What to do when your Auto Park brake is in a over-travel alarm condition?

What to do when your Auto Park brake is in a over-travel alarm condition?

Its important to realize that the AutoPark brake shoes, adjuster, etc., all sort of float on the backing plate.  Shoes themselves pivot (when applied) on the ends of the adjuster at the top.  The long lever which attaches to the cable, when pulled to the left by the cable, spreads both shoes outward with that horizontal bar plus the lever also applies equal pressure to the left shoe as well as the right shoe – – you can’t see the lower end of the lever where it pushes on the right shoe.  If the shoes are badly worn, and they are farther from the drum, that lever has to move farther to apply the brake.  By the same token, if the adjuster is not properly set, the shoes will also be farther from the drum and again, the lever has to move farther to apply the brake.  Because of the geometrical design of the system, the lever position with the brake released (which is what you see in the photo) does NOT move hardly at all as you widen the adjuster to move the shoes closer to the drum.

So – – when you look at a picture of the actuator shaft with the brake applied,  the amount of space you see between the jam nut and the actuator bracket face DOES DEPEND on the star wheel adjustment, AND the condition of the brake shoes.  This is exactly why worn shoes and/or lack of adjustment will lead to over-travel.  If you guys examine the back end of your actuators, you will see a switch mounted there.  It is called the “over-travel alarm switch.”  If you get too much slack in the cable due to EITHER worn shoes or lack of cable OR star wheel adjustment, then the actuator shaft will go in too far when the brake is applied.  This will result in:

1.  The jamb nut moves closer to the actuator bracket face when brake is applied

2.  The hydraulic piston will at some point over-travel and BOTTOM OUT which will in turn, set off your over-travel alarm.
The fix for this is to replace the shoes if they are badly enough worn – – If you still have enough lining, you can take up the slack with the star wheel adjuster.  EITHER OF THESE ACTIONS WILL PULL THE SHAFT BACK OUT OF THE ACTUATOR IN THE BRAKE APPLIED CONDITION.  Pulling the shaft back out will shut off the over-travel alarm and the piston will no longer be bottoming out on the switch.

Question and comments are always welcome, oldusedbear

 

 

Posted in: Category 6 - - AutoPark Brake; Bleeding the brakes, adjusting the shoes, cable adjustment, troubleshooting and understanding how it works.

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