My 2001 F150 has the ABS light on. I can startup and it runs thru the continuity test, light off, in park, no problem. Push on the brake pedal, no problem. Put it in gear and the light ccomes on, before you start to roll. Should mean its not the speed pickups, i checked them for ohms and 5 volt supply to each. I ran the scanner and got P0703 on the extended codes. That normally means bad pressure switch on the brake booster. I kew that would effect cruise contol and external leaks can be a problem, but could that be related to the 703 code ? Both "brake" related ? Im tempted to buy a switch and install. I do have the factory recall harness on the switch. And check for continuity thru the swith. Has continuity, but only a few ohms, not sure what it sould be.

 

The switch on the master cylinder should be zero ohms. I've never heard on one causing an ABS fault though since it's nowhere near the ABS system's circuits.

The most common thing that ails in the ABS system is the rear differential sensor located on top of the pumpkin. Pull it out and check its resistance. Bad ones usually go open. Good ones, from distant memory, should read around 1.5 k ohms.

However, since you pulled a P0703, it's more likely that you might have a bad brake pedal position switch. That's the one on the brake pedal, a double pole switch. One half of the switch operates the brake lights, the other half is used to signal the various subsystems as to the position of the switch (brake pedal pressed or not).

 

I wil check the switch tomorrow. THe brake lights do work, and i did change the rear axle sensor without looking into the problem much. I now know that the light comes on when i push on the brake and put the truck in gear, so i do think the truck is not getting a signal that the brake is working, and the truck is in gear. I dont have to start moving to get the ABS light... The 703 code might be related or i was wondering if it was store last year when they put the new harness on the master cylinder ? I erase the OB II codes when that light goes on, but not sure if the "erase" cleans out the KOER codes... thats where i found the P0703 and i rarely check it.

 

The P0703 and the ABS light are probably due to the same fault. If you could pull the ABS code(s), you'd likely find a code related to the BPP switch input which would definitively identify the fault.

The BPP switch would also affect speed control operation and disengagement if faulty.

To check, look for ALWAYS HOT power on the WH/PK wire, constant ground on the BK/PK wire, and the RD/LG wire to be grounded when the pedal is not pressed and to be HOT when the pedal is depressed.

Please note that this section of the switch is electrically separate from the operational brake lamp circuit but is mechanically actuated at the same time.

 

Have you checked your brake fluid? My ABS came on due to low fluid. Checked and found a micro leak at the left rear. Fixed that and no ABS light on.

 

Low brake fluid will illuminate BOTH the red BRAKE light and the yellow ABS light.

 

Well, i ended up with two intermittent problems. Brake switch on pedal had a small crack in mount. IF it dont make up right, you cant shift out of park into gear. I had noticed that a couple times in the past several months.. had to kick the brake pedal to get the shift lever to move. I think this also set off the ABS light on occasion. After fixing that i noted the light ON CONSTANT... right after turnignt he key on.. That means the sensors flunked the continuity test. I took an old hub from last replacement and took the ABS sensor out of it. Disconnected the drivers side front harness and plugged the spare in... NO LIGHT !!! Looked at the sensor and one of the two pins inside was 1/4 inch deeper than the other. Seems like it can move in or out as you push or pull the plug apart. Momentary contact... enough to set off the ABS. These two problems together were why i did not have a consistant problem. One thing to remember, the brake switch has to be compressed and brakes on to get the trans to shift out of park. Thats a good test of the switch. I did have brake lights on the other half (at least when i checked). I dont see the switch at Auto Zone, but it is availabe for $20. on e-bay.... i repaired mine with epoxy.

 

Good followup. Thanks for doing so.

From what I see O'reilly Auto has the "brake light switch" from two different vendors - a store brand for $10 and a Borg Warner for $13. AutoZone has the "stoplight switch" for $12.

Let us know how things work out down the road a bit.

 

YEP... your right, AutoZone calls it a "stop light switch"..i didnt see that yesterday, would have bought one instead of repair !.. Drove 20 miles to work and 20 miles back.. so far so good.

 

If I may, brilliant problem solving to fix the problem. Glad you fixed it for you. So many times we start we starting wondering off on what it might be instead of what it is.