School me on transmissions, overdrive, and lockup converters

[wilkin250r]

I've done my research, I know the basics. I know the planetary gears in an automatic tranny, I know how torque converters work, lockups, ect ect.

What I DON'T know are the specific details of how things go "bad".

Don't tow in overdrive. Everybody says it, but what is the reason? Does it cause the transmission to hunt for the right gear? Too much shifting can be a problem. Does it cause excessive heat? Can I tow in overdrive if I have an external cooler (and temp gauge to monitor)?

How about a lockup converter. Why does it only lock up on OD? What happens if I have a manual switch to lock it, but I never switch it on? Is it the same problem, heat? Why can't I tow with my converter locked?

[D13]

Lock up can occur in many gears. As an example, the 700R4 feeds oil from 2nd gear to the lockup circuit. Generally this is not a desirable condition as you are either accelerating or going fast enough for 3rd. on m y 87 diesel, 2nd is 15mph, 3rd is 28 and lockup is 35, 4th is 40ish and lock up is 45mph.

Trans designed with TCC usually don't have a lot of cooling to spare. Slipping converters create heat. If towing in 4th, you are loading that tiny 4th clutch and the small TCC facings. If you are in the throttle enough to keep the TCC unlocked, a lot of heat is created by the converter. If you over ride this and 'force' the clutch on it tends to slip and fail. Either way the clutches get hot and fail. Unfortunately when the converter clutch fails the debris gets pumped throughout the trans.

2004R's were designed for 305's in cars. They can be beefed up but again the TCC and 4th clutches are the weak points for towing.

Electronic versions include PWM for 'partial' TCC apply and upgraded cooling and so are more robust. When you hit the 'tow' button it delays shifts and raises line pressures for better holding, and will drop out of 4th quicker to prevent failures.

So tiny clutch facings vs. high loads, and heat build up, are the big issues with towing in overdrive. If you want to tow heavy with overdrive 4L80E is the way to go, or one of the new 6 speeds.

[Mattchu60]

Towing in OD means the weight of whatever you are towing has greater leverage over the drivetrain. When towing in D your transmission is at 1.00 ratio, when you get into OD you go to something like 0.73 ratio, which increases the load on the engine/transmission and creates a greater chance for slipping.

At least that is the way I think about it.

[wilkin250r]

And I'm not dead-set on towing "in overdrive", I can easily just set it in drive when towing, I just want to know the REASONS. I like the idea of having overdrive, right now I'm debating whether a built 700R4 is worth the extra cost over a TH400.

I don't think I need to go to a 4L80E, not only would I need a standalone controller, but I don't believe I've ever seen a passenger-side drop T-case for a 4L80E. I'd be lucky to keep it under 2X the cost of a 700R4.

Basically, my entire goal is to build my truck into a fun, lifted offroad toy, AND be able to tow a trailer with it.

[D13]

The passenger drop 4L80E was in 3/4 'burbs and 1 ton squares post-88 IIRC. the adapter is kind of rare, was set up for 241 case.
My cost analysis for swapping in overdrive trans in a reasonably geared truck was not as cost efficient as rebuilding ad driving what you have. You have to save a lot of gas to offset the price in parts, particularly when talking driveshaft length changes. If you already have the 700, it's about $900 in hard parts to get it upgraded, plus labor. There's a pretty good thread in the Drivetrain forum about it.

[wilkin250r]

Actually, my full train of thought is this: I currently have a TH350 that will not hold park (no difference between park and neutral), and a NP203. At a bare minimum, I need to repair that TH350, and I would also like to swap out the NP203.

But why repair a weak transmission, AND also replace the transfer case? Why don't I just replace the whole set with something more desirable?

Thus begins my decision process. I could easily go with the tried-and-true TH400 and NP205, but I'm willing to spend an extra $1500 for a good 700R4 just because I want the overdrive (and low first gear), but I have no delusions that I'm going to get that money back in gas. It's not a cost/benefit analysis. I just want it.