RPM limit for exhaust brake engagement?

[formula]

Had an interesting last night when engaging the ebrake. I had it wound up to about 3300 rpm in 5th and let my foot off the throttle to kick on the ebrake. It didn't engage till the rpm got down to about 2600 or so. So, I decided to try it again in 3rd so I wouldn't be going as fast. Took it up to about 3400 rpm, no ebrake till about 2600 rpm again. Humm... ... ... so, I tried it a few more times increasing rpm from about 2500. At about 3000 rpm and up the ebrake takes far longer to engage. It also seems to be sensitive to how quickly I take my foot off of the throttle. If I take my foot off quickly, it engages faster, if I roll out of the throttle, engagement is slower.

I'm guessing that this may be a safety built into the ECM to prevent smashing exhaust valves. Has anyone else experienced this behaviour?

Sean

 

[robertyoke]

both of my trucks pac brakes will come on at any engine speed.

-robert

 

[mattymac]

Thats why I like to just hardwire it to a switch. Its a pain when I need it to be on and off but I live with it.



I think most E-brake manufacturers say (on the 24v engines) that they will hold from xxxx-32-3300 RPM?

 

[klenger]

I wonder if it might be caused by the increased exhaust flow at high RPM's and not electronic control.

 

[BHolm]

Mine, with an auto, works at the higher rpms no problem. I would check the vacuum line and pump to make sure the canister is getting the vacuum it needs. Sure sounds like the exhaust flow is overwhelming the amount of pressure the vacuum canister needs to close the flap. When the dealer installed mine, the vacuum line to the can was twisted and kinked, still worked, but I feel much better about it now that the line is open.

 

[jelag]

A couple of points... your exhaust brake should come on above the 2600 rpm you mention..... if its ECM activitated the wait time might be longer or you might by chance be resting your foot on the throttle... . and weight of your foot will prevent the turn on of the exhaust brake... .



You should not be able to create enough back pressure with a stock turbo at any rpm range to push the exhaust valves open and hit a piston... with an after market turbo that is possible and you need to retest your back pressure... .



There is a delay... built into the ECM that waits for the fuel rail pressure to drop to idle pressure before the brake will come on... . you mention pump mods on your signature... if this pump mod is holding up rail pressure above 2600 rpm that could be the reason... the ECM sees the high pressure and leaves the brake off until the pressure falls...



If you need more help let me know... . We've sold/installed hundreds of them... .



Jim / Pacific Clutch

 

[formula]

Ah, I was unaware that the ECM was also monitoring rail pressure for brake engagement. Makes more sense now. I'll try this again when I have some weight behind me so the rpm doesn't drop so fast.

FWIW... ... . the pump mods are on my 1st gen. With the exception of the ebrake and the muffler, the '05 is completely stock. The vacuum lines are not kinked and the solenoid appears to be functioning properly.

I don't think exhaust flow or pressure would affect the closing of the butterfly valve (assuming that it's getting proper vacuum, etc. ) given that the valve is pretty well balanced and the vacuum canister exerts considerable force on the actuator arm. How's that sound?

Sean