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Cummins 400ISL with Allison Jake Brake issues


r3meyer
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So I figured I would ask this question here because IRV2 has been no help and there are some really smart guys on here that know this combo.

I got a new to me 03 Monaco Dynasty that has the 400ISL cummins and allison combo.  This coach has a true 2 stage Jake brake on it.  Low activates the first 3 cylinders and then high activates the remaining 3 giving you 6.  At least that is what its supposed to do.  When you activate the jake brake via the switch the allison selects the 2nd gear and then the coach will start to down shift.  I thought everything was working fine until coming home from Moab.  The jakes are not working at all.  The coach will snow ball down the hill making it not fun at all.

I have checked both of the solenoids and they both work if I give them 12V.  I had a friend drive while I tested both banks and when you activate them its very noticeable as the coach really slows down.

What I cant figure out is why the transmission is down shifting but I am not getting a 12V signal back in the engine.  Anyone know this system?

Edited by r3meyer
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You have a real Jacobs engine compression brake and can select the number of cylinders it will use?  How loud is that?  I drove trucks for years and remember a Detroit 2 stroke 8V92 was LOUD AF when set to High.   I only have an ON/OFF switch for my exhaust brake in my 2001 Holiday Rambler Cummins 350 ISC.  I wondered if my exhaust brake was working correctly because when it's ON the display shows "2".  I did some research, I think on iRV2, and found out that the 2 is preset from the factory.  I think I remember it meaning it is trying to downshift to 2nd gear.  It can be set to 4th by a dealer or shop.  

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5 hours ago, L.R.S. said:

You have a real Jacobs engine compression brake and can select the number of cylinders it will use?  How loud is that?  I drove trucks for years and remember a Detroit 2 stroke 8V92 was LOUD AF when set to High.   I only have an ON/OFF switch for my exhaust brake in my 2001 Holiday Rambler Cummins 350 ISC.  I wondered if my exhaust brake was working correctly because when it's ON the display shows "2".  I did some research, I think on iRV2, and found out that the 2 is preset from the factory.  I think I remember it meaning it is trying to downshift to 2nd gear.  It can be set to 4th by a dealer or shop.  

Yes I have an actual Jack brake on this coach.  My old coach had Pac Exhaust brake with the air actuated flap.  It operated almost the same way.  It would hold me @ about 50-55 with a trailer going down just about any grade.  Maybe tap the brakes once if it was getting close to wanting to upshift.  This coach there is nothing.  Complete snowball and in theory it should have much better slowing ability.

The trans selects 2nd and then begins to downshift as long as you are in 5th gear or lower.  That aspect is working correctly.  I have a Low / High button.  The low activates the first 3 cylinders and high activates the remaining 3 giving you all 6.  This is how it should work.  I cant get the 12v signal back in the engine bay for either low or high.

I jumped both solenoids with a 12v hot and you can hear them engage so I know the engine portion is working.  You can hear the typical jake sound coming from the exhaust.

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Follow the wires. Maybe you lost a fuse somewhere or a rat gnawed on that wire for you. Or see if you have 12v at the switch in the cab. 

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On a manual transmission setup you have cut out switches on the clutch and throttle pedals. 
 

since you don’t have a clutch pedal don’t worry about it. However even if the throttle is slightly pushed it can deactivate the jakes. I am assuming your pedal is a potentiometer. So the range of voltage will be minimal to tell the engine turn off the jakes because I’m pushing the gas. However it could be as simple as the pedal sticks slightly or the potentiometer is failing or not going to the off/decel voltage. 

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I had the exact opposite problem on mine, i.e. it was always on the "High" setting (could not turn it off or set it to low) and it would about throw you through the windshield every time you let off the throttle.  I took it to an Allison shop and they found a setting in the computer that controlled it and made the correction.  Apparently the prior owner had asked for it to be set up this way.  I also had them change it so that it stays in whatever gear it is in unless I shift it down manually.  I like the control and less extreme impact when it comes on but everyone is different on something like that.  I hope you are able to find it and get it resolved.  The Allison shop in Salt Lake City (Smith Power Products) only charged me a little over $200 to find it and take care of both items.

