Coach & Powertrain: Grade Braking
May 15, 2012
Filed under RV Tech Q&A
I have a 2006 Winnebago Suncruiser motorhome with a gas engine and a grade brake switch to the left of the instrument cluster. Would you please explain how the grade brake works? Also, would the grade brake affect a Brake Buddy? I used the grade brake intermittently coming down a lot of mountains for a couple of hours and the Brake Buddy started sounding several different audible alarms. After a quick stop and retesting the unit, all was fine with no more alarms.
— Sandy Cimarolli, Wolverine Lake, Mich.
The grade-braking system is part of the Allison transmission, and its primary purpose is to downshift to provide engine braking to heavy vehicles on steep downgrades, to reduce overheating and wear on the brakes. The system overrides the gear selector position and selects the next lower gear automatically. The transmission’s software monitors several factors before directing a downshift: throttle position, brake application (brakes must be applied to activate grade braking), vehicle acceleration/deceleration, gradient/load and vehicle speed.
The transmission control module (TCM) calculates the effect of the added mass of a trailer or towed vehicle based on the effects of it on downhill speed and required braking. Your auxiliary braking system may be applying too much stopping power. Ideally you want the dinghy vehicle to neither push nor pull on the tow bar during braking, but rather to match the decelerating motorhome’s braking.
— Ken Freund
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