Exhaust Brake and Tow Haul Mode

[RSchwarzli]

When towing in Tow/Haul with the exhaust brake on, does the truck only use the e-brake or does it also use the service brakes at its own will?



Why I ask is it seems that when decending a good hill, the trans will shift down to mabee 4th or 3rd, and you can feel and hear the e-brake, but then there is a sudden slow down almost as if the brakes came on even though I was not on the brake pedal. Does that even make sense that the truck is applying the brakes at will??



Any thoughts??



Thanks!



Robert

 

[snuff]

i would not think so, you might be shifting lower in gear again... .....

 

[HBarlow]

No, the ECM has no capability to apply the hydraulic service brakes.



I think what you are describing occurs after the ECM calls for a downshift to third gear. That's when the serious turbo braking occurs.

 

[RSchwarzli]

That was my initial thought as well, except that the rpm does not change. This is definetally after the 4-3rd shift.



My only other conclution would be after shifting into 3rd, there is normal deceleration from the e-brake. Then there is a delay of about 5-10 seconds before max retarding backpressure is created thus giving the feeling of the brakes coming on. The only issue with that theory is that it is not a slow build up, it is a sudden deceleration, hence the reason I figgured brakes.



I know it is not just me. Dad was driving the truck and he asked me the same question!



Robert

 

[snuff]

what about the turbo braking i have read about on the 6. 7, that might be it. .

 

[HBarlow]

what about the turbo braking i have read about on the 6. 7, that might be it. .

That's what we're talking about. 6. 7 engines have turbo braking only, not exhaust brakes. Many call the turbo brake an exhaust brake, myself included, out of habit or because it is a familiar term but it is no longer accurate.

 

[Hoefler]

It will down shift and then lock up, that is when you really feel the effect of the exhaust brake.

 

[jelag]

The exhaust brake has NO ability to apply the service brakes... its all in how it down shifts, and locks the torque converter...

Our 08 5500 seems to work better with the tow haul button depressed... it locks up the transmission and torque converter and words very well...

Because the ECM has mapped the speed and rpm range it knows what gear and when to lock up the torque converter to get the best performance from the exhaust brake... .

 

[DSTEPHENSON]

I just returned from a trip to Creede, Co. and noticed the sudden downshift braking effect going down the Raton Pass. It slowed the rig down so much I turned the TH off and left the EB on and it slowed the rig enough to safely descend the mountain grade. Only used the TH on the upside of the mountain pulls.

 

[RSchwarzli]

I just returned from a trip to Creede, Co. and noticed the sudden downshift braking effect going down the Raton Pass. It slowed the rig down so much I turned the TH off and left the EB on and it slowed the rig enough to safely descend the mountain grade. Only used the TH on the upside of the mountain pulls.

Exactly! That is the exact same thing. It REALLY slower down the truck so much I had to go back on the throttle.



Robert