I have a ZF6 transmission and have no trouble controlling my descending speed when towing heavy cross country. This includes places like CO, MT, UT, VA, TN, AZ, WA and I think you get the idea. I drive like an old man with nowhere to be though, so maybe that aids my control of speed going down a grade with 12,000 lbs strapped to the *** of the truck.

My question is, why is using the 4R100 perceived as being such a poor method of controlling speed going down a grade?

As I understand it, there is a "coast clutch" that is available when OD is off. Again, as I understand it, the coast clutch can and should do a decent job of providing engine braking and controlling speed. Perhaps I am completely wrong though and the coast clutch allows for.... well... coasting.

I don't think the ZF6 is "the all mighty" transmission, and this is not meant to be a pissing match or any other contest associated with the male anatomy. If the 4R100 could provide a similar level of engine braking that the ZF6 provides, then that is a strategy that could be and should be used. Am I wrong?

This is just geewhiz information for me and attempt at trying to learn something about the other transmission in our trucks. I did some research and the information out there on other forums and technical sites leads me to believe the coast clutch is a viable option for controlling speed going down a grade. I also did some research here and came up with mixed information from some well respected FTE'rs that could not agree and left it at that.

I understand that some SD owners have a decel tune, exhaust brake tune, or whatever tune, but that is not what this question is in relation to and I would like to leave that area of the truck for another thread.

I look forward to someone providing information so that at the very least I can learn why the coast clutch is inadequate or is perceived as inadequate.

 

No, you are not wrong.

Many people seem to think that if the torque converter clutch is unlocked there will be no engine braking. That is not correct. From actual measurements, I know that locking the converter clutch adds 10% to the braking torque. So even with the converter clutch unlocked you get 90% of the maximum possible engine braking. Most people would never be able to tell the difference.

 

The Transgo Tugger instructions say that if you want to use an exhaust brake, you should upgrade the coast clutch to the PTO 3 clutch version ( all 4R100 non PTO's only have 2, I think?) or a custom 4 clutch version. My truck has the 2 clutch version. I think it does not have enough holding power in the stock configuration.

 

Understood and noted for future readers. Although, my question is in relation to the coast clutch alone having the slowing power or resistance power to hold back a truck and load going down a grade.

I am attempting to understand how different the "engine braking" could be in a ZF6 truck vs. a 4R100 truck when the transmissions are utilized properly on a descending grade.

 

My limited understanding of the DP decel tune is that it locks the coast clutch via line pressure to allow the retard from the EBPV to transmit fully. The extra 10% I guess. That's absolutely what it feels like. Switch to decel, EBPV closes and you feel that retard transmitted to the rear wheels. Hits pretty hard in some circumstances. So it's like holding a gear on the ZF6 held back by engine compression plus the EBPV closed. ****'s awesome. I expect 1 or 2 more clutches will help durability and/or softer engagement, but if the tune can dial up enough pressure that the stock clutches clamp sufficiently to prevent slippage and corresponding wear... why?

 

No, that's not what it's doing. The coast clutch is already turned on by the stock programming. The tuner can't turn it on more. What it can do is lock the torque converter clutch to get the extra 10%.

Compression doesn't hold you back. That's why diesels don't have engine braking unless you modify the SBPV or add an aftermarket exhaust brake.

While it does take work from the rear wheels to power the pistons up to compress the air in the cylinder, once the piston passes top dead center you get that work back (minus a small percentage in losses) as the piston is pushed back down in the hole by that same compressed air. So you will have very little holdback.

Now when you block the exhaust there is A LOT of pumping losses and that provides holdback. That is why the EBPV is so effective in providing engine braking.

That should say harsher engagement, not softer. More clutch discs equal harsher engagements.

 

I haven't towed extensively with this truck and not at all through mountains. I do have some hills around here and do exercise the OD off switch a bit. I haven't been able to use it enough to fully understand it but...when descending a hill with the OD off I do get what seems to be full lock up, the RPM's and MPH's appear to be locked together. However, on the one hill where I use it the most I find that at some point the thing unlocks and the RPM's go down to idle. In my experience you get some braking from the engine but not a whole lot. Gearing down and staying in a lower gear does force the driver to keep his/her speed down though and probably helps prevent some runaways. I don't fully understand how the engine brakes on heavy trucks work but I believe that they manipulate the valves so the engine does provide compression braking.

