View Full Version : speed switch

01-22-2009, 12:47 PM
I am troubleshooting a tag axle auto engage issue. I am down to the speed switch (that is what the electrical drawing calls it). The Prevost electrical drawing states that at 20 MPH, a contact closure in the speed switch will output (24v) to the tag axle engage solenoid?

Where is the speed switch located?? What does it look like?



Joe Cannarozzi
01-22-2009, 06:43 PM
What is going on that shouldn't be?

Never heard of such a device. Have you looked for wires and or solonoid comming off your air toggle for the tag?

01-22-2009, 07:49 PM
On my vintage coach, the 'speed switch' is an electronic circuit board that receives a speed signal from the Allison computer and creates a pulse signal to drive the speedometer and odometer. Additionally, an output changes from 24 volt to 0 volt once the coach reaches 20 MPH. This signal is used to disengage (unload) the tag axle when the steering wheel is turned sharp enough (do not know the degrees) to activate a microswitch mounted on the steering gear located below the the drivers seat (bottom compartment). The micro switch in mounted such that when slight turns are made, the tag will not unload. However, when turns are great enough to activate the micro switch, and the coach is under 20 MPH, the tag will unload thus allowing the coach a smaller turning radius. Above 20 MPH, the tag does not unload. Coaches with rear stabilizers (not many) use this circuitry, with a few additional components to operate the stabilizers.

This little piece of circuitry, the speed switch, is located in the lower compartment, under the drivers seat. It is a small (4" X 5") plactic box, with a clear plastic cover.It is mounted on the bulkhead in front of the front tire wheel well.

As simple as this may seem, it has taken me all day to figure this out. Whew!

What started this exercise was the discovery of a leaking solenoid (very minor leak) on the tag axle engage/disengage solenoid. The solenoid for my coach is no longer offered, thus figuring out how to retrofit a norgren lead to all this detail.

It only took me all day to figure this out. But today in Livingston, Texas the weather was beautiful. The sun was shining, with a high temperature of 76 degrees. A nice day to be out.