The trick towing loaded/on a grade is to apply the brakes ever so slightly to get the switch to trip and it will lock the converter for engine breaking but won't be enough pressure to heat up the brakes near as bad as actually trying to use them.

Iirc the wires for the tcm are separate from the brake switch so with the correct diagram you could tap into it and have a switch mounted to do exactly what that aftermarket box does you'll just have to remember to turn it off.

 

In fourth gear (overdrive) there is ALWAYS engine braking. It doesn't matter if the torque converter is locked or not, there is engine braking.

With the converter unlocked there is 10% less braking force than there is with the converter locked. I've measured this and know it to be a fact. A diesel doesn't have any compression braking because there is no throttle plate. Also, the engine RPM won't be very high in overdrive. Add all of these up and there won't be enough engine braking to notice if you leave it in overdrive.

If you turn off overdrive engine RPM will go up about 29% (0.71:1 to 1:1.) That will provide more braking. Once again, locking the converter will add about 10% braking force.

 

As always, Mark, I respect your opinion and knowledge, but I thought this was odd. I can't remember if my friend's pickup was in OD or not. However we were probably going about 55MPH and as I said, when he let off the throttle the truck didn't slow at all, felt just like he slipped the lever to neutral. But when he slowly pressed back on the throttle again barely off idle, the truck momentarily slowed road speed slightly, and you could feel it was due to engine compression (felt like the converter re-engaged and locked?) then with further throttle pressure of course road speed increased. He did this 3 or 4 times going down the level highway. He was a fellow heavy truck mechanic, just bought the Ford and came by the house to point out this feature he didn't really like. Wish I'd have thought to look at his tach, that would have told the tale. What say you?

 

If it is in overdrive it has engine braking. The diesel doesn't provide very much braking at all because of no throttle plate. And many of these trucks came with a 3.55:1 axle, so at 55 the engine isn't going to be revving very much over idle when coasting. This leads to no holdback from the engine. If he slowly pressed the pedal the torque converter will relock and cause the engine speed to increase be a hundred or so RPM, so there will be some feel to that.

If overdrive had been turned off with the button, then I expect that the coast clutch is not working.

 

Locking the converter may only add 10% engine braking, but it sure feels like a lot more.

I keep reading that diesels have poor engine braking because of the lack of a throttle plate, but mine sure feel pretty strong. My IDI slows pretty hard when I downshift and it has 3:43 gears & weighs almost 9k lbs. My Excursion weighs about the same as the F-350 V-10 it replaced, it has higher gears, and the engine braking feels much more impressive than the gasser.

 

I'm a little spoiled because I'm standard, but I agree our engines do make great braking for what they are, I imagine the large displacement and the higher than normal compression are the main factors. Not having a turbo supposedly also helps but I imagine at a no fuel setting the exhaust restriction would overcome the intake pressure, there's no such thing as free power.

Iirc 04.5-07 6.0 diesels had an electronic throttle plate, I wonder if that was there to provide better engine braking.

 

It was there to provide some intake manifold vacuum to help draw the EGR into the engine.

 

Right, I've got the manual 5 speed and towed heavy for a lot of miles over the Continental Divide and other long grades in the mountain states. I've always felt the 7.3 had just as much engine braking as a comparable sized gas engine. IMO the high diesel engine compression more than makes up for no throttle plates.