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Old 07-08-2019, 10:59 AM   #1
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GaDawgFan's Avatar
Join Date: Mar 2007
Location: Dacula, Georgia
Posts: 309
Engine Brake/Downshifting Issue

I have a Caterpiller 3126B engine with an Allison MD3060 Transmission in my 2000 Itasca Horizon. Recently I have noticed that the engine brake (periodically) has not been engaging when I let off the gas and the engine does not downshift to 2nd gear as it used to.

This is not a continuous issue. Last week we left our home in North Atlanta to go to the north Georgia mountains. The engine was not downshifting coming down the mountains so I manually downshifted to second. Then, when we returned home, the engine brake worked correctly and everything downshifted properly.

I need some advice as to what may be the issue. I will be honest, I have very little knowledge when it comes to the engine and transmission so please dumb it down for me, lol. This is a 19 year old coach with less than 60,000 miles on it and I just want to address this before it leads to bigger problems.

2000 Itasca Horizon 36LD

300 HP Cat Diesel
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Old 07-08-2019, 11:11 AM   #2
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Join Date: Apr 2011
Location: Powell River, B.C.
Posts: 18,515
Exhaust brake operation depends on several parameters .

The power through switches and, a zero reading from the TPS is the most important . I've had both rough trans shifting and exhaust brake issues traced back to a rusted hinge ( from carpet shampooing ) on the throttle pedal pivot.

Hopefully resolved , now that there's vinyl tile in the passenger area of my coach and around the pedals.

I hope that's all your problem is.

99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
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Old 07-10-2019, 05:43 PM   #3
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Location: Sarasota, FL
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It sounds like you don't have an Engine Brake but instead have a PacBrake Exhaust Brake - 2 different animals.
I too have the PacBrake (PB) and have inconsistent operation. When I start off in the morning the PB works perfectly but later on, maybe a couple hours, it doesn't work when I need it to but does work when I don't really need it, like to slow down the coach while on Cruise Control. I have to start downshifting and then I'll see the tranny show a 2 and I know that now it's working. I suspect that the Throttle Position Sensor (TPS) is flaking out, even though it is relatively new. My coach still has a cable going from the fuel pedal all the way back to the engine - yours may be electronic (I have Cummins).

What I did was cut the correct wire from the Transmission Control Module (TCM) and insert a SPDT switch so I can route the TCM signal or a ground thru a foot pedal to the Vehicle Interface Module (VIM) that controls the power to the PB. When the PB system isn't working correctly I flip the switch and step on the foot pedal while starting the downshifting process. The system usually kicks in during this process.

If you want to learn more let me know.
If you want to have your TCM (inside your tranny control on the dash) checked out contact https://www.transmissioninstruments.com/. I sent mine in and they tested, found no problem and there was no charge except for shipping. Turn around was quick.
Rick and Larrie Dee
1997 40' Newmar London Aire DP CTA 8.3 (Mechanical) 325 Spartan MM
Bringing her back to her glory.
'08 Jeep GC Overland.
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brake, engine

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