

Carey

Carey

[Perry]

Art,

Based on my recent experience towing my 6000# enclosed car hauler, you should be fine leaving it in OD for rolling hills/flat lands. As long as the OD was locked, which is was all the time on the flats and small hills, my tranny temps in the pan stayed around 160-180 and hit 195 in a section of California where the ambient temp shot up to 100 deg. I was crusing mostly around 65-70mph. My post turbo pyro stayed between 500-700.

Going up the mountain passes was a completely different experience. For cooler running trans, I discovered it was better to leave in 3, and be agressive with the throttle maintaining at least 50mph up the steep grades. My trans temp stayed below 240, but the pyro shot up to 950 at the tops of the hills.

Initially on the way to California, I tried to take it easy, going slower up the mountain passes staying behind the semis at 35mph, and manually shifting it from OD to 3 to 2 as necessary, and my trans temp kept shooting past 270, but my pryo never went above 850.

So based on the terrain you'll be travelling, you should be fine leaving it OD.

[tanker]

Art, I go along with Carey. I don't downshift unless it starts to hunt back and forth. You should be fine down I-95, not much for hills.
Use common sense, and all should be fine. Keep your fluids changed often. Oil is cheap insurance. I just hit 147,000 miles on my transmission without a problem. Change my fluid each year, and have been using Amsoil synthetic the past two years. Keep it cool. ;)

[TurboDiverArt]

Thanks guys!

I'll update ya when I get back to let you know how I made out. Since it's been getting warmer and the A/C is almost always on I now hear my fan kicking on and off and see the temp gauge actually move. Never gets to 210 on the gauge but gets close on hills but always quickly comes down. I've been thinking of replacing the fan clutch, might actually have a reason now... smile.gif

Thanks again for all the quick responses!

Art.

[moose1]

I was always under the impression that when towing itwas nessesary to take it out of OD. I tow a 24.5 foot 5th and almost never put it in OD. I had an experience with a Yukon towing a medium size boat when the tranny overheated boiled out the breather tube onto the muffler and the oil caught fire almost causing a vehicle fire. The tranny was in OD.

Any comments? Thanks

95 ext cab 6.5 TD 411

[CareyWeber]

Originally posted by moose1:
I was always under the impression that when towing itwas nessesary to take it out of OD. I tow a 24.5 foot 5th and almost never put it in OD. I had an experience with a Yukon towing a medium size boat when the tranny overheated boiled out the breather tube onto the muffler and the oil caught fire almost causing a vehicle fire. The tranny was in OD.

Any comments? Thanks

95 ext cab 6.5 TD 411 If you have a 4L60E aka t-700 GM says no towing in OD.

All the 6.5TD truck IIRC have the 4L80E (a T-400 with OD basically). GM says you can tow in OD with the 4L80E.

Carey