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Discussion Starter #1
What is a dump valve? As far as brakes are concerned. My ABS light has been on on my 95 since I owned it. Pulled the code today flash flash pause flash flash flash flash flash, code 25 right? Haynes says defective right front dump valve. How to fix this? Thanks!!

J
 

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yo,
outlet valve is called a dump valve; it and the other valves are in the 4WABS Hydraulic Control Unit (HCU)
One of the Troubleshooting procedures is to test that valve's coil for resistance; see below for ohms.

25 Front Right Outlet Solenoid Valve Concern Diagnosis
Possible Source(s):
� Intermittent open Circuit 498 (PK).
� Terminal backout in anti-lock brake electronic control module connector Pins 17 and 15.
� Terminal backout in anti-lock hydraulic control unit connector Pins 1, 4 and 5.
� Open or shorted outlet front right valve coil.
� Open or shorted driver in anti-lock brake electronic control module

Go thu this by Steve83 http://www.supermotors.net/registry/media/280776
4WABS Closed System it also shows where the HCU is under hood, driver's side

In the 4-Wheel ABS the red brake light is not monitored. This is because the system is a "closed" hydraulic system.

If you are in a 4WABS stop and a wheel goes to lock, the isolation valve will activate, just as with the RABS. No more fluid will be directed to the wheel that may lock up.

If it still appears that a wheel is going to lock, the dump valve will open, allowing the fluid to bleed to an accumulator similar to the one in the RABS.

However, in this "closed" 4-Wheel ABS there is a pump that will move fluid from the accumulator, back to the line between the master cylinder and the hydraulic control unit, where it may be recycled again through the isolation and dump valves.

Since the fluid in this closed system is still available to both front and rear circuits, the number of cycles is unlimited. The closed 4WABS isn't shut down when the low fluid light comes on, as was the case with the open system.
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by Steve83 (Steve, That dirty old truck) at SuperMotors.net - Use this & to check for:
r Intermittent open Circuit 498 (PK); & Open or shorted outlet front right valve coil; Terminal backout in anti-lock brake electronic control module connector Pins 17 and 15. & Terminal backout in anti-lock hydraulic control unit connector Pins 1, 4 and 5.

4WABS Circuit
The front inlet valves should read 5-8 Ohms each; all other valves should read 3-6 Ohms each. Sensors should read 800-1400 Ohms across the pins, and more than 10 Ohms from the metal shell to either pin. The pump motor sensor should be 5-40 Ohms. The acceleration sensor is a group of switches which should be closed when the truck is stationary & level. When the sensor is tilted ~1" up in the rear, pins 2-3 should go open; when tilted ~1.5" up in the front, pins 1-3 should go open.

4WABS Wiring Diagram in 93-96
Source: by Steve83 (Steve, That dirty old truck) at http://www.supermotors.net/registry/media/280767
 

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Discussion Starter #4
Thanks! Very helpful!!

J
 

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yo,
I forgot to mention that for all Resistance tests, battery needs to be disconnected; remove Negative cable at battery
 
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