Ghetto 4WABS HCU Bleed Procedure

[SigEpBlue]

For the longest time, I've felt that my brakes were too soft and not as responsive as they should be. Particularly during emergency stops, when the 4WABS would become active, the pedal would drop to the floor, requiring me to pump the brakes occasionally. After replacing almost all of the brake system components that were faulty and/or worn, the only thing left was to bleed the HCU. I had removed it once from the truck to rebuild the electric motor's brush holder, so I knew there was air in the HCU from that.

I also know Ford has a special tool that goes between the ABS controller and harness, and it has nifty little switches for grounding the appropriate circuits according to the bleed procedure. Taking Ford's HCU bleed procedure, understanding how the system works, and using a few schematics, I've come up with a ghetto way to bleed the HCU, and I confirmed that it does indeed work just as well as the factory-approved method.

A couple of notes: You will need an assistant, and speed bleeders will be a bonus. And don't be afraid of sparks when connecting this stuff. Check out Steve83's SuperMotors.net page OR download the Ford Service CD and figure out which circuit is which. I don't want to copy all of the stuff from there for ya, as it's not too difficult to figure out if you can understand what's going on in this bleed procedure. If you can't wrap your head around it, please don't try it. Remember, this is the brake system you're dealing with, so not only YOUR ass is at stake, but the asses of those around you when you're on the road.

1. Perform conventional brake system bleed.
2. Prepare an appropriate method of connecting individual circuits and wires to ground. This connection must be sound, with very low impedance to the battery negative terminal (I used some stainless hat pins and alligator clips on test leads).
3. Turn the ignition switch to the ON position.
4. While an assistant presses down on the brake pedal:
   • *a. ground circuit 599 [PK/LG], pin 19. (4WABS Relay #1)
   • b. ground circuit 510 [T/R], pin 27. (Rear Dump)
   • c. ground circuit 498 [PK], pin 15. (Right Front Dump)
   • d. ground circuit 496 [O], pin 26. (Left Front Dump)
   • -- the pedal will fall.
5. Disconnect the ground connections made in parts 4b, 4c, and 4d. *Leave the connection made in part 4a. Allow the brake pedal to return.
6. Repeat steps 4 and 5 once more.
7. Ground Circuit 539 [PK/LB], pin 29 for one minute. The ABS pump motor should run for the entire minute.
8. Perform conventional brake system bleed.

Thanks go to Steve83 for his diagrams on his SuperMotors.net page. Hopefully this will help others with HCU issues and/or soft pedals for no other reason than the HCU needs to be bled. I will argue against Ford's advice about bleeding these units: they should probably be bled regularly, and bled/flushed when replacing the rest of the system fluid. In my opinion, you're not going to get the old stuff out of there unless you perform this procedure, and since the fluid is hygroscopic, it may lead to failure of the HCU from corrosion. Based on what came out of my bleeders, I think the fluid in mine was probably the original stuff!

[stan the man]

Great trick Sig! Do us late model guys have to bleed the ABS HCU if we install a new master cylinder? Logical sense says yes, but its always better to SEARCH and ask

[Steve83]

There's absolutely NO need to bleed the HCU unless you KNOW it has air in the ABS sections. For all regular brake service situations, the HCU is effectively NOT there,and can be ignored. Air in the ABS sections of the HCU will NOT affect pedal feel or normal brake performance.

[bluezy_susie]

THanks for the help. I'll make sure who ever does mine knows this.

[jimford6]

Much appreciated Sig, this info will be a great help........