I have a 1998 silverado four wheel drive truck. I have notice the brakes are ok during the time it is in the two wheel drive. When I put the truck into four wheel, the brakes will start to pulsate to a point that it is not stopping the truck. Once I put the thing back to two wheel drive it does not do it.Any Idea what may be the problem? Four wheel drive is for driving in the snow but if the brakes do not work properly then it is worthless. I have seen other sections on the net showing that chevy trucks during that era is well known for this as other owners has dumped several thousand dollars in it to find that the problem is still there.  thanks  

without codes from the abs computer it is hard to say...

abs uses sign waves and voltages generated by tone rings as they pass the abs sensors...

there have been problems with the wheel sensors failing..

if you can find a near by shop with a tech 1 or tech 2 scanner   they and a few others can look at wheel sensors voltage outputs during test drives.. all 3 or 4 depending on the system..

change the front wheel sensors if you can..  they are not hard to change..   sometimes metal from the brakes get on the magnets and throws the sensors off.. this can be cleaned off...but if there is any damage to the cables... change them..


looks like the 98 4x4 sensors are a lot harder to change...        autozones  repair info pages.. says the wheel speed sensor in in the back side of the wheel hub and bearing assy.. so the caliper and rotor have to come off to get to it...  

Thanks. The ABS light never came on during the time it started to pulsate. I bought a scanner and hooked it up to the system but it never showed any problems with in the system. The brakes will only pulsate while the truck is in the four wheel drive. The other option that the boy done was take out the fuse for the ABS and it did not do the pulsating. But of course the ABS light did come on. I will check and see what all it takes to change the front sensors. The body shop tech sent the truck to a garage after he crashed the truck based on the pulsating brakes as they changed some sensor in the transfer case. I am guessing it was a speed sensor. That change did not help. I am just lost when it comes to the ABS things.  

read up about antilocks over at the autozone.com web site...  look for repair info...

http://www.autozone.com/shopping/repairGuide.htm      enter your year and model... then look at brakes... and then down near the bottom for abs brakes...

and did your scanner come with abs software????    not all ***

the tech 1 and tech 2 scanners were designed to work with these systems...  not just to read codes...  but to look at live data.... and display it...  

Thanks for the info. I will check out autozone's site and see what I can find. When I am home on leave I spend many trips to Autozone. For some reason when I go once there is always two more trips before the day is over. I guess I get side tracked while in there with the items and forget what I went for. I will have to e mail my boy and ask him about the scanners I bought him. I am thinking that it did have the info on the abs, but I could be wrong. I did not buy the cheap ones as it had the internet up-dated equipement to stay current.  thanks again.  

Just got the right side hub off and found the boots were bad. After re[;acing the boots, I found that Napa wanted 100 bucks for the sensor or 185 bucks for a new hub and sensor. Got the right side hub and sensor replaced. Road tested it and found it was much better. As I got to about ten MPH during breaking the brakes did surged a little. With the time I had,,,,,i put a new hub and abs sensor on the drivers side. It works good now. I did talk with a mechanic and he mentioned that it is best to replace hub and all as it could be something internally with the hub causing problems. I did find that the hubs are pressed together. The hub on the passenger side did come apart when i pulled it. So I am sure the hub was past replacment a long time ago. But for about two hundred a side with the new hub and sensor,,it took care of the problem, .