How does the Jacobs Extarder work?

Ivylog wrote:
OP, it sounds like you did not manually downshift to a lower gear when you tried Wolfe's test. You need RPMs (2000+) to get good braking from our engine/exhaust brake. At 60 mph you need to turn it on and manually down shift to 5th. Once the rpms get below 1800 you need to go to 4th and the same for 3rd. Does this get you slowed to say 20 faster than just taking your foot off the gas at 60? Until you time the difference you have not proved anything.

I'll give it a try Ivylog. Thanks

OP, it sounds like you did not manually downshift to a lower gear when you tried Wolfe's test. You need RPMs (2000+) to get good braking from our engine/exhaust brake. At 60 mph you need to turn it on and manually down shift to 5th. Once the rpms get below 1800 you need to go to 4th and the same for 3rd. Does this get you slowed to say 20 faster than just taking your foot off the gas at 60? Until you time the difference you have not proved anything.

Remember, even though the shifter pad shows 3rd or whatever gear, does not mean the tranny has downshifted. The Allison is designed not to over rev your engine, so it will not downshift until your speed is reduced to where the engine will not over rev. Often you might have to tap the brakes to get it to actually downshift. Normally a diesel will not slow simply because your foot is off the go pedal. It really sounds like your EB is defective...Dennis

Well, I took it out today and tried the instructions given. At 40 MPH, I flipped the switch does nothing and it stays in the same gear until my speed gets down to a point to where it naturally downshifts. I tried it at 60 since my tranny up-shifts into 6th gear at 55 MPH. Again, it does nothing until it hits 55 MPH, which is the speed it always downshifts to 5th. There is no effect whatsoever on the speed other than slowing because my foot is off the accelerator. I tried it at lower speeds, higher speeds and I get nothing and the gear does not change until I get to the speed to where it naturally downshifts.

The flapper arm is moving with the switch but I'll have to check to see if it is moving and actually closing the butterfly. I guess I'll have to take it to someone after some more checking that some of you have suggested. Jacobs still has not replied to my email.

Normally the signal goes from dash switch to the Allison ECM and from it to the air cylinder. And, yes this is exactly how our 1993 Foretravel was wired (Cat 3116 and Allison MD3060). If so, when you turn on the exhaust brake, your left shift pad will go from "6" to whatever the preselect gear is for the exhaust brake. 2nd and 4th are common choices.

Could it be wired without including the Allison-- sure. Then you have to use the down arrow to get the desired gear.

The cruise is not on when I have used it.

Is the Allison supposed to automatically downshift to a pre-selected gear even though I have a mechanical engine. If there's some type of an ECM I suppose it would have to be somewhere in sync with the transmission. I thought it was strictly manually operated via the small compressor and tank but I also know there is wire running back there because I flip the switch and the pivot arm actuates the flapper arm. I'll check the exhaust brake with Wolfe's instructions given next time I'm out and see if I can see what the deal is. OH, and I have emailed Jacobs to get detailed instructions for it's use but they have not replied yet.

If I find the flapper has eroded, can it be repaired or does the whole exhaust brake have to be replaced?

wolfe10 wrote:
I agree with Ivylog. BUT, you do not want to confuse the braking provided by a transmission dowhshift with a downshift AND a properly functioning exhaust brake.

Exactly!..and if it needs replacing, it's an easy DIY job. I replaced the one on my Dynasty, which was doing basically the same thing as the OPs with the PRXB and WOW! what a difference!..Took me all of 45 mins while at a campground to do it.....Dennis

I agree with Ivylog. BUT, you do not want to confuse the braking provided by a transmission dowhshift with a downshift AND a properly functioning exhaust brake.

More RPMs equal more braking and the reason it should be automatically downshifting when you turn it on and take your foot off the gas. Try manually downshifting to keep the RPMs around 2200 with occasional brake use.

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.