I appreciate the response. However, after checking the connection at the master cylinder, I found all connections were fine. Thank you for trying.

 

Let me do some digging through the service manual and get back to you again. I didn't have the factory manuals back when this thread was started. It may be a day or two, I've got a busy afternoon today.

 

I just glanced at the FSM and it says to connect the diagnostic wire to ground, I assume throughout the test. Maybe momentarily connecting it to ground is what I was doing wrong. Give that a try and let me know what you find out.

 

I have an early 1994 F250HD XLT 4x4 extended cab w/ the turbocharged International 7.3L idi engine.

I recently jumped on the bandwagon and decided to change out the 194 incandescent bulbs behind my instrument cluster. I discovered one burned out illumination bulb on the left side and one in the ABS Warning light socket. Naturally, I assumed I had a long-standing ABS problem, since before I bought the truck ~14 mos. ago. Unfortunately, I was Gung Ho enough to change out ALL the bulbs, illumination AND warning and other indicator lights.

As soon as I put it back together, I noticed that my ABS light was on all the time. With help from these forums, I was eventually able to locate the RABS-II Control Module, the RABS-II control Valve/Accumulator and the rear differential ABS-II [rear axle average] Speed Sensor.

The RABS-II Control Module on this vehicle is located directly behind (aka just fwd) of the glove box. It is very easily accessed simply by opening and emptying the glove box and then squeezing the upper outside edges of the glove box horizontally toward the center of the box, until the opening stops clear the edge of the hole in the dash panel. Then the glove box can be rotated further open, until the hinges can be easily slipped free from the rectangular cross-section hinge axles built into the lower edge of the dash panel. The ABS Control Module is mounted vertically on the ventilation duct structure. The diagnostic code readout triggering wire (Blk w/ Org stripe, coming from port 12 on the RABS-II Control Module) is connected to the red KeepAlivePower wire (coming from fuse 13 in the interior/instrument panel fuse box located just above the driver’s left knee).

To trigger the RABS-II diagnostic code readout, separate this connector and ground the Blk-Org wire for 1-2 seconds while the engine is running with an active RABS-II warning light. After an ~1sec “on” flash, the light will blink on and off the number of times equaling the active error code, with the last flash being a longer one. This flash sequence will repeat indefinitely until the ignition is turned off with the red KAP wire still disconnected. This appears to be the essential effect of removing the ground wire(s) from the Battery(ies) that is prescribed throughout these forums for clearing RABS-II diagnostic codes.

The problem I had at this point, though, was that the “ABS” light was still lit when I turned the ignition back on after supposedly clearing the code and reconnecting the Red-Blk/Org connector. And when I read the codes again, I still got code 16, not some new code indicating a problem.

And it was a little more complicated than just that. The “ABS” warning light lit semi-brightly for ~1-2 sec. immediately upon turning the ignition key to ON, and then it went dim. This lasted until the glow plug controller timed out and then began it’s normal cycling of the glow plugs on and off until the engine is started and warms up slightly. During this intermittent glow plug cycling, the brightness of all lighting is modulated when the glow plugs are on, due to their large current draw. More to the point, once the RABS-II diagnostic code readout was triggered as described above, the flashing output of the codes was interleaved with the effect of the cycling glow plug operation, until the engine warmed up enough for that to stop. Then the brightness of the “ABS” light varied between bright and semi-bright, delivering the code 16 message, but it never went entirely out between ON flashes. After terminating the readout, the light was still on semi-brightly.

So, I decided to try eliminating the new LEDs from the picture, leaving them in the simple illumination and indicator spots, but putting new 194 incandescent bulbs back into the “ABS” and “Check Engine” spots, where they might be getting grounded thru solid state control modules.

Sure enough, the “ABS” light now stayed off. Apparently, these dimmable LED bulbs have such a low current requirement that they illuminate dimly due to even a slight current leakage into port 12 on a healthy RABS-II Control Module with no active diagnostic errors present.

I’ll be double checking the code reading and clearing process next, in the process of replacing the original RABS-II Speed Sensor pigtail that now has a broken locking tab on one side from removing it to try a new sensor, as indicated by most troubleshooting guides.
Turns out there is actually a test port on this vintage of Ford trucks, conveniently located adjacent to the brake master cylinder/power brake vacuum booster and the OBD-1 connections. This port allows an ohmmeter test from this test port. It is a simple 2-wire connector with a cap, fed by a LtGrn/Blk and a Red/Pink wire. A reading of ~1400 ohms indicates a good sensor and wiring harness to it, although a buildup of wear particles from the rear differential gear oil can cause this sensor to lose sensitivity, as can also occur with the engine rpm sensor located near the engine oil filler port on this vintage 7.3L diesel. I believe the speedometer on this vintage truck receives its speed signal from the transmission and not from the RABS-II Speed Sensor, but I’ve not verified this yet. I will try to test this also, while I have the speed sensor disconnected to replace the pigtail.