towing with auto transmission question

[Texasdeere]

I have a stock 24V, 2500 truck with an automatic

transmission, and 3. 54 gearing. I sometimes tow a trailer weighing around 5500lbs. When towing this load I turn the overdrive off when traveling around 60-65mph or less. When I get on the open highway (flat, no hills) I turn the overdrive back on ( around 65mph ) and cruise around 70mph (1900rpms). My question is, If I use overdrive for highway cruising, with no aggressive throttle application. Will I be hurting the transmission?

 

[Bob Wagner]

Eric,

you will have no problem towing on the flat in overdrive if you keep the RPM's above 1700.



When towing lower than 1600 lock out OD



Do you have temp gauge for the transmission?

 

[Dieselnerd]

Bob is correct. As long as you can maintain your speed it's OK to tow in OD. The reason for keeping the RPM's up over 1700, or some even say 1800-2000 is the strong pulses from the diesel engine are hard on the internals when it's all locked up. At higher engine speeds the pulses even out and are not as likely to hurt anything.

 

[SlyBones]

Dont you also get higher line pressures, with the higher RPM's? I was thinking -- too much load, too much power/torque, at low line pressures ( IE too low RPM ), and you will trash the TC lockup clutch.



There are a lot of variables when you decicde you need to push your foot into it that will device if your transmission likes you or not.



So the easy way is to follow Bob's advice. Dont have the TC locked up with less than 1700 RPM. And when its not locked up and you are applying load, make sure to monitor that transmission temp gauge. Dont let the temps get too high.



Then you will be fine.

 

[Dieselnerd]

SlyBones, your pressure may or may not be higher at higher speeds. It's controlled by a lever on the left side of your transmission. That lever moves a valve in the valve body when you step on the accelerator so if you give it more throttle you get more pressure. If you are going downhill with less throttle, even if you are going faster, you will have less pressure. That's the reason people put on devices like a BD Pressure Loc with an auto and an exhaust brake. That way you don't have low pressure that will let the TC clutch slip when using the brake. When you add a modified valve body it still has higher and lower pressure depending on throttle position but the starting pressure is higher and the final pressure at WOT is higher than stock.

 

[icarus33]

Here are the transmission pressures I measured last week. Running stock 245's and a 3. 54 rear. Valve body has the older Transgo TFOD mod's. Truck was unloaded.



2000 rpm with O/D on @ 71mph = 100 psig

2000 rpm with O/D off @ ?????? = 90 psig



Strange that for the same engine rpm's that there was such a pressure difference.



ic

 

[SlyBones]

icarus33,



Read Dieselnerds response. Even though you were running the same RPM you probably had different throttle positions in the two cases. and therefore two different pressure readings.

 

[Gunny]

About three months ago, the wife and I where on Vacation in S. D. going from the upper north western part of the state down to Rapid City. The truck was a new to us (less than 6 weeks) 12 valve 1998 3500 auto transmission 4X4. We had trouble on this trip from the very begining. Broken axle in Winnemuca NV. And to damn many things to mention. When we finally arrived in Rapid City, and it took us all day to go 240 miles. Bad head wind truck never got over 50 miles per hour. We where going up a pretty steep grade about 8% at less than 25 mph and the trottle was all in, transmission was in 2nd gear. transmission exploded, case split, valve body and many other sordid parts where every where. Ended up buying a new 2001 3500 4X4, HO 6 speed. It is a great pulling truck. But the dealer we traded with in Rapid City, called and told me that the auto transmission in my old truck got waaay too hot, and this caused the falure. My advice NO O. D. when towing below 50 and a good transmission temp guage. Gunny

 

[PZicha]

MY TRUCK HAS 4. 11 GEARS ,NO PROBLEMS TO DATE TOWING IN O. D AT ANY SPEED ,GEARS SOME TIMES MAKE A HUGE DIFFERANCE

 

[OILBRNR]

Bad luck for Gunny, that is abnormal, but Bob is right for a stock rig. I've kinda taken the rule of below 2k and a grade I was out of OD in advance of the grade if possible. Now that I have DTT's upgrades I'm not so concerned.



As I've read in the past, and it could be wrong, but the stock line pressures below 2k are not all that good, given the torque potential output of our Cummins.

 

[Dieselnerd]

OILBRNR, you are also correct. The more "Bombed" you are, the LESS throttle it takes to maintain speed. The way the transmission works is Less throttle = Lower pressure. Another reason you should have a modified valve body with power upgrades for towing. They have higher pressures over the whole range. Some are set up to ramp up the pressure very quickly at the beginning of your throttle movement and others are more linear and just start higher and end higher. Depends on your needs and how firm you want shifts. Good transmission rebuilders/upgraders will match the VB for your specific use.

 

[RedRamer]

Deiselnerd:

Good reading stuff.

Would just a DTT valve body help a lot for a stock transmission? Keep presure up and all.

I find myself at 7500 rpm most times up here and still a bit over speed limit of 90kph on our 2 lanes.

Seems my TC is working fine, but you make sence about low presure in OD.

thanks

 

[Dieselnerd]

Goober, I think Bill K. would be the most qualified to answer your question about the DTT VB. For a generic answer, just a modified VB alone with a stock transmission will help to keep things running better. Most provide better lube circuits and higher pressures to keep your TC clutch from slipping. You can slip the TC clutch with stock power if you are towing a fairly heavy load. If you put in a modified VB now and plan more power later, get a VB that is designed for the power upgrades and what you will be towing or hauling later. That way you can always upgrade the transmission internals and get a better TC without also having to change the VB again.