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WW Williams would be the Allison shop in Phoenix.  They're off the 17 truck route and 19th ave, or they have mobile service.

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If you haven't already checked you might want to make sure you are getting a signal from your high low switch if so as others have said could be throttle position sensor. If you can get a wiring diagram of the inputs out puts at the ECM you could check there also, or get some one to plug into it it, should show if it has a bad sensor or no input.

Good luck some times these problems can drive a person crazy.

 

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On 5/30/2021 at 6:20 PM, Joblo said:

On a manual transmission setup you have cut out switches on the clutch and throttle pedals. 
 

since you don’t have a clutch pedal don’t worry about it. However even if the throttle is slightly pushed it can deactivate the jakes. I am assuming your pedal is a potentiometer. So the range of voltage will be minimal to tell the engine turn off the jakes because I’m pushing the gas. However it could be as simple as the pedal sticks slightly or the potentiometer is failing or not going to the off/decel voltage. 

I thought about the potentiometer maybe failing but I had ruled it out because the smallest press of the gas pedal and the transmission will go back to the current gear. Off the gas and it will go back to selecting 2nd and start its downshifting again.

I will take a look at it closer now that you have brought it up.  Maybe I can find a diagram on how to ohm it out.

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4 hours ago, GBR said:

If you haven't already checked you might want to make sure you are getting a signal from your high low switch if so as others have said could be throttle position sensor. If you can get a wiring diagram of the inputs out puts at the ECM you could check there also, or get some one to plug into it it, should show if it has a bad sensor or no input.

Good luck some times these problems can drive a person crazy.

 

I’m for sure going to find it.  There is a China version of the insite Cummins software that’s about $250.  Figured I would order that up after exhausting all my other options and see if maybe the PO turned it off.

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6 minutes ago, r3meyer said:

I’m for sure going to find it.  There is a China version of the insite Cummins software that’s about $250.  Figured I would order that up after exhausting all my other options and see if maybe the PO turned it off.

I didn't think about PO turning it off that could very well be a possibility. We have Cat ET at work as most of our trucks are Cat powered but the new ones have Cummins so I will be looking into Cummins programing, we have it on our Snap On reader but just isn't complete enough. Also if your transmission shifts down to much they can reprogram it to auto to 4th and then manual below that, had to have mine done to stop from slamming into 2nd and then upshift because it was over revving engine. If you end up with the China programing post up what you think of it, cost is more attractive than Cummins Insite.

  

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2 hours ago, r3meyer said:

I’m for sure going to find it.  There is a China version of the insite Cummins software that’s about $250.  Figured I would order that up after exhausting all my other options and see if maybe the PO turned it off.

Do you live in the San Diego area? If so, I have access to a scanner

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15 hours ago, Snap-On Chris said:

Do you live in the San Diego area? If so, I have access to a scanner

Im in Phx AZ.

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  • 2 weeks later...
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Davenport. Diesel in phx does all my moho stuff. Fair priced and knows there stuff. He just adjusted my valves and my Jake is working a little better don't know if it's me or that has something to do with it

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On 6/1/2021 at 1:09 PM, r3meyer said:

Im in Phx AZ.

I have Insite if you know what to look for, or need to change.

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  • 3 weeks later...

Ok so trying to think outside the box while I figure this issue out.  Would I do any damage to the engine while going down a grade if I were to manually give the engine brake solenoids the 12V they need to operate?  I have a remote controlled 12v relay that I could activate from the driver seat and then disengage before applying gas.

I floated this idea over on IRV2 and you would think the world was coming to an end.  The transmission and engine are both doing what they normally would when trying to slow down I am just missing that 12v signal to the solenoids and think this could solve it for short future while I hunt for the main problem.

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If you manually close the solenoids does the trans downshift too? Or does it downshift like it’s supposed to already but you just don’t actuate the solenoids. 

