Troubleshooting stalled AutoPark parking brake pump syndrome – – Symptoms of a stalling pump
When the RGS (aka pump motor switch aka Rotten Green Switch) fails, it most often (maybe 90 percent of the time) does so with the switch contacts “locked” ON. Only on rare occasions do we see the switch fail in the “open contacts” condition.
Normally, when the gear shift lever is moved from PARK to some other position (reverse, drive, neutral), the RGS will switch the pump on until the system pressure reaches 1600 psi and then the RGS will go to the open contact condition, and shut the pump off. The pump will stay off until the pressure has dropped to 1200 psi or lower, and then it will cycle and the pressure runs back up to 1600 psi. This hi pressure is needed to RELEASE the parking brake so the coach can roll.
When the RGS fails in the locked ON condition, the AutoPark lite on the dash will stay ON as the pump continues to run (or tries to continue running) beyond the 1600 psi point when it would normally shut off. We don’t know just how high the pressure goes under these circumstances, but expect it could be in the 1800 to 2000 psi range. In the huge majority of cases, this very high pressure will soon rupture the hydraulic integrity of the RGS, and it will begin to leak oil – – eventually leading to total loss of the ATF in the reservoir. At some point in this process there will no longer be enough ATF to keep the parking brake released, and it will apply the parking brake – – Regardless of where you are (fast lane, driveway etc.)
The above more or less describes a typical RGS failure. BUT, sometimes (maybe in 10 percent of the failures) the system will exhibit a quite different set of failure symptoms. We call this “the stalled pump syndrome.” In these cases, the AutoPark lite on the dash goes ON and stays on, but the pump can be heard to be cycling – – perhaps running for 30 seconds, then stopping for 2 or 3 minutes, then running for 30 seconds etc. NOTE: These times are likely to be somewhat different in each individual case – – Actual times are somewhat arbitrary.
Regardless of the exact times and intervals, these are usually symptoms of a stalling pump. What is happening is that the pump pressure (because of the locked on switch) is going way beyond the normal shut-off level, and the motor is simply stalling under the excessive load. When the motor stalls, the current applied to it is greatly increased – – This extraordinarily high current will cause high temperatures in the motor, which in turn will trip the thermal overload protection switch that is built into the motor. The motor will cease trying to run as long as the thermal switch is tripped – – BUT, after a few minutes or so, the motor will cool sufficiently to allow the thermal protector to RESET. At this point, the motor will again run for a very brief period until it stalls – – thus perpetuating the on/off cycling. All of these comments pertain to AutoPark systems using the Parker Oildyne T-108 pump. Coaches equipped with the MTE pump DO NOT have a thermally protected motor – – As such, stalling the pump motor will rapidly lead to total motor failure with the MTE system. MTE pumps are recognized as being machined from a solid aluminum rectangular block. Pictures available on request.
During this period of cycling, the AutoPark lite on the dash remains ON. Sometimes, close observation of the lite will reveal that it dims during the periods when the motor has stalled but is trying to run. The dimming can be directly attributed to the extra high current being drawn during the stalled condition. A more sure fire way of confirming the stall is to actually place a hand on the motor as it is cycling – – You can often hear/feel the motor trying to run while it is stalled – – until the thermal protector trips.
In many cases, simple replacement of the RGS will resolve all the problems and the system will again operate in a normal fashion. Sometimes tho, the thermal protector in the motor will have failed due to the repeated cycles of abuse. A good electric motor shop can often replace the protector for much less than the price of a whole new motor – – something to consider if such a shop is within a reasonable distance.
So – – – If your AutoPark lite is staying ON as you travel down the road, be watchful for the above symptoms. Regardless of the symptoms noted however, ANY TIME YOU HAVE AN AUTOPARK LITE STAYING ON, YOU HAVE A PROBLEM THAT NEEDS ATTENTION.
Comments and questions are always welcome, oldusedbear