Drive vs Over Drive

joshua32064 · 10-16-2003, 05:39 AM

When pulling your trailer, do you use drive or over drive?

Tikihut · 10-16-2003, 05:58 AM

I pull in 3rd on my trail blazer that is what the manual says

markjoandall · 10-16-2003, 07:03 AM

I think a lot depends on the rear drive ratio you have. Before I changed the rear end our our "Burb to a 4:10 from a 3:73, I had to tow in 3rd. Now with the new drive ratio, we pull in drive no problem. I stongly suggest a 4:10 rear drive ratio.

Silvertwinkie · 10-16-2003, 07:41 AM

Right now I have 2.93s in the back and will be going to 3.73s shortly.

Here is the reason for not using OD in a non tow/haul equipped vehicle:

Heat

When you use OD (on an non equipped tow/haul vehicle), it puts the trans into "fourth" gear. When that happens, the RPM are decreased. As a result, the torque converter in some cases will not be locked. An unlocked torque converter will generate 2x the heat that a locked torque converter will generate.

Depending on your vehicle the options are increase from 2.93 to 3.73 or 4.10 and/or finding some 3rd party that can re-program your powertrain control module to lock the torque converter at a given speed (say 51mph) rather than an RPM.

Also the tow/haul limits the downshifts whereas simply re-programming your PCM or getting different gears won't help. If using overdrive and the vehicle downshifts, that too adds heat and heat as we all know is not a tranmissions best friend.

Eric

85MH325 · 10-16-2003, 08:07 AM

I have always been of the "never overdrive when towing" school; however, checking the owner's manual for our Excursion, it says that the only time you have to take it out of overdrive when towing is when it begins to 'hunt' or shifts excessively. So, the question is: Is the tranny tough enough to take it, or is Ford looking to sell new trannys a few years out...

Roger

Silvertwinkie · 10-16-2003, 08:34 AM

I think that as long as the torque converter is locked and it's not gear hunting, life in overdrive is fine. I say this as I have already placed almost 1000 miles of towing in OD. I have a trans temp gauge and it only once got above 175 degress when I towed up a long steep hill.

I also have had my computer re-programmed to lock the torque converter at 51mph rather than the between 1600-1800rpm the factory set it at. My car when in OD would be between 1600-1800 going 55-60. With no way to really tell if it was locked or not (because it locks based on torque output not an actual RPM in that range), it was easier to just tell the computer to lock it at 51.

The only thing I need to look out for is that since I don't have the tow/haul feautre, is if it starts to hunt gears.

Of course, if it's a Ford, all bets are off anyway by default.

Eric

overlander64 · 10-16-2003, 08:35 AM

Greetings joshua32064!

I have had two overdrive equipped tow vehicles. With one it was never tow in overdrive, but with the current one 90% of my towing has been in overdrive. My first guide was to check the owner's manual and then checked with my dealer's Service Manger. Both said "No" with the '95 K1500 Z71 Chevrolet club cab pickup (it was loaded within 500 pounds of its GCVWR with my Overlander in tow). Both said a qualified "Yes" with my K2500 Sburban (it is at least 4,000 pounds below its GCVWR with the Overlander) provided that the transmission is shifted into Direct Drive if it begins to hunt. (The Suburban has the 7400 VORTEC with 4.10 differential gearing.)

I have been towing with the Suburban since April of 1998 and now have more than 117,000 miles on the truck (over 50,000 towing), and have had nothing more than general maintenance issues with anything in the drivetrain other than the electronic controls for the transfer case. The transmission is serviced every 30,000 miles by my GM dealer and has experienced absolutely no problems with overheated fluid or other issues - - it does have the factory issue heavy duty transmission fluid cooler with supplemental electric cooling fan.

Kevin

RoadKingMoe · 10-16-2003, 09:24 AM

The OD gears on the Ford 4R100 and 5R110 are rated for full input torque level (engine torque times converter torque multiplication (1.86:1)) and it is safe to tow in OD with them. I'm sure the Allison is the same.

On the Fords, and I believe the Allisons also, the shift points are determined by mph and throttle position. The mph comes from the output shaft of the transmission, and is adjusted for axle ratio and tire diameter in the ABS computer.

The tow/haul mode on the 5R110 raises the upshift point mph for any given throttle position, so it hangs onto lower gears until a higher mph before upshift. This helps prevent hunting. It also does not disengage the coast clutch when you lift the throttle in Drive and OD, so you have some engine braking... what little a diesel does provide.

I've also heard, but don't know for sure, that the tranny downshifts to Drive during deceleration in OD, and that you can force a downshift to Drive by holding down the Tow/Haul button rather than just punching it to engage Tow/Haul.

On my pre-tow/haul 4R100, I don't wait until it begins hunting to punch OD off. At that point, it's downshifting under load, and the torque converter takes longer to relock after the shift. If I feel the grade starting to increase, I lift the throttle slightly, and punch OD off, then reapply the throttle after the torque converter locks again. Punching OD off also keeps the coast clutch engaged in Drive for engine braking, so I leave OD off until cresting the hill to see if there's a downhill on the other side.

As long as I stay above 45 mph, it will stay in Drive with the torque converter locked regardless of throttle position. The torque converter will remain locked on deceleration in Drive down to 36 mph IF OD is punched off, IIRC.

Hope this helps,

**john hd** · 10-16-2003, 07:57 PM

josh

i tow in overdrive.

as you will be able to with your 1500HD.

john

**jcanavera** · 10-16-2003, 10:22 PM

My last two tow vehicles have been vans. The older vehicle was a 5.7L 3.73 half ton Chevy van. Manual stated all towing was to be done in 3rd. No OD.

My current vehicle 6.0L 4.10 3/4 ton GMC van. Manual states that towing in OD is acceptable unless the tranny is doing a lot of hunting. I also have the tow/haul switch which sets the shift points at a higher speed.

I have always followed the recommendations as stated in my owners manual and have followed the manufacturer's transmission oil change recommendations. Up to this point with over 25 years of towing, I have never had a transmission failure or repair.

Jack

joshua32064 · 10-17-2003, 02:25 AM

Hi Everyone,

Thanks so much for all of your opinions. It kind of sounds like there is no 1 perfect answer and everyone is going to have their own opinion about which way is best.

As soon as I get my trailer, I'll have to see what the tachometer is doing and then go from there.

Josh