Explanation of towing in overdrive

[dragonvan]

I found this nice explanation of why you shouldn't tow in overdrive for all of those who are confused on this one.

Towing in OD

Couldn't have said it better myself. This is especially true with the 700R4 as line pressure drops in OD. I would have to look up specs but that might also be true of the 4L60E. If anyone knows for sure, give a shout.

On the same page is a blurb about the Pressure Regulator Boost Valve on 4L60E's. I posted on this in the Trans forum if you want a direct link.

Later

[tripplec]

I have read input from numerous people who tow in OD on the other Astro site. No problem (within reason of course and not up hills or noteworthy grades but that should be evident to most) on the flats. There are mention of increasing the number of clutches in the clutch pack ( I presume there must be room for more than GM uses which eliminate the problem for the great majority) but requires a rebuild.

I have a two sled snowmobile trailer and brought that up some time ago as I towing in 3rd and the gas millage dropped like a rock. I will likely be moving some furniture at highway speeds > 100kph for about 500km. OD a must as rev's would be too high.

More input (#5 is alive, remember that one, more input) ha ha

[dragonvan]

That's what happened to that washer motor Tripp... short circuit.

I also know that the 700R4's and some of the 4L60E's had the line pressure drop pretty drastically in OD which makes the clutches slip even on the flat if there is much load. This causes a localized overheat in the clutch pack that doesn't show up on a temp gauge that is monitoring the fluid temp in the pan. I "think" that GM changed this in the later 4L60E's though and bumped up the line pressure a bit in OD (hence the change in the recommendation in the owners manual). There are aftermarket shift kits for the 700R4 and programmers for the 4L60E that offer a solution to the OD line pressure problems.

Also the 4L60E has a PWM solenoid for the TCC. This lets the clutch slip at times. For example engagement and disengagement. My understanding is that the system can start to let the clutch slip to get some torque multiplication without fully disengaging it causing a constant slip condition if it is trying to do this a lot. Of course, in that case I would think that you would see this on the temp gauge. I have thought of looking into the possibility of running an LED on the TCC PWM circuit to tell me when the TCC is fully locked. If it is staying fully locked when you are in OD you are probably pretty safe to stay there as you are producing much less heat with the converter locked.

Sorry guys but I still have some more learning to do on the 4L60E before I can say for sure what situation is ok to tow in OD in. I would very much like to find how to make use of OD while towing as most of my mods are geared toward my holy grail of MPG. Bottom line though is that if it is shifting back and forth or the converter is locking and unlocking you need to shift down a gear no matter what gear you are in. Since I can monitor calculated engine load with my scan gauge I do look forward to doing some testing this weekend on the flat in MN and SD, as engine load can tell me quite a bit about the situation. It is just to hilly around here to be able to do that and I haven't gone anywhere with the camper yet this year that is flat.

My $0.10 - Until I reprogram mine or find out for sure that GM did bump up the line pressure in OD I'll pay for the extra gas. Gas is cheap compared to trannies and the time I don't have to rebuild them.

[dragonvan]

Almost forgot... If I was going to pull anything over about 2500lbs in overdrive I would definitely upgrade the 3-4 servo to the corvette one for better holding power.

Often the gas mileage will be the same or even drop by towing in OD vs. D. The reason is that there is more load on the engine. Fuel consumption is based on engine load (needed hp) not simply RPM. Depending on the power band of the engine and the speed being traveled D can keep the engine in a more efficient portion of the power band also.

[tripplec]

Yeap yeap, but with lighter 1 to 2000 lbs towing should not be a problem and still better than in 3rd all the time. I have a Scanguage to monitor some of this. But no temp guage in the tranny.

Oh dear, maybe I can drop a big block of ICE in through the doghouse and hit the highway. Oh crap did I give someone and idea. Dammm... Next they be routing the A/C duck out onto the tranny.... oh damm I did it again. shutting up now.

[dragonvan]

My scan gauge gives me trans temp so I can keep an eye on things, but you could still have the OD clutches get overheated without the pan temp getting too high.

I wouldn't worry about towing in OD with 2000lbs or less. I towed an approximately 2000-2200 lb camper for a summer with my transport in OD when on the level (interstate etc.). Granted I did have a good size trans cooler. If the TCC wasn't staying locked I shifted down to D. With the bigger camper I didn't even try.

[tripplec]

I don't think my tranny has an internal sensor monitored by the computer which would be required to pickup from the it to the Scanguage. Maybe in the new series and not in a 4L60 which I what I am told the AWD 1996 has. A port exists to screw in a temp guage to pickup the temp but of course I don't have that installed. I do have a series tranny cooler put in a few months ago.

I will have another look at the scanguage but doubt that.

[dragonvan]

If you have a 4L60(700R4) and not the 4L60E then no there is no temp sensor. Not sure without looking it up what is in a '96 AWD.

[tripplec]

If you have a 4L60(700R4) and not the 4L60E then no there is no temp sensor. Not sure without looking it up what is in a '96 AWD.

That was what I was told. I also found out they have a weak valving system and minimal clutch pack by design which is our immediate problem for original trannies. Rebuilders can really fortify those weak points.

But I don't want to rebuild the tranny to fix short comings neither do I want blow it so I can rebuild it. I just hope to use it within it document two and load rating but alias it likely would blow if I were to go that far.

On another note.
I towed a very heavy camper trailer (crank top years ago with that caravan 3.0 liter, 3 speed auto) 10,000km round tripe to the rockies, up and in them and back. Max speed through the US was 115kph with wind drag. It had at least 400lb hitch weight and had to be around 3000lbs w/piston brake on the tow extension. I had a tranny cooler and beefed springs on the back. Did not blow the tranny and still running over 100k later. Valvoline took out the engine but the trailer did not take out their tranny ha. Most people said it couldn't be done, too heavy. I could just see over the top barely in the rear view. Sleeps 8 tight, 6 confortably. We all know the those trannies on 3 litres were a problem anyway.

[chesehd]

What about the Tow/Haul button?

So what about that feature, for those having it?

What does it do for those towing on OD, if anything at all?

If it's still not a good idea to tow in OD then why have it there at all?

[tripplec]

What about the Tow/Haul button?

So what about that feature, for those having it?

What does it do for those towing on OD, if anything at all?

If it's still not a good idea to tow in OD then why have it there at all?

Well I may not be entirely correct but I believe it changes the shift point to higher RPM's which is better suited for towing. But it could if programmed (since the auto tranny is electronically controlled) disable it's ability to lockup the TC and even lockout OD. But then again that I am not sure about.

OD makes a big difference in gas savings on what we might call super highways >100kph or 60mph on relatively flat road. It does not do well even on the Astro, on hilly roads with long climbs unless you like working the pedal to force down shifts to get up to speed. I prefer to manually pull it down to 3rd then back to D when I get to the top or going down and keeping the trottle pressure similar. As mentioned in other posts frequent shifting back and forth or tranny hunting generates more heat and if the clutch pack get too hot and overheated (parts themselves they will break down), but the overall tranny will take longer to acquire this heat throughout. Seems to me that a vehicle such as this with it fairly high tow rating would have a better supported tranny high gear mode under load for that purpose and heat dispersion. After all there is a oil cooler and large lines on the tranny to rad cooler. A number of vehicles especially AWD and 4WD come built with tranny coolers (Explorer). We can add it easy enough but the transmission is no up to the challenge I am affraid. The have been posts from tranny shops and rebuilders stating that a rework is necessary to fortify these 4L60E trannies even at half the rated towing load.