Hey everyone,

I posted this last week but haven't heard anything. Trying one more time to see if I can get some help.
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I have a 2002 3.0 2WD. I just replaced the brake rotors due to them being warped. I also replaced the pads and bearings of course. During my installation, the brake fluid reservoir overflowed some due to me compressing the calipers (I never bleed them, only compress them). Also, I sucked some of the old brake fluid out of the reservoir to replace with newer fluid (DOT 3) but didn't suck all fluid out as I didn't want to get air in the system.

Anyways, after this was all said and done I went for a drive. Immediately after moving 10 feet from my parked position the ABS light turned on (no physical signs of ABS actually engaging). I tried resetting the battery but the light still came back on next time I drove. Also cleared the code with a scan tool and it came back right away.

The code it's giving me is C1939 - brake pressure switch input circuit failure. It's definitely related to the job I just did. However, I'm not sure if it was caused my the fluid overflow, rotor replacement, or fluid replacement.

I'm not sure what the brake pressure switch even is or where it's located. Is this what controls your brake lights? I can't find anything online about this so I don't know how to go about diagnosing it. Any help is much appreciated!

 

If memory serves me right, the brake pressure switch is part of the ABS module. I am not sure if it is a serviceable component or if you will have to replace the entire ABS control module.

What I would do, before anything else, is bleeding all 4 corners of the truck, do both the rear brakes first (if drums) and then do either front wheel. You want to move at least a pint of fluid through each corner, a quart is more preferable if you've never replaced your fluid. If you haven't flushed it in your ownership, I guarantee the brake fluid will come out BLACK, as it did for mine the first time I flushed it.

If you have 4 wheel ABS with disc brakes on all 4 corners, do the passenger rear, driver front, and driver rear, then passenger front. Vehicles with 4 wheel independent braking systems are often "cross linked" to keep one wheel on either end of the truck braking in the event one wheel's brakes fail for any reason.

 

Thanks for the info. I've always avoided a flush/bleed just because I'm not super familiar with break systems. I'll do as you said and see if it resolved the issue. If anything, at least I'll get a long overdue break fluid change.

 

If you're worried, just gravity bleed them. It's very passive and works well for one person. By doing it this way, you also can watch all the gunk come out and can see when you hit clean fluid.

Get yourself a run of silicone hose, large enough to fit over the bleeder, but small enough to grip it. You want about 2-3 ft of it. I think the stuff I used last was 5/32nds I.D. but others will work.
Then raid the kitchen/pantry for all the unused and empty glass jars. You'll need a few. Then, get yourself a box or a stool that you can set the jar on, that keeps the jar above the brake bleeder, but no higher than that.

Then, place your wrench on the bleeder, and then put the hose over the nipple. Place the remainder of the hose into the jar, try and curl it around so the hose remains at the bottom.

Then, crack the bleeder. You can wait, or you can get in the cab, and very slowly depress the brake pedal. Try and stop about an inch from the "floor", if you go too far you could potentially over-extend the master cyl, and you don't want that. Place a block of wood on the floor if you have to, to prevent yourself from doing so. You should only need to push the pedal down once, but feel free to do it twice if you'd like. The only thing you are doing here is getting the fluid flowing a little bit. Once you get fluid through the hose it'll continue to draw itself out. Keep an eye on your master cyl and continuously top it off until you've ran through at least 1 pint on that corner.

When you get through that pint, switch your jars to another "clean" jar, and a clean hose, and then set the system up again. This time, put enough new, clean fluid in the "clean" jar that the hose end is submerged. While this is recommended to do any time you gravity bleed, I find we move so much fluid through a "major" flush like this that it is unnecessary. If you are very slow with the pedal movements when initially getting the system to self-bleed, you shouldn't draw any air in through the open bleeder. After you have set up that corner with a "clean" jar, crack the bleeder, hit the pedal once, to get fluid movement, and then check the fluid. Anything going into that jar should not change color. If it darkens or comes out with any debris, continue to bleed that corner (and any subsequent corners) until you get clean, fresh fluid.

While this is a slow process, this allows you to inspect what comes out, as well as top the reservoir off as often as needed. I personally leave the cap off the entire time I do the job, and only lay a clean paper shop towel over top to keep any potential debris from falling in.

I cannot guarantee that this will fix your issue, but I feel like you may have pushed an air bubble or debris back into your ABS module in some capacity, so it cannot properly complete its self test when you key the truck on.

 

Thanks again for the help, Turismo. Unfortunately the flush/bleed still didn't solve my issue. I flushed the system twice ensuring no air is in it but the code still remains. I also noticed I don't have that great of a stopping distance and the brake pedal barely touches the floor on hard stops.

I did some research and found that there's an aftermarket fuse installed inline between the wire harness and brake pressure switch (aka cruise control release switch) that was installed due to a recall. However my multimeter shows continuity through the fuse, the switch, and voltage from the harness.

Not really sure where to go from here? The fluid is fine, switch has continuity and works properly, harness has voltage, and fuse has continuity. Yet, the light is still on and my braking distance is longer than it should be.

If you have any other advice it would be much appreciated.

 

In my opinion, if you still have poor brake performance, and you're working with new components, I would suggest possibly having the ABS module activated with a scan tool, and then bleeding it again. If you do not have the appropriate level scan tool, I would bring it to someone who does. They can activate the ABS pump and this should force any potential air bubbles out of the ABS system, and down into the lines, where they can be bled out.

Another trick is to go down a dirt road and slam on the brakes. It should also activate the ABS if the truck allows it, which should push the bubbles down the line.

The unfortunate part about the ABS module code you have, is the truck may or may not activate ABS, so the "dirt road" method may not work.

I am not nearly as well versed in the ABS module codes as @RonD is, hopefully he can come and chime in, and correct anything I've said that might not be accurate, if I'm not on the right track. I'm sorry I don't have much more help to offer you with this.