How does the Jacobs Extarder work?
My m/h is a 1994 Allegro Bus, 5.9B 230 hp (mechanical engine) Cummins DP. It is equipped with a Jacobs Extarder. From what I understand, to stop confusion in what it's called, it is now just called ...
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OK, exhaust brakes are not all the same. The one on Wayne's coach is probably the most advanced-- a PacBrake PRXB. It generates the full spec back pressure at lower RPM than the earlier PacBrakes or other "single stage" exhaust brakes.
And, yes, the Allison should automatically downshift to whatever "pre-select" gear is programmed for exhaust brake application. 2nd and 4th are the common choices, though as Wayne said it can be any gear you choose to have an Allison dealer program in. No perfect choice-- when coming to a stop, 2nd is a good choice. 5th gives the driver the most control in the mountains (there are some grades where exhaust brake and 5th gear give the right amount of braking HP and 4th or lower would give too much braking. I40 westbound into Albuquerque is a good example. You can descend the whole grade in 5th gear exhaust brake, but if a lower gear is used, you would be constantly turning off the brake and applying throttle to upshift, then back on exhaust brake..... Again personal choice.
Now, to test the exhaust brake. Get up to 40 or so MPH. Turn on the exhaust brake and take your foot off the throttle. Note what gear the transmission downshifts to. Use the down arrow to lock the transmission in that gear.
Now toggle the exhaust brake on and off. Since the transmission is now a "constant", it will tell you how much additional braking HP your exhaust brake itself generates.
Particularly if not used frequently, they can rust up to where the butterfly valve does not close completely.
Also, any exhaust leak upstream of the exhaust brake will materially diminish its performance. Look for signs of black soot: head to exhaust manifold, manifold itself, manifold to turbo and turbo to exhaust brake. And, yes some are mounted further "down stream" of the turbo.
I know on the PacBrakes, they have a test port so you can put a gauge on it to verify back pressure. I do not know if the Jacobs has one. Worth a look.
And, yes, the Allison should automatically downshift to whatever "pre-select" gear is programmed for exhaust brake application. 2nd and 4th are the common choices, though as Wayne said it can be any gear you choose to have an Allison dealer program in. No perfect choice-- when coming to a stop, 2nd is a good choice. 5th gives the driver the most control in the mountains (there are some grades where exhaust brake and 5th gear give the right amount of braking HP and 4th or lower would give too much braking. I40 westbound into Albuquerque is a good example. You can descend the whole grade in 5th gear exhaust brake, but if a lower gear is used, you would be constantly turning off the brake and applying throttle to upshift, then back on exhaust brake..... Again personal choice.
Now, to test the exhaust brake. Get up to 40 or so MPH. Turn on the exhaust brake and take your foot off the throttle. Note what gear the transmission downshifts to. Use the down arrow to lock the transmission in that gear.
Now toggle the exhaust brake on and off. Since the transmission is now a "constant", it will tell you how much additional braking HP your exhaust brake itself generates.
Particularly if not used frequently, they can rust up to where the butterfly valve does not close completely.
Also, any exhaust leak upstream of the exhaust brake will materially diminish its performance. Look for signs of black soot: head to exhaust manifold, manifold itself, manifold to turbo and turbo to exhaust brake. And, yes some are mounted further "down stream" of the turbo.
I know on the PacBrakes, they have a test port so you can put a gauge on it to verify back pressure. I do not know if the Jacobs has one. Worth a look.
