Vibration When Braking? It’s not the rotors, could it be the modulator?
My P38 has a strong and distinct vibration in the front end when braking from 50-60 MPH. The vibration goes away as the vehicle slows through 20-30 MPH. Sounds like warped rotors, right? Not so fast…
The problem started right after I replaced upper and lower ball joints with factory parts. I used the correct press and had no problem pressing out the old ball joints or pressing in the new ones. Since the front hubs were off anyway, I went ahead and installed new rotors and pads (Lockheed) from RN. The vibration was immediately apparent during the first test drive. I checked both rotors with a dial indicator and they had less than .001” runout. There was no variation in the rotor thickness when checked with a caliper. Nevertheless, I put the old rotors back on, but the vibration remained. The mating surfaces of the calipers, hubs, rotors and wheels have been checked and rechecked and they are perfectly flat. Finally, I installed another set of new front rotors from AB. No difference, still had the vibration. Again I checked the rotor runout and thickness but found no problems. So I pulled the front hubs again and this time separated them from the axle shafts. Back when I had pulled them to replace the ball joints, I had left the axle shafts attached to the hubs. With the shafts removed, there was some ratcheting evident in the bearings that hadn’t been obvious with the shafts installed. Two new front hubs ($$$) and very careful reassembly - but the vibration remained. Tires (Mich 255/55 18 XPC) were changed front to rear with no effect. Replaced all tires with new Pirelli Scorpion 255/60 18 ATR XL. Balancing checked, alignment checked (twice). Steering box checked and adjustment was correct. Tightened it up a little anyway.
The vehicle is a 1998 P38 4.6 HSE with 180,000 miles. No lift. Air suspension is excellent shape with Arnott Gen III air springs. All of the track rod/tie rod ends are fairly new and have been checked. They are tight and have no play. Shocks and bushings are relatively new Bilsteins and are in good shape and have been checked. Steering dampener checked. Steering column u-joints checked. Front radius arms have new bolts and factory bushings that were correctly installed with the factory tool. Panhard rod bushings and bolts are new. Rear brakes are new (rotors, pads and calipers). During the troubleshooting I found a worn u-joint in the front driveshaft which has been replaced with no effect on the vibration when braking.
Could I have damaged the modulator when I replaced the pads? I pushed the pistons back into the calipers with a press. I’ve done it this way three previous times on this vehicle with no problems. Is it possible that forcing the pistons back into the calipers could have damaged the modulator internally?
I have no other brake symptoms. No warning lights, no “clicking” sound in the pedal, the brakes are strong and work on the first application. The accumulator is only a year old and is holding pressure. All of the caliper pins are new and correctly lubed. The brakes have been bled using the lengthy and complicated procedure in the workshop manual (twice) with no obvious problems.
What am I missing?
My P38 has a strong and distinct vibration in the front end when braking from 50-60 MPH. The vibration goes away as the vehicle slows through 20-30 MPH. Sounds like warped rotors, right? Not so fast…
The problem started right after I replaced upper and lower ball joints with factory parts. I used the correct press and had no problem pressing out the old ball joints or pressing in the new ones. Since the front hubs were off anyway, I went ahead and installed new rotors and pads (Lockheed) from RN. The vibration was immediately apparent during the first test drive. I checked both rotors with a dial indicator and they had less than .001” runout. There was no variation in the rotor thickness when checked with a caliper. Nevertheless, I put the old rotors back on, but the vibration remained. The mating surfaces of the calipers, hubs, rotors and wheels have been checked and rechecked and they are perfectly flat. Finally, I installed another set of new front rotors from AB. No difference, still had the vibration. Again I checked the rotor runout and thickness but found no problems. So I pulled the front hubs again and this time separated them from the axle shafts. Back when I had pulled them to replace the ball joints, I had left the axle shafts attached to the hubs. With the shafts removed, there was some ratcheting evident in the bearings that hadn’t been obvious with the shafts installed. Two new front hubs ($$$) and very careful reassembly - but the vibration remained. Tires (Mich 255/55 18 XPC) were changed front to rear with no effect. Replaced all tires with new Pirelli Scorpion 255/60 18 ATR XL. Balancing checked, alignment checked (twice). Steering box checked and adjustment was correct. Tightened it up a little anyway.
The vehicle is a 1998 P38 4.6 HSE with 180,000 miles. No lift. Air suspension is excellent shape with Arnott Gen III air springs. All of the track rod/tie rod ends are fairly new and have been checked. They are tight and have no play. Shocks and bushings are relatively new Bilsteins and are in good shape and have been checked. Steering dampener checked. Steering column u-joints checked. Front radius arms have new bolts and factory bushings that were correctly installed with the factory tool. Panhard rod bushings and bolts are new. Rear brakes are new (rotors, pads and calipers). During the troubleshooting I found a worn u-joint in the front driveshaft which has been replaced with no effect on the vibration when braking.
Could I have damaged the modulator when I replaced the pads? I pushed the pistons back into the calipers with a press. I’ve done it this way three previous times on this vehicle with no problems. Is it possible that forcing the pistons back into the calipers could have damaged the modulator internally?
I have no other brake symptoms. No warning lights, no “clicking” sound in the pedal, the brakes are strong and work on the first application. The accumulator is only a year old and is holding pressure. All of the caliper pins are new and correctly lubed. The brakes have been bled using the lengthy and complicated procedure in the workshop manual (twice) with no obvious problems.
What am I missing?
