1988 3500 4x4 brake problem

[NewHere77]

Hi there, and thank you for letting me join.
I have a 1988 3500 4x4 with brake issues.
The indicator light on the dash is flashing 3 short flashes 1 long and then repeats. When I apply the brakes there is no slow down time, it just slams to a stop.
Can anyone tell me what the flashes mean, and what needs to be done to fix my brakes?
Please and Thanks?!

[ChevyTech]

The RWAL / ZRWAL system is blinking the dash brake light.
RWAL stands for REAR WHEEL ANTILOCK. RWAL was standard equipment starting in 1988 on full size trucks.

What you describe is a code 4
Code 4 = Grounded or closed RABS or RWAL valve switch

Look at the two threads in the links below, to give you some background.

http://67-72chevytrucks.com/vboard/s...d.php?t=399153

http://67-72chevytrucks.com/vboard/s...d.php?t=302690

Let me know if your truck is a dually with rear disc breaks, or if it has rear drum brakes. There is two different systems depending on if it has rear drum or rear disc brakes.

You can search my user-name and RWAL to find information on this topic.

Welcome to the board!

[NewHere77]

Thank you for your quick reply!
My truck is not a dually, and has rear drum brakes.

[ChevyTech]

You're welcome.
With your truck having drum brakes you can use the RWAL information in the links and in other posts of mine.

GM RWAL code 4 testing:
NOTE: the Isolation/Dump valve is out by the master cylinder. It is an aluminum block with two round things and has brake lines going to it.
Key OFF, unplug the Isolation/Dump valve, test the resistance between terminal C of the Isolation/Dump valve and the body of the valve. This tests the switch. Also test the resistance between terminal C and terminal D of the Isolation/Dump Valve. Both tests should measure greater then 50,000 ohms. If they measure less than this the Isolation/ Dump valve is bad.

If it tests ok, with the connecter still unplugged, turn the key on, measure the voltage between terminal C and terminal D of the wire harness that connects to the Isolation/Dump valve. It should be 4 volts or more. If it reads 0 volts replace the RWAL ECU (flat black plastic box by the master cylinder). If it tested 4 volts or more, and it sets a code 4 again, after clearing the code, replace the Isolation/Dump valve.

Let us know how it goes.

[ChevyTech]

Isolation/Dump valve Wire colors
Terminal A = Green wire
Terminal B = White wire
Terminal C = Blue wire
Terminal D = Black wire