Brakes locking up on 2009 Jeep Liberty 4x4

[Wbcjeep nut]

The brakes on my 2009 Jeep Liberty 4x4 are locking up. I think it is an electrical issue. No DTC's, no lights on dash. With the key on, engine NOT running, Jeep will roll, stop when brakes applied, roll when brakes released.... this would indicate the hydraulic components on the system appear to be working fine. Now.. with engine running...brakes lock up. Pulled vacuum line to booster..no change. Disconnected Dynamic sensor, removed ABS Pump fuse, ABS Valve fuse, (all separate times) brakes still locked up. Can anyone offer any suggestions.... other than a match and 1 gallon of gas? LOL

 

[u2slow]

When the engine is running, the brakes work more aggressively because of the power assist.

At what speed are you experiencing lockup? All 4 corners skidding?

 

[Wbcjeep nut]

When the engine is running, the brakes work more aggressively because of the power assist.

At what speed are you experiencing lockup? All 4 corners skidding?

They are applying pressure while the vehicle is sitting still in the driveway. If you cut the engine off... Jeep will roll freely.

 

[u2slow]

Sounds like something haywire with the booster.

 

[Wbcjeep nut]

Sounds like something haywire with the booster.

Pulled the vacuum line going to the booster and it's still applying pressure with engine running. Cut the engine off and Jeep rolls freely. Thinking it may be ABS module or something with the "Hill Start Assist" function. I unplugged the "Dynamic Senson" (Yaw and lateral acceleration) and still staying locked up. If I cut the engine off, the jeep will roll, stop when brakes applied, and roll when I take foot off the brake.

 

[u2slow]

Disclaimer: I have no ABS or "hill start" experience with the Liberty, so I don't know what that system is capable of.

From what you've described, there may be something besides the brake pedal that can apply the brakes.

 

[Wbcjeep nut]

I think it's something with the ABS module, Traction Control System, or maybe even the PCM / Main Computer. They run freaking everything through the PCM now. Hill Start Assist kicks in when you apply the brakes on a hill or grade greater than 7%. The brakes will hold for 3 -5 seconds to supposedly give you time to get your foot from the brakes pedal to the gas pedal and keep the car from rolling backwards. I grew up driving a manual transmission... didn't have o need all that fancy crap.... just makes it harder to work on. I put a cheap Amazon rear wheel speed sensor on a month or to ago... thought maybe it was sending bad signal.... not enough to throw a code... but enough to screw the data and therefore the computer or ABS module up. Going to replace the cheap wheel speed sensor and see if that changes anything.

 

[frankjc]

You could try pulling the ABS fuse, or unplugging the ABS sensor to disable the system, then see if will roll when the vehicle is running. That will tell you if the ABS system is to blame.

 

[Wbcjeep nut]

Tried unplugging the ABS fuse. The car has to be running to unplug the fuse. If you unplug the fuse before starting the car, it will turn over but not start. It didn't make a difference. It seems like the brakes will drag until it gets out of first gear. Once the transmission shifts into second gear it feels like the brakes quit dragging until you come to a complete stop... Then the brakes drag again until you hit second gear again.

I checked the reading for the lateral acceleration from the dynamic sensor. While sitting still, longitudinal acceleration is zero, but lateral acceleration was around 1.6 m/sec2. Wondering if Dynamic Sensor could be sending bad info and causing brakes to apply pressure. Brakes apply and release fine if engine is not running.

 

[Wbcjeep nut]

UPDATE! Still having issues... but noticed that what I think are brakes dragging could be transmission related... not sure. It drags through 1st gear... but when ****** shifts into second gear it quits dragging. You can shift into neutral and the car will coast... until it gets to about 5 mph... then it acts like you hit the brakes and car stops. Drags again until you hit 2nd gear and dragging or resistance stops. Sitting still you have to really give it gas to get it to move. Sitting still with engine running in neutral on a slope... car won't move... but turn off the engine and the car immediately rolls freely. Hit the brakes (with engine off) car stops. Release the brake (with engine off) and car rolls freely again. I think it's HAUNTED! LOL

 

[DadOSix]

Is your speedometer reading at all? Is it correct?

Do you have access to a scan tool with live data?

Kind of curious as to what the wheel speed sensors read. R & L front , rear was a single sensor on the kj’s. Maybe there is a mis-match here applying the ABS

 

[Wbcjeep nut]

Speedometer is working correctly. Checked data from all wheel speed sensors with scanner... at the same time. All read the same. Checked brake pressure sensor reading... appeared correct. I have just about decided it is something with the transmission.... like first and reverse working against each other. Every now and then it will act like nothings wrong... then acts up again. Talked to a transmission guy and he said sounds like first and reverse "welded" together. Drained and flushed the transmission, changed the filter, still acting up. Have just about decided to drop the transmission and replace it.

 

[John P.]

Have you replaced the rubber brake lines? Sounds silly but when they go bad they can act like a one way check valve and hold pressure on one position or all positions causing the brakes to drag or lock up. It's an issue that happens on Wranglers quite often.

 

[Wbcjeep nut]

Have you replaced the rubber brake lines? Sounds silly but when they go bad they can act like a one way check valve and hold pressure on one position or all positions causing the brakes to drag or lock up. It's an issue that happens on Wranglers quite often.

Didn't check the flex / rubber lines.... but have decided it's transmission related. If you don't have the engine running the car will stop when brake pedal is depressed, and will begin rolling immediately upon release of the pedal. Start the engine and will Jeep sits still in neutral. Jack all 4 wheels of the ground and all turn freely when not running. Start the engine and put in drive and back tires don't turn until the rev the engine up. It's something either in the ****** gears / clutches or the valve body. Working on swapping the transmission now. Will hopefully have a positive post soon!