why does my abs light came on and off

If only the ABS light comes on, that usually indicates there is a fault condition somewhere in the ABS
that the ABS computer is detecting. It is usually an electrical fault, such as a defective, defective power relay for the ABS module or pump, or, in some cases, a lower than normal level in a dedicated ABS fluid reservoir. If there are no other brake system warning lights on, the vehicle will have a non-operational ABS ,. The vehicle should be safe to drive to a shop as long as the vehicle owner/operator keeps in mind that in the case of an accident, there will be no ABS operation. With no ABS system, the vehicle can skid out of control in a panic stop, especially on wet or slick pavement. lights on with the ABS light, then there could be something wrong with the basic service. It could be something serious, such as a hydraulic problem or a serious and/or pressure loss. In this case, it is very likely that the vehicle is not safe to drive to a shop. The safest thing to do would be to have the vehicle towed to a shop. If there is a basic service problem, then there is the real possibility that the vehicle could experience a brake failure that could endanger lives. You have two options when your ABS light stays on. The first one is to find a shop with a scan tool that will talk to your ABS controller.


The dealer will have one, as will some aftermarket shops. For a modest service fee ($50 to $100), a technician will interrogate your ABS controller and look for a trouble code stored in memory. This code will at least give you some idea of where to look. But if you'd like to tear into it yourself and maybe find something simple, you'll need a service manual specific to your car or truck, and simple shop tools, including a high-impedance multimeter. The service manual is extremely important if you can't get a paper manual, try subscribing to alldatadiy. com for service data you can download. On some vehicles, you can access diagnostic trouble codes stored in the computer without using a scan tool. Usually this means pulling a connector and shorting two pins with a small jumper. The ABS warning light will blink on and off in a pattern, corresponding to a trouble code or lack thereof. Obviously, you'll need a shop manual to attempt this. Odds are you won't be able to talk the service manager at the dealer into letting you photocopy his manual but it's worth a try. Much of this data is available online check popularmechanics. com/autodata for suggestions on where to look for service manuals. Let's say you don't have the trouble code and no information to go on other than an illuminated ABS light.


You did check the fuse for the ABS unit, right? This fuse may be in the fuse panel inside the passenger compartment or underhood. Let's not skip the obvious. Fuse okay? Check the service manual for the voltage and resistance values on various pins and sensors. The main harness to the ABS controller is one place you'll be checking, so while it's apart, give it a shot of aerosol contact cleaner. Inspect carefully for any signs of corrosion remember that the signals traveling down some of these wires are only millivolts and almost any resistance is a major impediment. If you can, check the resistance across the wheel speed sensors. If the ABS controller looks A-Okay, and unplugging and reinserting the main harness didn't help, it's time to eyeball the wheel speed sensors especially if your ABS issue started right after a trip through that snowbank at the mall or after a high-speed trip down a gravel road. You may have damaged the wiring leading to the ABS sensors or even the tone wheels or sensors themselves. Tone wheel? The ABS controller needs to know how fast each wheel is rotating. Somewhere on each wheel bearing assembly or axle is a toothed wheel, and there's a magnetic pickup positioned immediately next to it. (Exception: Some rwd vehicles use a three-channel system with the tone wheel built into the rear differential. ) Because this assembly is out in the open, it's prone to damage from foreign objects.


Loosen the lug nuts or bolts, block the opposite wheel and jack up the car. (You pickup truck owners may be able to crawl under and check. ) Inspect the wheel speed sensor's wiring harness and the sensor itself. Some sensors are integrated fairly well into the hub and aren't prone to damage. Others are simply bolted to stamped brackets. If the sensor is loose or missing, or the wires are damaged, you've found your problem. Check the air gap between the sensor and the tone wheel if it's adjustable the shop manual will give you a specific distance the sensor is supposed to be spaced from the tone wheel and directions on how to properly set it. On rare occasions, the tone wheel itself will show damage, such as missing teeth or teeth damaged by road debris. On front-wheel-drive cars, this usually means replacing the stub axle, because the tone wheel is machined directly onto the axle. If the harness to the sensor has a connector in the wheel well, pull it apart and check the sensor for continuity with an ohmmeter. The resistance value can be found in the shop manual.


Check for shorts to ground as well. Repair any bad wiring, remembering that there's plenty of mud, snow and salt water flying around in the wheel well. TIME'S UP If you've checked all the sensors and wiring and the ABS light still glows, it's time to throw in the towel and start looking seriously at the ABS controller, an amalgam of hydraulics and electronics that has, as they say, no user-serviceable parts inside. It is, predictably, expensive. Replace it as a last resort. Consult your service manual for a chart of specific values and pinouts to check against, which should help you narrow down the problem to a specific wire or sensor. This chart is typical. Item To Be Tested Ignition Mode Measure Between Pins Tester Scale/Range Specification Pinpoint Test Power Feed Battery Check batt. Terminals ABS ECU Power Pump Motor Power Valve Power Sensor Resistance LF Sensor Resistance 1. 28-1. 92 K ohms RF Sensor Resistance 1. 28-1. 92 K ohms LR Sensor Resistance 1. 28-1. 92 K ohms RR Sensor Resistance 1. 28-1. 92 K ohms Sensor Continuity to Ground LF to Ground RF to Ground LR to Ground RR to Ground Sensor Output Voltage LF Sensor Output RF Sensor Output LR Sensor Output RR Sensor Output ABS Warning Lamp Check Stoplamp Switch Check Brake Pedal Applied Brake Pedal Released

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