Hi mastertechhvac. Welcome to the forum. You've already checked the common stuff I was going to suggest, but you made a dandy observation with the application of the brake pedal. The ABS computer performs a series of self tests right after starting the engine but those tests are delayed when the brake light switch is turned on. If you were to continue to drive that way, the computer would memorize a diagnostic fault code related to a shorted switch. Instead, since the light stays on, you know a problem has been detected during the self test sequence. You're going to have to find a mechanic with a scanner that can access the ABS computer to read those codes. They are only meant to get you to the right circuit or system with the problem, not necessarily the specific defective part. The Dodge dealership in my area will read codes for other shops at no charge. To help cover the cost of the expensive equipment and the mechanic's time, your dealership will likely charge you something, but having the codes read is the most effective way to determine the cause of the problem. Many of the aftermarket scanners used by independent repair shops can not access the ABS computers, but they're getting better so it doesn't hurt to ask. Reading the code(s) should only take about 15 minutes or less.
As a generalization, if the yellow warning light comes on by itself before you even start driving, the problem is electrical in nature, most commonly an open sensor or its wiring including corrosion in the connector. If the yellow light only turns on after the vehicle has been driven up a mile or more, suspect a cracked toothed tone ring by one of the wheels. Ford has also had some problems with metal filings building up on the magnet of the speed sensor on top of the rear axle. You would have seen that when you had it out, and none of these things should be related to the sudden stop.
It is more likely there is a problem inside the hydraulic control unit. The valves and pressure switches are tested during the self-test procedure, but after that they don't do anything until the anti-lock function is needed.
If the red brake warning light is on at the same time, the yellow ABS light will be on and the ABS system will be disabled, not because of an ABS defect, but because a problem in the base brake system will compromise the operation of the ABS function. There are only three things that can turn the red warning light on. If it is on, we can cover that, but I suspect you would have mentioned it.
To continue with your testing, use an ohm meter to measure the continuity of the three sensors. A typical value should be around 500 to 800 ohms. The two front ones should read the same. When a sensor is out of specs it will almost always be completely open, (infinite). If the resistance is higher than normal but not completely open, suspect corrosion in one of the electrical connectors. The resistance of the sensor itself is determined by the diameter and length of the coil of wire. Those things can't change. Some of the loops of wire can short together. The partial short results in a lower than normal resistance reading but that isn't common and won't result from sudden braking.
Sunday, October 3rd, 2010 AT 3:51 AM