I've recently swapped out some 05 Lariat wheels for my 99 XLT wheels. Since then I've noticed that the ABS light has come on. Just the same it might have been on before then but just didn't notice. The tires are LT285/70R17 making them about 5 % larger in circumference than stock of LT265/75R16. Question: could the 5% larger tires somehow confuse the VSS into thinking there is an out of range condition that would cause a logic fault based on the now different odometer, tranny shift and abs speed sensor input deferences?? Also when I was checking abnormal abs condition the abs light was not illuminated one of the times after shutting off and starting back up so when the brakes were applied the abs kicked in for no reason when I applied the brakes and essentially could not easily stop, chatter pulsing the brakes - but sounded abit different than the few times it has kicked in under real skidding conditions. It has only done that once - thank god
Yeah the the speed sensor's output no longer matches what the wheel sensors say. The ABS system expects the wheels rpm to be X when the vehicle is going Y mph. the system can be recalibrated by the dealer.
Ya I thought it might be a potential issue. As it turns out the stock '05 tires just slightly rub the frame in a full tight turn and the stops are not adjustable on a 99 so while it's down at the shop I just told him to put the stock wheels on and go from there just to eliminate the larger tires causing the fault. Will get back to the group with the outcome.\ Thanks, Toby
Wheel size was not an issue in this case although it could have been if the size difference was larger - according to my tire dealer. The rear ABS sensor was faulty and no other indications other than the ABS light on was a factor, that is, no weird stuff going on with the speedo etc.
good to hear the issue is resolved, and thanks for posting it. Pet peeve of mine, too many come asking for help and never come back with what the fix was.
The size of the tires does not matter to the ABS if all of the tires are the same size.
ABS looks at each tire and expects to find it going the same speed as the others, and it looks at the speed sensor feeding the speedometer expecting them all to match.
When the ABS light comes on, this means that one or more speed sensors have stopped supplying input, and the system is no longer able to understand accurately if any tire has stopped while the others remain turning. Normally, a failed sensor will cause the ABS light to come on in a random fashion. When you turn the key ON, the ABS does a self test including the light ON, then turns the light OFF. The light remains OFF as long as the inputs are present. In a faulty sensor scenario, it is typical that the input will exist for a random duration then go away, when the input is gone, the ABS light comes on telling you that the system has become unstable.
When the system works properly, one wheel will stop turning while the others remain in motion. The system detects this and releases the braking pressure on that tire so that it can turn. There could be a lock up on more than one tire, but the same result happens in all cases -- braking pressure is released on the locked tire. The speed sensors send a clock pulse (square wave) to the computer, and a stopped tire will have a very long square wave relative to the tires that are still turning. If all of the tires are the same size, then all of the square waves will also be the same size, so the ABS would not care.
A faulty speed sensor will generally not be known until the vehicle starts moving, so the ABS light should turn on with the ignition, turn off after the self test, then turn on again once you start moving. A completely failed sensor may cause the ABS light to remain ON after the self test, but in my case the light came on at random points in any given trip. Some trips it would not come on at all, other times it would come on as I pulled out of my driveway. Frequently it came on after I got on the freeway. It only came on when the vehicle was moving, and I could turn it off by killing the ignition then restarting the car, even while the car was moving.
Another point where the ABS light can be set ON is the valve body where the ABS action actually takes place.
I don't recall if the OBD II system keeps track of ABS issues or not, but it is a good place to start.
The OBDII does not keep track of ABS issues. The only reason I'm fairly certain about this is that for the last year, my ABS light comes on and stays on after the truck starts moving. I'm pretty sure the sensor is shot, but due to funds and the fact the truck only sees about 2K a year I haven't gotten around to getting it fixed. I live in NYS and if anything is recorded in the OBDII, the vehicle does not pass NYS inspection. When I scheduled an inspection, the guy at my garage asked over the phone if I have any SES or Check Engine lights on, because if I do, don't bother with the inspection. I told him only the ABS light was on, he said "that's ok, that doesn't get counted because it doesn't get recorded"....
Well, the ABS will not throw a Check Engine light, that much is true. OBD II does track stuff other than the engine, but I was not sure if that included the brake system or not.
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