QUESTION: I drive a 2000 Chevrolet Silverado 4 X 4, V8 automatic transmission 1500 extended cab truck, with 200,000 km.
Lately, when coming to a stop, my antilock comes on at a very slow speed and I have to stand on the brakes to come to a complete stop. Could this be in the computer, something mechanical or faulty sensors? Is this an expensive repair? Thanks.
ANSWER: The antilock brake system (ABS) looks at individual wheel speeds to determine when a wheel is about to lock up. When a wheel locks, the ABS activates and releases the pressure from the brake caliper so the wheel can start rotating again. As soon as the wheel starts rotating, the ABS then allows the brake caliper to apply again, slowing the wheel.
This cycle repeats several times a second till the vehicle slows to only about three to five km/h. Then the system will typically stop activating and the wheels lock up. At this speed on pavement, the vehicle will stop almost immediately. If the road is covered with ice, the vehicle may slide a few feet before it stops.
From your description, it appears the system is activating at very low speeds. Because it is activating, the computer should be fine. There could be a problem in the hydraulic unit of the ABS module but the most common problems are wheel speed sensors. The wires to the wheel speed sensors flex as the suspension moves. The wires are designed to be very flexible, but ice, mud and cold temperatures all place additional stress on the wires and they can break internally in the harness.
The ABS computer performs a wheel speed sensor test to determine if there is an open circuit. But I have seen situations in which the wires have a good enough connection to pass this performance test but then flex and open during a braking situation. The broken wire causes a loss of wheel speed signal, which the computer interprets as a locked-up wheel and then activates the ABS. I suspect this is the problem with your truck.
There may be a code set for a wheel speed sensor in the ABS computer, so a scan tool can be used to help locate the problem. If there is no code, the wheel speed data can be monitored during a stop to see if all the signals are equal as the vehicle slows. If one signal drops out before the others, that's the problem area.
QUESTION: I have a Dodge Journey 2WD with Traction Control. A couple of weeks ago, the morning after a semi-heavy snowfall -- a couple of inches or snow and moderate wind overnight -- I volunteered to do some driving. My friend's street had a fair bit of snow on it, but I figured, "Traction Control. No problem!"
Well, I got bogged down. I found that when both front wheels lost traction and began to spin, I lost all my rpm. My engine wouldn't rev over 1,000 rpm. My car wouldn't move at all. I had to shut the traction control off so I could get any revs at all. Fortunately, I was able to navigate three long blocks with the TC off to get to a clear street.
My question is this: When both front wheels lose traction and spin, does the accelerator shut off altogether or could my traction control be faulty?
ANSWER: Your traction control is working properly. On most vehicles, the system will apply brakes on the spinning wheel to provide traction. This causes the torque to transfer to the other drive wheel. At the same time, the engine power is reduced during a traction-control event to lower the brake temperatures.
On extremely slippery surfaces when all drive wheels have limited traction, the power can be reduced to the point the vehicle may not move through any snow. Turning off the traction control will allow full engine power, but it's then up to the driver to operate the throttle to reduce wheel spin.
Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.