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Old 04-06-2015, 02:02 PM   #1
Syl
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Jake Brake

Hi. I have a 2002 Country Coach with a C12 cat 6 speed trans.
My problem is that when the jake brake is applied when traveling in 6 or 5th gear it down shifts to 4th gear instantly. Has anyone had this problem.
My electrical diagram shows that when the jake switch is turned on the wire goes to the trans ECM. After that I don,t know where it goes. Any help would be appreciated. Thanks in advance.
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Old 04-06-2015, 02:04 PM   #2
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Does it actually shift down or does it preselect that gear and then downshifts as you slow? Also, it's speed dependent so the dash control may show it as 4th but your speed is still too high so it won't downshift till the speed goes down.
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Old 04-06-2015, 02:09 PM   #3
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Hi Mister D. It goes to 4th gear and over speeds.
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Old 04-06-2015, 02:12 PM   #4
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It shouldn't drop to 4th gear instantly. It will display 4th gear but that is merely the target gear. If it shifted from 6th immediately to 4th it would over-rev the engine which is not good. Depending on the RPM you are cruising at when you depress the engine compression brake (Jake Brake), the transmission may or may not downshift at all. If the RPM's are up near 2000~ish it will merely engage the engine braking system depending on whether you are on Low or High (3-cylinders or 6-cylinders). If you are running lower RPM's then it will drop down one gear when the Jake Brake is depressed. The Allison ECM communicates with the engines ECM and shouldn't allow it to over-rev. That said, if not paying attention and not stab breaking with the service brakes on large grades it is possible for the weight of the coach to cause the RPM's to creep up where you may get an alarm.

The larger engines such as your CAT C13 may have a 3-speed Jake Brake, meaning on LOW it will use 2-cylinders, MED it will use 4-cylinders and HI it will engage all 6-cylinders.

Many of the Allison ECM's have gear 2 as the target. I beleive 2 is the default target gear, that is what mine is.

Mike.
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Old 04-06-2015, 02:35 PM   #5
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Hi Mister D. It goes to 4th gear and over speeds.

What RPM do you see when the downshift occurs? Jakes work best at highest allowable RPM. A diesel with closed throttle can operate a higher RPM than under power. You should ask Cat what RPM is allowable under "no load" conditions.

My ISM is governed at 2100 RPM under power but is allowed 2400 RPM with closed throttle.


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Old 04-06-2015, 02:47 PM   #6
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Many of the Allison ECM's have gear 2 as the target. I beleive 2 is the default target gear, that is what mine is.

Mike.
Our first Dutch Star had athe CAT 3126 and preselected 2nd. I had to reprogrammed to go to 4th and hated it.
The 2002 Dutch Star had the ISC 350 and it was programmed to go to 2nd.
The Magna is programmed to go the 3rd and has a three "speed" engine brake. It has the variable vane turbo which also comes into play when the engine brake is on. Not sure what it does though but I don't need to touch the brakes on any hill we found.
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Old 04-06-2015, 02:58 PM   #7
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Quote:
Originally Posted by Syl View Post
Hi. I have a 2002 Country Coach with a C12 cat 6 speed trans.
My problem is that when the jake brake is applied when traveling in 6 or 5th gear it down shifts to 4th gear instantly. Has anyone had this problem.
My electrical diagram shows that when the jake switch is turned on the wire goes to the trans ECM. After that I don,t know where it goes. Any help would be appreciated. Thanks in advance.
Quote:
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Hi Mister D. It goes to 4th gear and over speeds.
Syl
Me thinks you have a problem since your coach is actually downshifting immediately when you turn in the Jake.

On most coaches when the "Jake Brake", (or an "exhust brake" if so equipped), is turned ON the Allison shift pad displays either 2nd, 3rd or 4th gear as the "selected gear"... however the Allison does not actually shift into the "selected" gear until the road speed drops to a speed that is safe for both the Allison and the engine.

(BTW, my coach has a Pacbrake... and it "selects" 2nd gear....I like it that way).

