Info on retarder foot pedal, pls

[cranetruck]

This image is from the M915 operator's TM...I'm interested in installing a similar foot pedal (item #2) in my 8x8, which has mounting holes for an additional pedal, but it was never used by the military and not covered in the TM's.
So, I'd appreciate some info, is it electric or pneumatic? How is it used with the "range" switch and is there an interlock to disable it when the "gas" pedal is depressed, etc?

TIA

 

[wreckerman893]

The pedal is basically an electrical switch that activates the jakes.
As long as the pedal is depressed, and your foot is off the fuel, the jakes will be on.
The range switch on the dash selects how many cylinders are used in the braking process(2-4-6).
Not sure about the interlock issue.

 

[cranetruck]

Thanks.
The 8x8 uses a hydraulic retarder, so the range selector wouldn't apply, was wondering about that...it's enabled by a lever on the transmission. Don't know if it's an on/off type of thing or if it's a gradual engagement. More fun stuff to learn.

 

[area52]

I don't know if it applies to your application or not but the Jake brake on my 816 has a switch on the cable from the accelerator pedal so when the pedal is resting on the stop, the contacts are made up to allow it to actuate - basically no gas = Jakes can brake.

 

[WillWagner]

http://www.jakebrake.com/service/pdf2/015626.pdf

Page 20 has a good diagram of the foot activated switch installation.

 

[M543A2]

Bjorn:
Hello! I am assuming that you have an Allison transmission. The lever to a valve on the side of the transmission activates the valve, controlling the retarder action. The valve lever was usually connected to a pedal on the floor board that, when depressed, actuated the retarder. The retarder is a vane "pump" as shown in the diagrams. In the exploded view, it is item 2. In the cross-section view, it is item 4. When the retarder is activated, oil is pumped into the retarder cavity, where the rotating vanes cause a resistance by churning oil against vanes on the housing it is in, resulting in the braking action. Releasing the pedal evacuates the retarder cavity, allowing normal operation.
The diagrams are from the cast iron versions I have here, but I will assume later models worked the same way.
Regards Marti

 

[cranetruck]

Thanks Marti! The operation matches what I have from the TM, so it's an ON/OFF operation then?
The retarder was not used according to conversations I have had with the drivers of these vehicles, but the trucks had prepunched holes for another pedal which is the same hole pattern as used for the accelerator pedal.

I'm thinking about using a small air cylinder to activate the retarder, any thoughts on that?

Edit: Added images to show similarity of MT-31 and the TX-200 series. Item #12 in the TX-200 pic is the hydraulic retarder rotor.

 

[M543A2]

Bjorn:
Yes, the operation is on/off, sorry, I should have answered that. The TX-200 appears to have the same construction for the retarder.
We are considering an air cylinder shift for a conversion we are making in a GMC 5500 truck. It will be a lot simpler than trying to get mechanical linkage to work on a cab-over truck! One definitely wants to use an air valve that automatically returns to the original position (retarder cylinder shifted to retarder off position) when pressure is released off it with the foot. This will prevent the retarder being inadvertently left on. Foot valves of this kind were used to activate punch press clutch dogs.
Regards Marti