 

Lots of 2nd gear riding if it’s a bad one otherwise if I want to go 40 I hit the brakes hard to bring me to 30 then when I get to 45 hit them again to 30 so I’m not using them the whole descent.

 

I'm still trying to figure out how to word things but I'm going to respond at some point. I'll just wait for more responses that I can quote and counter. I already have a novel worth of rebuttals in my head....just gotta put 'em on paper. No rainbows and unicorn farts here. Put on your big girl panties.

 

When are you writing the novel for the 2011 rear end swap the pamphlet you wrote me last went something like figure it out lol

 

What I have gathered so far from the few replies is that the TC must be disengaged by the tuning and OD off in order for the coast clutch to be engaged.

The coast clutch is on/off, not 25%, 50%, etc application.

Some owners feel that if the TQ is disengaged, there is no braking applied, which is incorrect. In actuality, the TC only provides 10% of the braking power where as there is 90% braking power from the coast clutch.

Even when the coast clutch is applying engine braking, this should not be considered the value of a manual transmission holding a lower gear in order to control descend a grade and controlling speed.

There is a point at which RPM and MPH match each other and a point at which the RPM drops and the MPH has potential to increase if still on the downward slope of a grade.

Exhaust braking is a useful feature of diesel trucks, but was not a factory installed feature on our trucks. We have the ability to modify the way the EBPV works, or install an aftermarket EBPV in order to work similarly to an exhaust brake.

Based on the information above, the coast clutch is capable of maintaining a set speed of an unloaded truck to a point, but not a loaded truck?

 

Your experience and knowledge is appreciated, but just keep in mind this is not a daytime TV Divorce Court show.

Not looking for Rainbows or unicorns, just looking for some clarity on how the coast clutch works in a 4R100 truck since both internet and FTE searches didn't lead me to a conclusive answer.

When I am laying awake in a dark room at night, these are the sort of things I wonder about.

This has already been a great discussion and clarification on a major component of the 7.3L PSD. Let's keep it on the polite, educational and respectful side.

 

Hopefully a good example of engine braking with an automatic.
Coming out of Boone N.C. there is an 8% grade to descend heading East. When towing my 9k camper I lock out overdrive and start down hill at 45MPH (posted speed limit). The truck will build speed to 60MPH and I then apply brakes to decrease speed. On this grade I have to get on the brakes about eight times to scrub off speed. Last trip I warped the right front rotor braking. I have an F-550 with BTS transmission. I have not tried pulling it down to 2nd gear. This is the only grade that i have problems going up to Boone and coming back from Boone. I have no problems maintaining downhill speed on other grades on I-40, I-80, I-77.

 

Thank you Mark K, ebbnflow, aawl, RigCity, Walleye, Cody (pending) and CGMKCM for your comments this far.

I look forward to further constructive discussion on the topic at hand.

 

That's not right. The torque converter clutch is totally independent of the coast clutch. Neither one has any bearing on the other.

The coast clutch is engaged when the OD OFF light is on, or if the shift handle is in the 1 or 2 position. It's also on in reverse, but that doesn't help in this case.

Correct.

Yes.

Let me explain a little further. The extra engine braking when the torque converter clutch is on is due to the higher engine speed when the torque converter clutch is on. When it is off there is slip across the converter. When it is locked there is essentially zero slip (the system will always read a few RPM slip, but I believe that is due to errors in reading the speed sensors, not actual slip.)

It's very, very close. If the ratios of the gears being used in the descent are the same I expect that the braking force available from an automatic will be within 10% of that of a manual trans.

That will happen if you are in too high of a gear for the grade.

Yes.

I disagree. I've used engine braking with a 4R100 equipped vehicle loaded to the GCWR. It worked fine.

You should have pulled it down to 2. That's what it is there for. It would have prevented a warped rotor.

I believe BTS builds all of their transmissions with at least three coast clutch plates. It has plenty of torque capacity to handle engine braking in your F-550.