Another ABS light puzzle

[StewieNC]

I have a 99 Ranger 2wd, 4L engine, 5 speed manual, rear wheel ABS. When I start the engine, everything seems fine until I drive away and reach 20 mph. The ABS light comes on and stays on. The brakes seem to work fine. I will list below all the parts I have replaced, but nothing has changed this condition. I am running out of things to check. I found the diagnostic connector, but it will not flash a code. I disconnected the rear speed sensor to see if it would flash a code for that and it did. I don't think the ABS module is the problem. Does anyone have any ideas? The truck is in great condition 100k miles, I have owned it for 20 years and it has not been wrecked.

Parts replaced include:
Rear speed sensor, new
RABS valve, salvage
ABS control module, salvage (I put the original back in the truck)
Brake pedal position switch, new
I also checked the diode and resistor and I think they are ok

 

[Bobnjo]

I have a 99 Ranger 2wd, 4L engine, 5 speed manual, rear wheel ABS. When I start the engine, everything seems fine until I drive away and reach 20 mph. The ABS light comes on and stays on. The brakes seem to work fine. I will list below all the parts I have replaced, but nothing has changed this condition. I am running out of things to check. I found the diagnostic connector, but it will not flash a code. I disconnected the rear speed sensor to see if it would flash a code for that and it did. I don't think the ABS module is the problem. Does anyone have any ideas? The truck is in great condition 100k miles, I have owned it for 20 years and it has not been wrecked.

Parts replaced include:
Rear speed sensor, new
RABS valve, salvage
ABS control module, salvage (I put the original back in the truck)
Brake pedal position switch, new
I also checked the diode and resistor and I think they are ok

 

[Bobnjo]

StewieNC
I currently have the same issue with the ABS light coming on but have not been able to diagnose on my 94 Ranger w/RABS.
However, I had a similar issue with a front well drive car with ABS that would activate the ABS system when rolling slowly under 5 mile an hour.
I determined the wheel sensor would act as if the wheel was not rotating and activate the ABS system. It would scare the hell out of me.
I discovered the output in ohm value generated by the sensor was erratic (low or zero) when rotating the wheel. I eventually found it wasn't a bad sensor but the distance from the tip of the sensor to the gear tooth ring in the differential. If the running gap is too large the ohm signal can be weak or erratic and cause the ABS system to activate. I too bought a new sensor and found same problem. I compared the length
of the two sensors from the sensor tip to the mounting surface, the were identical. I examined the exciter ring gear through the hole in the casting all looked clean. What I found was the casting surface in the area of the sensor mounting developed severe corrosion under the mounting surface of the sensor. This buildup of corrosion increased the sensor running gap causing erratic ohm values. By cleaning the mounting surface allowing the sensor to seat slightly lower by closing the gap to the exciter ring gear corrected the problem.
Something you might want to check.
Bob

 

[StewieNC]

StewieNC
I currently have the same issue with the ABS light coming on but have not been able to diagnose on my 94 Ranger w/RABS.
However, I had a similar issue with a front well drive car with ABS that would activate the ABS system when rolling slowly under 5 mile an hour.
I determined the wheel sensor would act as if the wheel was not rotating and activate the ABS system. It would scare the hell out of me.
I discovered the output in ohm value generated by the sensor was erratic (low or zero) when rotating the wheel. I eventually found it wasn't a bad sensor but the distance from the tip of the sensor to the gear tooth ring in the differential. If the running gap is too large the ohm signal can be weak or erratic and cause the ABS system to activate. I too bought a new sensor and found same problem. I compared the length
of the two sensors from the sensor tip to the mounting surface, the were identical. I examined the exciter ring gear through the hole in the casting all looked clean. What I found was the casting surface in the area of the sensor mounting developed severe corrosion under the mounting surface of the sensor. This buildup of corrosion increased the sensor running gap causing erratic ohm values. By cleaning the mounting surface allowing the sensor to seat slightly lower by closing the gap to the exciter ring gear corrected the problem.
Something you might want to check.
Bob

Thanks for the input, I will keep that in mind. I have arrived at the conclusion that my problem is with the brake light switch. I am getting an ABS diagnostic code 11, which indicates brake pedal position switch issue. This is after I replaced the switch. The brake lights work, but it is still tripping the ABS light. I bought the replacement switch at Autozone, I might try buying a Ford Motorcraft switch next. The brakes work fine and I am confident that the other components are good. I am just ignoring the light until the weather warms up a bit.

Here is a tip for anyone trying to diagnose an ABS issue. The parking brake must not be engaged, if it is engaged it will not flash a code. My driveway is on a grade, and I wasted some time before I figured out the parking brake needs to be off.

 

[StewieNC]

Thanks for the input, I will keep that in mind. I have arrived at the conclusion that my problem is with the brake light switch. I am getting an ABS diagnostic code 11, which indicates brake pedal position switch issue. This is after I replaced the switch. The brake lights work, but it is still tripping the ABS light. I bought the replacement switch at Autozone, I might try buying a Ford Motorcraft switch next. The brakes work fine and I am confident that the other components are good. I am just ignoring the light until the weather warms up a bit. I still have the old switch and I connected it and left it hang under the dash without being connected to the brake pedal. Obviously, the brake lights won't work, but I wanted to see if the ABS light would trip. I drove it and the ABS light did not trip, so I am pretty confident that that the problem is the brake light switch.

Here is a tip for anyone trying to diagnose an ABS issue. The parking brake must not be engaged, if it is engaged it will not flash a code. My driveway is on a grade, and I wasted some time before I figured out the parking brake needs to be off.