BACKYARD MECHANIC: ABS system might require bleeding

Hey there, time traveller!
This article was published 29/04/2011 (5248 days ago), so information in it may no longer be current.

QUESTION: My dad bought a 2003 Chev Silverado, 1/2 ton, two-wheel drive in December 2010. This truck has ABS with all disc brakes. The brake pedal goes almost to the floor and the brakes do not hold like they should. The garage that did the last brake job said “those brakes are like that.” We took it to another garage and again were told that there is nothing wrong with them. Please, can you offer some advice?

ANSWER: This truck could have come equipped with either a hydroboost or vacuum brake booster. From your description of vehicle operation, I am guessing it has a hydroboost brake booster. The hydroboost system uses the power steering pump to provide oil pressure that helps the driver press the brake pedal.

One of the characteristics of this system is that when the vehicle is stopped, the harder you step on the brake pedal, the more assist is provided and the pedal will go nearly all the way to the floor. This only occurs when the vehicle is stopped and there will be several hundreds pounds force helping press the pedal down and applying the brakes. Even though the pedal will be low, the brakes will be fully applied. Normally it only takes a little pressure to hold the vehicle stopped, so you don’t have to press on the brake pedal that hard.

There could also be a problem with your vehicle that wasn’t identified at the garages. Air in the brake system could be causing a soft and low brake pedal. I would assume the brake system has been bled to remove air, but they may not have bled the anti-lock hydraulic unit at the same time. Every time you start the vehicle and drive it over 13 kph, the ABS system cycles as a self-test. If there is air in the ABS system, it could then be let into the base brake system.

To bleed the air out of the ABS system, a scan tool is needed to operate the ABS system through an extended operation cycle. Then the base brakes need to be bled again.

QUESTION: I have a 1989 Toyota Camry and the doors will not open (it’s a four door). The wire harness has been reworked twice and replaced once. The door-lock stem cannot be manipulated for any of the doors to open, nor can the doors be unlocked from outside. The vehicle has more than 350,000 km on the odometer. Help.

ANSWER: The power door-lock system on your car has a lock solenoid motor in each door that moves the door-latch mechanism to either the lock or unlock position. When the door locks are not being operated electrically, the lock solenoid motors are not energized and the lock mechanisms can be manually operated, either with the key in the outside front doors or by moving the lock knob manually on each door. Because you say you cannot unlock any door either with the key or the lock knob or stem, I think the door-lock solenoids are remaining energized all the time in the locked position, making it difficult to move the lock mechanism.

Because all door locks are affected, the best place to start is at the door-lock relay unit. This relay unit has a 13-pin connector with 11 wires connected to it and should be located under the dash on the driver’s side. When you press the lock and unlock buttons, you should hear the two relays inside the unit click as they operate. Unplugging the relay should allow you to operate all the door locks manually.

If you wish to have power locks, test the light green wire at terminal 6 of the relay connector. This wire should have 12 volt power only when the lock buttons are pressed to the lock position or the key switches on the doors moved to lock the doors. If there is power there all the time, then the problem may be in one of the lock switches but more likely a wire shorted to power in one of the door hinge areas. If there is power only when a lock button is pressed, then the relay is at fault.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists Association of Canada.

kerr.jim@sasktel.net