Blinking brake light...89/3500

[LeeG]

Ok, have 89/3500, have just replaced the fuel pump, water pump, and the radiator. Runs like a top!!! 3 or 4 days ago I noticed the light on solid, now its does this 3 blinks then solid for a second then cycles all over again. I cannot find any reference to this in the manual that I have. Any help would be greatfull!

[ChevyTech]

I see it’s your first post. Welcome to the message board.

It’s the ZPRWAL system that is turning the dash brake light on, and making it blink
ZPRWAL is the system on 1 ton trucks that is much like the RWAL (REAR WHEEL ANTILOCK) system on ½ and ¾ ton trucks.

I have a lot of experience with the RWAL system, but not with the ZPRWAL system. I do know the systems are very similar.

Proceed knowing I cannot remember what all the differences are between the systems and what differences there are in procedures for repairing them.

The system can store trouble codes, but it is only capable of storing one code at a time.

You can retrieve codes without a scan tool.

If the light is blinking the manual states you should still check the codes.

To retrieve codes connect a jumper wire between terminal H and terminal A of the ALDL connector with the key on (engine not running). Watch the dash brake lamp and count the blinks. Sometimes system send out a phantom blink before the code blinks start. It can take up to 20 seconds for the code to start coming out.

Checking the code with this method can set a false code 9. Checking for a code when no code is present sets a code 9.

All that being said your truck has a code 4 by your description of the light blinking.

Code 4 = Grounded or closed RABS or RWAL valve switch
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.


ZPRWAL on 30 series HD models have a different procedure for clearing codes. Pulling fuses will not clear the code.

Disconnect the battery for ten second to clear the trouble code, and if that does not clear the code let me know, and I can give you another procedure to try.