Rabs Valve

[MTKirk]

Any chance your brake fluid is low in the reservoir ? that too wil bring on the dash light.

I'm never gonna be that lucky, fluid is perfect. Besides my red brake light is not staying on, just the yellow "rear anti-lock brake" light.

Funny thing is; the anti-lock light comes on for the correct 2 second start up, goes out, then comes back on again after a couple of seconds (like normal). But; the damn thing won't give me an error code when I ground the diagnostic lead. All I can think of at this point is to tighten the contact at the module pin (I believe it's pin 12) & see if I can get the thing to flash a code. If that doesn't work it's gotta be the control module has crapped out? Hopefully I can find a junkyard unit to try before I shell out $125+/- on something I'm not really sure I need.

I'm really starting to hate this system.

 

[79jasper]

I think you're doing it wrong.

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[MTKirk]

I think you're doing it wrong.

Sent from my SAMSUNG-SM-G890A using Tapatalk

Entirely possible, but; I've always been able to get codes to flash before by momentarily grounding the diagnostic lead.

 

[icanfixall]

Might try removing the speed sensor from the differential and clean off any metal film they can pick up from gear wear. It may look like the same fuzzy metal around the magnets in the trans oil pan. Other than these simple suggestions.. I'm not much help.

 

[MTKirk]

Might try removing the speed sensor from the differential and clean off any metal film they can pick up from gear wear. It may look like the same fuzzy metal around the magnets in the trans oil pan. Other than these simple suggestions.. I'm not much help.

Well; it turns out you were on the right track originally. It was the wires to the master cylinder. The electrical connector to the master has gotten old & crumbly and wasn't making contact. There are three wires there; one goes to the idiot light (past a diode & then back to the battery I believe), one goes to pin #2 on the rabs control module, and one is a chassis ground. So if the wire going to the rabs controller isn't getting 12V+/- it lights the antilock light permanently.

I couldn't fix the old connector, so I just crimp,soldered, heat shrunk the two "hot" leads together, and just snipped off the ground. All is right again

Well; 'cept I have to learn how to live without an idiot light telling me when to add brake fluid.


NOW BACK TO YOUR ISSUE! I was thinking; On 87's through 89's there are 2 sizes of RABS valves. I think one is for duals & one is for singles. The one for duals has a larger accumulator, if you installed that on a single axle system it would probably have enough volume to make your pedal go to the floor. Maybe double check those part numbers?

 

[79jasper]

Still weird that it wouldn't flash the code though.

Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[MTKirk]

Still weird that it wouldn't flash the code though.

Sent from my SAMSUNG-SM-G890A using Tapatalk

Yeah; that's gnawing away at me a little bit, I think I will pull the ground from the RABS valve (to create an obvious error) and make sure the controller picks it up & generates the correct error code. Still; The brakes are working great for a 29 year old truck. (should be fantastic after I put in new pads & rotors up front).

 

[ifrythings]

This thread on fte has a lot of good info http://www.ford-trucks.com/forums/1383931-91-f-150-abs-light.html

Again if your abs kicks in and your peddle goes instantly to the floor the the isolation valve is bad and you need to replace the abs valve.

 

[jpw]

I just scanned this thread briefly and I'm not sure if it was mentioned but reverse bleeding often helps. use a syringe to pull the master down to the minimum level then use another syringe to push fresh brake fluid in through the bleeder screw and up to the master cylinder, repeat. This some times works for clutch cylinders, wheel cylinders, calipers, ABS valves excetera when nothing else does. Tapping lightly on the component you are bleeding while moving fluid also helps. When I do abs repair involving the hydraulics of any type I do five to ten full lock up stops on a loose surface and then bleed again a second time if need be.

I think it was mentioned, try get a false code to set by driving around with the rear sensor unplugged. This allows you to see if the control module can respond properly. If you don't get any codes you could have poor inputs/outputs including sensor, actuator, power and ground circuits to the module or a module bad. Based on experience solid state does not age well and reman parts even from forward including modules and actuators like the wraps valve you can be iffy quality.

Sometimes Plumbing a jumper line around the Rabs valve can help you to narrow the cause of your issues down. If you have normal breaking with the valve by past then you know to focus on the wraps. If you still have braking issues with the bypass in place sometimes the Pistons in wheel cylinders stick and cause crazy pedal issues.

HTH