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3 minutes ago, Jtmoney714 said:

If you manually close the solenoids does the trans downshift too? Or does it downshift like it’s supposed to already but you just don’t actuate the solenoids. 

Correct.  When I activate the engine brake switch at the drivers seat the trans starts it down shift program.  IE its trying to head to 2nd gear.  I had a buddy drive the coach and activate the switch.  I manually jumped the solenoids while in the engine bay with the hatch open and the engine brakes activate and you can feel and hear the coach slowing.  I disconnected it before he hit the gas peddle and everything seemed to work as normal.  Trans went back to 6th gear selected and operated normally.  This is why I think I could I could jump around this for the time being.

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On 5/25/2021 at 1:33 AM, L.R.S. said:

You have a real Jacobs engine compression brake and can select the number of cylinders it will use?  How loud is that?  I drove trucks for years and remember a Detroit 2 stroke 8V92 was LOUD AF when set to High.   I only have an ON/OFF switch for my exhaust brake in my 2001 Holiday Rambler Cummins 350 ISC.  I wondered if my exhaust brake was working correctly because when it's ON the display shows "2".  I did some research, I think on iRV2, and found out that the 2 is preset from the factory.  I think I remember it meaning it is trying to downshift to 2nd gear.  It can be set to 4th by a dealer or shop.  

My Dutch Star with the ISL and now my Mountain Aire with the ISX both have 'real' Jake brakes with low/medium/high switches.  You can barely hear it.  I assume they go 2/4/6 cylinders for low/medium/high but never looked into it much.  I do know that they work really, really, REALLY well.  

You are correct about the "2" meaning the Allison is being told to go to 2nd, when RPMs allow it.  That way it'll downshift as aggressively as possible when the Jake brake is on, bringing the coach's speed down as quickly as possible.  I don't think I'd want to change it to 4th...

 

-TJ

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4 hours ago, r3meyer said:

Correct.  When I activate the engine brake switch at the drivers seat the trans starts it down shift program.  IE its trying to head to 2nd gear.  I had a buddy drive the coach and activate the switch.  I manually jumped the solenoids while in the engine bay with the hatch open and the engine brakes activate and you can feel and hear the coach slowing.  I disconnected it before he hit the gas peddle and everything seemed to work as normal.  Trans went back to 6th gear selected and operated normally.  This is why I think I could I could jump around this for the time being.

I’d say that it wouldn’t hurt it. I’d almost rather have a momentary switch I could press to trigger when I want. I’d just make sure there isn’t any goofy timing or tune thing that happens. 

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On 6/1/2021 at 10:57 AM, r3meyer said:

I thought about the potentiometer maybe failing but I had ruled it out because the smallest press of the gas pedal and the transmission will go back to the current gear. Off the gas and it will go back to selecting 2nd and start its downshifting again.

I will take a look at it closer now that you have brought it up.  Maybe I can find a diagram on how to ohm it out.

As stated earlier  the output to Jake solenoid is from the ECM. If you are not getting  the output you will need to verify output with a scanner/computer. It is possible one or more parameters are not met to engage Jake. Also the Trans must engage lockup to allow engine braking. If the RPM fall below around 1000 it will not engage.

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10 hours ago, tjZ06 said:

My Dutch Star with the ISL and now my Mountain Aire with the ISX both have 'real' Jake brakes with low/medium/high switches.  You can barely hear it.  I assume they go 2/4/6 cylinders for low/medium/high but never looked into it much.  I do know that they work really, really, REALLY well.  

You are correct about the "2" meaning the Allison is being told to go to 2nd, when RPMs allow it.  That way it'll downshift as aggressively as possible when the Jake brake is on, bringing the coach's speed down as quickly as possible.  I don't think I'd want to change it to 4th...

 

-TJ

Out of curiosity, and extreme boredom, I did some more reading on iRV2.com and found a couple threads with guys that had the dealership/maintenance shop switch the Allison PAC brake to 4 and they hated it and eventually switched it back to 2.

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