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Old 04-06-2015, 03:38 PM   #8
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I installed the Silverleaf VMSpc so now I can see what gear is selected and which is engaged. When my Pac Brake engages it will show 4th selected & 5th engaged until the speed slows enough to engage the lower gear. Without the computer screen I can only see what is selected.


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Old 04-06-2015, 03:41 PM   #9
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What RPM do you see when the downshift occurs? Jakes work best at highest allowable RPM. A diesel with closed throttle can operate a higher RPM than under power. You should ask Cat what RPM is allowable under "no load" conditions.

My ISM is governed at 2100 RPM under power but is allowed 2400 RPM with closed throttle.


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Exactly, and for comparisons sake, the Cummins ISL has a governed RPM of 2200 RPM and if memory serves 2500 with closed throttle or non-fueling/coasting condition.

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Old 04-06-2015, 06:00 PM   #10
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Syl, Your Jake is probably working OK. As soon as I turn the Jake on, the transmission immediately goes to the lowest gear it can without overspeeding the engine. As explained above, RPMs may go slightly above max, but that shouldn't hurt the engine. Older coaches like mine were programmed to downshift to second gear, but later coaches were programmed to go to fourth due to customer saying the downshifts to second were too rough. Engaging lower gears while on Jake brake must be done manually--At one point I went down to first gear in Jake brake mode.
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Old 04-06-2015, 07:55 PM   #11
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Thanks every one for the input. I will call cat and see what the top rpms are for the C12 under no load. I worked on heavy earth moving equipment for many years and we never let the rpms go over hi free idle. I think my c12 is 2100 hi free idle, When I apply the jake it goes past that. Scares me.
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Old 04-06-2015, 08:05 PM   #12
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Thanks every one for the input. I will call cat and see what the top rpms are for the C12 under no load. I worked on heavy earth moving equipment for many years and we never let the rpms go over hi free idle. I think my c12 is 2100 hi free idle, When I apply the jake it goes past that. Scares me.
I had that happen once with the 3126. Called CAT the next day and they said it was OK since the RPM was allowed to go higher on compression then under load.
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Old 04-07-2015, 03:59 PM   #13
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I will call cat and see what the top rpms are for the C12 under no load.
Good idea. I'll bet they say that the RPMs you're seeing are acceptable. After all, the lower the gear you're in, the more braking effort you get: both because the Jake brake is compressing more air and therefore absorbing more power, and also because the lower gear lets the braking effort of the engine have more effect on the momentum (just as being in a lower gear gives you more power for acceleration.)

But it's good to call them and verify. I recall a report in another engine braking thread where an engine was damaged from over-speeding. This was because the programming in the engine and transmission was not compatible, and the transmission shifted to a lower gear than was safe.

So it's good to verify. But if they say it's OK, then don't be afraid - heavy earth movers have a different usage profile than a motorhome, so their engine speeds are bound to be different (when was the last time you saw a grader trying to control speed while going down a steep grade at speed on the Interstate? )
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Old 04-07-2015, 06:55 PM   #14
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Syl,
Just wondering. I don't think I've read here and, I don't think I recall if anyone's asked you but, at what speed is your tranny downshifting so soon to 4th?

The reason I ask is this. We don't have a Jake but do have an exhaust brake. And, based on what the engine ECM and, Trans TCM are telling each other, and, what the speed sensor is telling both, our E-brake will engage as soon as I let off the throttle, if the switch is on.

But, here's the deal. If our speed is at or above, 63 mph, and, I'm presently cruising down hill in 6th gear, the e-brake will only down shift to 5th if, I'm in 6th at the time. But, as soon as the e-brake slows us down enough to reach 63 mph or below, it will then downshift to 4th and, of course, the tach will instantaneously hit around 2500 rpms. OH, by the way, our coach is an '04 Itasca Horizon 36GD with the C-7 330HP CAT and Allison MH3000 trans.

Now, the shift pad is showing "2". And, as has been stated, that is the target gear for the downshifting and ebraking, all dependent on grade and other influences.

But, if you're cruising at say, oh, around 70 mph and, start to descend a grade, then let off the throttle and your "Jake" switch is on, will it immediately down shift to 4th, from 6th, at 70 mph? If so, yep, I'd say "Houston, we have a problem".
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