I have a 1989 Cherokee Laredo 4WD automatic 4.0L (160K miles) that I picked up recently and before I fix the body damage, I noticed the ABS hydraulic pump comes on and runs for 10 seconds, then stops for 15 seconds and then repeats. This happens with the engine running in park or drive and whether or not you are stepping on the brake pedal. I figure that this is quickly wearing out the (expensive?) Pump, so maybe I should just part the car out.
Is this normal or acceptable for the pump to be running so frequently? If not, is there an easy fix (replacing the pressure sensor switch)?
I would check for codes before going too far. There is a pressure switch on the accumulator block the will signal the ECM to deactivate the ABS system if the pressure drops below 1050 PSI. The Pump assy. Will cycle on and off to keep the pressure within the accumulator between 1650-2050 psi. IF the switch deactivates the system from low pressure it will throw the yellow abs light. Unless the yellow light is not working, I'm not sure that there is a problem. The light will come on when you first start the truck and illuminate for a few seconds while it is performing it's self tests to determine if there is a problem. If no problems, it will shut off.
The worst case scenario is the ABS system isn't working, but your hydrolics brakes still will work.
August, 18, 2007 AT 8:21 AM
Thanks for the reply! I'm not sure of what you are saying. Are you saying that as long as the ABS warning light is out, that it's OK for the pump to cycle on and off frequently, even when the brakes aren't being touched? I wish I had a code checker for this (all I have is an OBD II code checker - would that work).
This car was given to me after an accident wrecked the front clip. I don't want to throw repair money ($300) on the damage if I know that a $1200 brake repair might be next. I can't drive it to a shop for a test (though maybe trailering it could be done).
Thanks again for your advice!
August, 18, 2007 AT 8:48 AM
I believe you have the Bendix type3 abs system in your truck. This unit has an actuator piston in the hydraulic unit that goes bad and creates excessive clearance. This will not allow pressure to build enough and or allows it to leak off creating the frequent pump cycling you mentioned. If you contact a Chrysler dealer you may find a lifetime replacement bulletin exists for this vehicle due to the ABS failure. If not, the fix it yourself route will be to replace the ABS unit or you may be able to replace the actuator piston in the unit. This will stop the pump fron constant cycling and restore proper abs operation.
August, 18, 2007 AT 11:35 AM
I'm not sure that system has the Bendix-3.
But that isn't to say that the same logical doesn't apply that Robert is saying.
The information I am getting is from Mitchell on demand software. Often I have found that infomation obtainable that the dealer releases is always accurate, especially from chrysler.
You would need an obd 1 scanner for the codes. Not to say there are definitly any in there.
Let me do a little research and I'll get back to this with what I found.