Hello,
I've been using this site for a while but this is my first time actually posting. I own a 98' Blazer LT 4.3L 6 cylinder 4wd. The problem I'm having is on occasion my brake light will come on, usually when I hit a bump. When the light comes on I will also notice on occasion when I brake I will feel a pulsating sensation in the brake pedal. Even though the light will be on the pulsating isn't constant. Now from what I've read in the forums, if I am understanding this correctly, is that the brake light coming on occasionally and the pulsating is from a malfunctioning speed sensor. I've read that them becoming dirty or have a problem with the wires is quite common. My question(s) is this...

1. Where is the location of the speed sensor?

2. Is the pulsating in the brake pedal in fact caused by a faulty speed sensor or could it be something else?

Right now my Blazer is a paper weight in my garage since the fuel pump went out, another common problem from what I've read. The brake light/pulsating problem has been going on for a while now but I figure as long as I'm waiting for the rest of my parts to come in for the fuel pump I'd take care of the brake problem as well. Thank you in advance to any one who can offer any advice and also to everyone for posting all the useful tips I've been using for the past year since I bought my Blazer.

 

You have a minimum of 3 speed sensors in your truck. There may be more depending on what transfer case you have. If you have the 4-button switch in the dash, then you will have 2 additional speed sensors on the transfer case.

If you are only having problems with the ABS system, I would suspect a dirty/faulty wheel speed sensor. The WSS is located in the front hub/bearing assembly at each wheel accessible by removing the caliper bracket (with caliper) and then the brake rotor. Follow the instructions found in the Tech Article for testing, cleaning, & possibly reworking the gap of the sensor.

Getting the ABS system scanned for codes would narrow down the source of the problem for you.

As far as other things which could cause brake pulsation... You could have cracked or warped rotors that are causing the pulsation, but that depends on whether it is a speed dependent pulsation or not.

 

I only have the 3 button switch on my dash. The rotors on the front and back were replaced this past summer so I'm pretty sure they are good. I had taken it to Autozone previously hoping they could scan it for me. Unfortunately they only scan your vehicle for engine lights. If you want your brakes done you have to buy the tester and do it yourself and the unit is far from cheap. Taking apart the hub assembly is going to be a royal pain but I'd like to have this fixed before it starts snowing really bad. I'm going to have to go through the Tech Articles again for the info on the sensors. I did a quick scan of them and the closest thing I saw was front hub replacement. I wasn't even aware until reading your post that you can rework the gap on them. Thanks for the information, it is greatly appreciated.

 

This is the one I was referring to:
Brakes - Low Speed (below 5 MPH) ABS Activation [SPEED SENSOR TESTING & ADJUSTMENT]