Downhill with your foot off the accelerator + little to no boost. I think it is just the work of turning the engine over slowing the truck down. Try pushing a manual truck in neutral in a level parking lot then try pushing it in gear. Which takes more effort?

 

Found this learning about the coast clutch (CC):

https://www.ford-trucks.com/forums/4...-solenoid.html

In short, the CC does not engage in high gear. So I need to try my hill descent while in low gear to find out if I get any engine braking before modification.

In review, I looked at my tach during a previous downhill run and I saw the engine at idle speed. That lead me to look at locking the TC, which then lead me to learning there was something called a CC.

Per the link, the coast clutch is controlled by the PCM. There will never be engine braking in high gear with the stock tune. Also per the link, engaging the CC for engine braking when in high gear will cause transmission damage.

 

Complete side thought...

For those in California or other heavily smog-tested region, the mod I am planning would probably fail visual inspection. Just a guess, but the inspectors would probably not be happy seeing splices around the EBPV, PCM, or any engine sensors.

 

On decel tunes, you just have to take it out of over drive to make the CC work. That's how it is on DP tunes anyway. And it will slow you right down. But the stock CC isn't heavy duty enough from what I've heard to stand this very long. That's why most good trans builders beef it up with extra clutch etc..

 

For what its worth, and I'm not discrediting any of the above information, but we just parted out a 2000 7.3 that was bone stock and had a pacbrake on it for over 380k miles, without stiffer springs or valve issues whatsoever... just food for thought. switch operated.

Ee have a 12v cummins with a pac brake since it was purchased near 300k ago. 0 issues.

Yes stiffer springs would be suggested, but fitting a pacbrake in would not be too difficult if you have a stock or aftermarket charger.

 

Did the test again, transmission shifter in low gear. I came to a full stop first.

Tach was now about 1,400 rpm at 20 mph downhill, could feel some engine braking but still had to repeatedly apply the regular brakes to slow down.

Once stopped at the bottom of the hill, I started driving without touching the gear shift. Now got about 2,000 rpm at 20 mph.

So does the tranny go to second gear when not under load (down hill with no throttle) even when the shifter is in low?

 

Found partial EBP sensor specs and some other useful info:

97-15A 7.3L DI Turbo

EBP sensor outputs:

• 0.8 to 1.0 V @ sea level
• 1.19V @ 14.8 PSIG (~1 atmosphere of back pressure).

So (check my math and [units]):

• (1.19 - 0.9) [V] / (14.8 - 0)[PSIG] = X = 0.02 [V/PSIG]
• 0.9 [V] + (40 [PSIG] * X [V/PSIG]) = Y = 1.7 [V]

If I'm reading the specs right, but I have a problem with these numbers. I used 0.9 to split the difference on the sea level reading. So that is a possible error of plus or minus 5 pounds which I think is a lot.

Wondering if either the PCM self-calibrates this input for ambient pressure, or if there is a calibrated point in the sensor output.

[EDIT To Add: See post 47 in this thread]

 

Which brings me to another question:

Can either TorquePro or ForScan read raw PCM data? Like the output of the EBP and MAF sensors?

This question suggested by a several thoughts banging against each other:

• The idea of a decel tune suggested earlier in this thread
• The info I found stating that the PCM determines sensor low or high values by comparing the EBP to the MAF sensor
• The need for analyzing failure scenarios for my hardware hack since it overrides the PCM

 

Why would you put it in low gear? Just take it out of over drive, and let it work. Mine towing my trailer slows it right down. Even slower then I'd like. If your going slow already you won't feel it. Go freeway speeds and you will notice it right away.

 

I'm going down a steep grade, 1 lane road, 20 mph max. Dogleg blind curves.

I go down it in my VW GTI 1.8T in 1st gear and engine braking keeps the speed sane.

Looking to duplicate that action with my 3-ton brick.

 

Try it in drive. You will be surprised.

 

"I'm going down a steep grade, 1 lane road, 20 mph max. Dogleg blind curves."

There is a road like that out of Sonora called Priest Grade. The old road from bottom to the top is about 1.5 miles. New Priest Grade road starts and ends at the same points, but it is about 6 miles. You only want to go UP the old road.

 

In drive, but turn off overdrive before starting down the hill? Will do and post results as stock test #3.

 

LOL I've driven it down! There is a stop sign at the bottom. It's kind of important to stop at it.

 

needs more jpeg

Here is the EBP chart from the PCED showing voltage vs psi. Not finely sliced, unfortunately, but that's evidently the level of discernment required by the PCM. The PCM has a built in barometer to compare to the signals from MAP and EBP etc.

Torque Pro can show exhaust back pressure in psi only, as far as I know. You can set up a gauge as a chart showing ExhPres psi in 4.5 psi increments, with an alarm to flash the gauge at a level you select.

I have the DP decel tune and I do not think it considers EBP. It's super effective and easy. My recollection is most folks using exhaust brakes either don't worry about back pressure, or freak out about it and install fresh higher rate valve springs. I've never had a problem using it. The biggest plus to the DP switch (aside from ease of use) is the trans line pressure increase to decrease slippage and thereby increase effectiveness. Without firming that up the effectiveness on an AT truck is greatly reduced. I am absolutely no expert on the tuning details, but I do know if you keep the revs up and hit the switch it works great.

I doubt it would happen, but if the valves float I bet you'll hear the cacophony and slap that switch off before you even think about it.

Having excess EBP cut off the system makes sense to me, I hope you can get that working. Otherwise I'd stick to using the IVS to turn off the system.