Engine/Transmission (1994 - 1998) Downshifting with converter locked?

[Grinder1]

OK, I've been getting great info about exhaust brakes. Next question, is it safe to downshift to 2nd or 1st gear with converter locked? I think the TransGo VB kit applies more TC clutch pressure, which would keep the clutch from slipping. The last thing I want to do is scatter my 47RH while enjoying a new exhaust brake setup. Really need some trans guys to chime in. Thanks

 

[Bill Stockard]

The transmission in my formerly owned '97 would not down shift with the torque converter locked. IIRC, the mystery switch will not force a lockup, but only holds the lockup. At the top of a grade, I would shift to 3rd gear, let the torque converter lock up, and then flip on the mystery switch to hold it in lock up. I don't remember the torque converter locking up in 2nd. I used the exhaust brake only when towing and only in the mountains of the western lower 48.



The torque converter in my present 48RE will lock in 2nd gear if it is manually shifted into 2nd gear, depending on the road speed. Since I no longer tow, I haven't used that feature and/or need an exhaust brake.



Bill

 

[BSMcCauley]

My mystery switch would lock the TC in the factory drive-train configuration just as soon as the transmission hit 3rd gear and the shift into/out of OD would proceed with the TC locked. However, in the factory configuration, the mystery switch had to be turned off before 3-2 downshift would occur. Turning the mystery switch back on again would immediately lock the TC unless the transmission wasn't in 3rd gear yet, in which case the TC would then lock simultaneously with the 2-3 upshift. As part of my transmission overhaul & upgrade, some tweeks were performed on the valve body and now I am able to lock the torque converter in every gear (yes, even reverse) and can let all the shifts occur with the TC locked if I want to.



That said, my observation is that TC locked with transmission below 3rd at engine RPM below 2000 and more than light torque load (either pulling or holding back) makes for a lot of drivetrain vibration and noise and therefore I usually avoid using the mystery switch below 3rd gear.



BTW: I don't yet have an exhaust brake, so can't comment on wisdom of using it in lower gears.

 

[KRStone]

Exhaust brake is configured to not be effective under 35 mph anyway (mine is setup that way), because the truck needs to unlock the TC in order to prepare to stop.



It would be nice in rare cases such as steep downgrades at a speed of 25-35 mph, but in most cases I can't think of a reason to keep lockup and use the EB that slow. Also, at low rpms you'll notice a significantly diminished EB power.



My Dad's '06 with EB on and TC locked up, will let you slow down, unlock TC, downshift, relock TC, and reengage EB... . it's awesome. My '96 doesn't have the computing power to make that happen.

 

[cerberusiam]

Transmisison will not downshift with TC locked unles the VB is modified. It also won't go to first if your over 35 mph, below that no reason to use EB as it is not very effective.

 

[Axtell3]

I don't think I agree with yah there. In my area we have short steep hills. Mostly a mile or so, with anywhere from 800 to 1200 feet of elevation gain. Obviously some sections are steeper than others. Point being, that an exhaust brake in first gear can be a great thing, when decending one of these with a loaded trailer behind you. I don't have one on my dodge yet, but have 2 cabovers, and wouldn't drive them if the eb didn't function.

One is an auto, and I just downshift to whatever gear is appropriate, and flip the eb switch.

 

[cerberusiam]

Are those actual EB's or Jakes? Big difference in how they function.



Gearing is also important and keeping the rpms up where an EB will be useful. Tougher to do with less gears and higher ratios.

 

[Axtell3]

They are eb's. UD is airpowered off the brake systems air, and the Isuzu/ GMC has it's own pump on location.

They are mounted a good 3 feet from the turbo, both factory installs.

I hope to get one next spring, for the cummins, got to budget my projects...

 

[KRStone]

The problem isn't with the EB setup, it's with the trans. The transmission overdrive is the only computer controlled gear. Any gear under 3rd won't lockup for EB to function. That's what my pacbrake info states.