Torque Converter Lock Switch
Dave and Helen Damouth www.damouth.com
Question: I just installed a torque convertor lock-up switch on my Dodge Ram diesel pickup truck. I was told you use one a lot and I was wondering if you could explain the best way to put this feature to use?
Answer: As background: The 1994 and newer automatic-transmission Dodge Rams, like most modern automatic transmission vehicles, have a clutch which locks the torque converter when driving at a steady speed above about 48 mph. This gives a straight-through connection from engine to wheels, with no slip, no power loss, and no heating of the transmission. Unfortunately, this torque converter clutch is automatically unlocked whenever the driver removes his foot from the accelerator, essentially letting the truck freewheel down a hill with little engine braking. For towing a heavy trailer, it would be nice to keep this clutch locked while decelerating, giving much more effective engine braking. It turns out that this is possible with a simple switch and minor wiring changes - an easy do-it-yourself project for anyone who can read a simple wiring diagram. Instructions for doing this are published in Turbo Diesel Register - a quarterly magazine by and for Dodge/Cummins diesel owners. Pages 52-56 of issue 19, Winter 1998 have all the information needed. Send subscription and/or reprint orders to Turbo Diesel Register, 1275 Elk Ridge Cove, Alpharetta, GA 30005. A one year subscription is $35. Back issues are $7/issue plus $5.00 shipping (for any number of reprints). I strongly recommend this magazine for any Dodge/Cummins owner. It's also useful for other diesel owners, and carries occasional information about other diesel engines, including the Ford/Navistar Powerstroke. Manually locking the converter has the side effect of preventing the transmission from down-shifting. Upshifts will happen with the converter locked, but are quite harsh (and probably are very hard on the drivetrain if done during hard acceleration).
When not towing, I use the switch rarely. One situation where it is useful is in level suburban driving, where the traffic moves fairly steadily in the 45-50 mph range. I'll lock the converter, in overdrive, during this kind of driving. This keeps the converter from frequently locking and unlocking on its own, and keeps the engine rpm down where the noise is less and the economy is better. The truck then feels like a manual transmission. And of course, like a manual, you have to remember to unlock and/or downshift if you want to accelerate, or if your speed drops too low and the engine lugs. Going down long steep hills, I may lock the converter and flip the exhaust brake on, to avoid riding the brakes. This isn't any different than towing - except I can stay in a higher gear without the trailer weight.
Juggling the converter switch, the overdrive switch, and the shift lever is an awkward thing to do, and takes some practice. It's worth practicing a lot, so that is will happen quickly and automatically in an emergency situation.
For towing, I lock the converter in two situations
a: On moderate climbs, in the 50-55 mph speed range, the automatic transmission seems to want to either lock up in overdrive or to remain unlocked in third gear, neither of which is quite right. Turning off overdrive and locking the converter often results in a better rpm range in this situation.
b: The main use is braking while coming down long hills. With the converter locked, the exhaust brake is much more effective in either 4th or 3d gears. (On my truck, the converter refuses to lock in 2nd - one of the TDR articles implies that some RAMS will indeed lock in 2nd). Interstate highways in the mountains often have mile after mile of fairly straight 6% downslope, and 3d gear with the converter locked will maintain a comfortable speed without touching the brakes.
I have the 3.54 gears. I have a BD exhaust brake, and I did *not* install the 60psi valve springs, so my brake is calibrated to generate much less braking force than would be possible with the heavy springs. On the other hand, my trailer only weighs about 8000 pounds (with perhaps another 700 pounds of stuff in the bed of the truck). With a heavier trailer, I'd recommend the heavier valve springs. The 24-valve engines, introduced in about 1998, already have the heavier springs.