Demco Towed Vehicle Brake System

[Kenkarast28]

Does anyone have experience with using a Demco DUO towed vehicle brake system for hydraulic brakes on a Class A coach with air brakes? I know Demco does not recommend it, but I do not understand why. The Duo system is activated by a brake light coming on and it determines how much braking to apply based up how much the towed vehicle is slowing; essentially the faster it decelerates the more brake is applied and vice versa. The sensor is called an "inertia" sensor. That will work whenever the brakes are applied irrespective of the type of brakes. The concern I have read about is when the "jake" brake is used. My vehicle is a almost new Frieightliner chassis so the brake lights come on then I decelerate using the jake brake. I see no reason the Duo system would "care" whether the deceleration is caused by the jake brake or the air brakes - it will activate when the brake lights illuminate, and the sensor will sense the rate of deceleration and apply the brakes accordingly. I could see there being a concern when using a jake brake if the brake lights did NOT illuminate, in which case the Duo system would not activate, but that is not how modern jake systems work - the brake lights do come on when the jake brake comes on. Thx in advance for your input!

 

[Kirk]

I highly suspect that you are right in your suspicion. As long as the brake lights do turn on, the inertial system should actuate. While it was not a motorhome with air brakes, I towed with an inertial brake system for years and liked it.

 

[Kenkarast28]

For posterity, on another forum it was pointed out to me that the issue is the toad brakes being deployed while the jake brake is activated on a long downhill. This can lead to the toad brakes being worn or even ruined. A solution suggested was to deactivate the brake lights turning on when the jake brake is deployed. Apparently this is something a Freightliner shop can perform, but there is a question of legality. Note: I have not confirmed the possible jake brake deactivation solution.

 

[Gary RV_Wizard]

The Duo inertial does indeed utilize the brake light circuit but the effect of that depends on where in that circuit it is connected. The wire connection from the Jake to the brake light wiring joins the circuit downstream from the air brake treadle valve. If you don't want the Duo activated by the Jake, wire the Duo to the portion of the brake light circuit that is upstream from the Jake connection and install a diode in the circuit so the Jake signal doesn't backfeed into Duo wire. If wired in that manner, the Duo can't activate unless the brake pedal is pushed.

If you haven't installed the Duo yet, make sure the installer understands what you want. And knows how to implement it.

However, I don't think this is a very worrisome thing with an inertial-controlled proportional system like the Duo. Some brake systems are time-based rather than inertial and a long downhill causes ever increasing brake pressure. That will indeed cause overheating if the grade is long enough. Might even lock up the toad brakes. Not so with the Duo, which will brake only lightly in most such situations.

 

[Heli_av8tor]

Holding the coach on a long downgrade at a relative constant speed with either the engine or service brakes involves no deceleration and therefore no braking on the toad with this system.
Whether that’s good or bad is another debate.

 

[Gary RV_Wizard]

Holding the coach on a long downgrade at a relative constant speed with either the engine or service brakes involves no deceleration and therefore no braking on the toad with this system.
Whether that’s good or bad is another debate.

When on a downgrade, the vehicle is being accelerated by gravity, so there has to be some deceleration applied by some type of braking to achieve that "constant speed". The inertial sensor (technically it's an accelerometer) will detect that.

 

[Kirk]

The inertial sensor (technically it's an accelerometer) will detect that.

I'm not familiar with the Demco Duo, but isn't it able to adjust the amount of braking versus the amount of deceleration? What I used was and I always kept it set so that very little braking was applied unless in a hard stop with rapid deceleration.