AEB False Activation After Lift Kit and Larger Tires
A 2019 pickup with factory AEB and adaptive cruise control (ACC) comes in after the owner installed a 75 mm suspension lift and 33-inch tires (up from the factory 31-inch). The speedometer was corrected with a programmer. The owner reports that AEB has braked unexpectedly twice at highway speed-once approaching an overpass and once near a large metal sign-and that ACC follows vehicles at a closer distance than before. No DTCs are stored. The forward radar is behind the front grille; the forward camera is at the top of the windshield. The owner wants you to 'just recalibrate it.'
1. Explain why recalibration alone may not solve this and what the lift/tires changed.
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Recalibration restores the sensor's aim relative to the vehicle as built; it does not correct for a vehicle whose geometry and dynamics no longer match the software's assumptions.
What changed:
- Ride height and pitch. A 75 mm lift raises the body and changes the static and dynamic pitch of the radar and camera. The radar's vertical aim and the camera's horizon reference move. Calibration can address aim, but the vehicle now pitches differently under braking and acceleration.
- Tire diameter and rolling circumference. Larger tires change actual speed versus indicated speed and change the rate at which the vehicle actually accelerates/decelerates for a given command. ACC and AEB compute time-to-collision from vehicle speed; if the speed signal is off, the computed TTC is off.
- Radar field of view. Raising the front end can point the radar higher, causing it to see over some targets and pick up overhead structures (overpasses) and large reflective signs-consistent with the reported false activations.
- Suspension geometry. Caster and camber changes affect steering angle sensing used by LKA and ACC.
Recalibration is necessary but not sufficient if the vehicle is outside the manufacturer's supported configuration.
2. What is your diagnostic and customer-communication plan?
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Diagnostic plan:
- Verify configuration. Confirm the lift and tire size against the manufacturer's supported range. If the vehicle is outside it, document that; the OEM calibration targets and software assumptions may not apply.
- Check speed signal accuracy. Compare indicated speed to GPS or a measured rollout to confirm the programmer corrected the speedometer and that the ABS/wheel speed signals are consistent.
- Inspect sensor mounting and aim. Verify the radar bracket and camera bracket are undamaged and correctly seated; check for debris or a grille change that affects radar transparency.
- Perform the manufacturer's calibration if the configuration is supported. If the procedure cannot be completed or the vehicle is out of supported range, do not sign off.
- Road test in conditions similar to the customer's report (highway, overpasses, signs) and note any false activations.
Customer communication:
- Be direct: 'The lift and tires changed how the truck sits and how fast it actually goes. That can make the radar look higher and the computer misjudge closing speed. Recalibrating helps aim, but it can't make the truck match the factory assumptions if it's outside the supported range.'
- Present options: return to a supported ride height/tire size, or accept that AEB/ACC performance may be degraded and document that the customer was informed.
- Do not disable AEB to 'fix' complaints without a documented, customer-acknowledged reason, and never present a disabled safety system as a repair.
3. The customer refuses to remove the lift. How do you document and hand off?
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Document thoroughly and hand off with informed consent.
- Record the as-built configuration (lift height, tire size, programmer used), the reported symptoms, the diagnostic steps, and the calibration result or inability to calibrate.
- State plainly in the RO: 'Vehicle is outside manufacturer-supported configuration for AEB/ACC; performance may be degraded; customer declined return to stock.'
- Have the customer sign an acknowledgment and give them a copy.
- Provide a written summary of the specific risks (false AEB braking, ACC following distance error) and advise them to report any further events.
- Do not clear codes to hide the issue, and do not represent the system as 'fixed' if it is not.
- If the manufacturer publishes a procedure for modified vehicles, follow it; otherwise, the honest position is that the system cannot be certified to OEM performance in this configuration.