The U.S. National Highway Traffic Safety Administration has initiated an investigation (NHTSA PE05020) of the front wheel bearing hub assemblies on 1.3 million General Motors pickup trucks built between 1999 and 2002.






Silverado
Yukon Denali
Sierra
Escalade
The investigation follows a recall GM conducted in several Canadian provinces over the same problem.

Like their Canadian counterparts, GM light truck owners in the U.S. have been complaining to NHTSA that the vehicles' antilock braking systems are being triggered at low speeds, rendering them unable to stop within a reasonable distance.

The trucks involved are 1999-2002 model year GM light trucks and SUVs -- Chevrolet Silverado, Tahoe, Suburban and Avalanche; GMC Sierra and Yukon; and the Cadillac Escalade.

In these light trucks, the wheel speed sensor is built into the wheel bearing hub assembly. A metal "tone ring" with a predetermined number of teeth sits between the inner and outer bearings. A Hall Effect sensor plugs into a carefully machined hole in the forging, locating it just above the tone ring.


ABS tone ring and sensor

As the vehicle's wheel turns, the tone ring teeth pass by the sensor. The gaps between the teeth trigger the sensor in direct relationship to their speed. Under braking, the anti-lock computer compares the signals from the wheel speed sensors. If it sees the wheels are locked or turning at different speeds (skidding), it triggers the ABS braking system to modulate the brakes.

Since there is no direct mechanical contact between the tone ring and the sensor, the air gap between them must be precisely maintained or it will give false readings. The air gap in this type of system is normally in the range of 1mm.

NHTSA is investigating the likelihood that salt used on the roads in some states is working its way into the hub assembly via the sensor hole, contaminating the hub assembly, fouling the all-important air gap and corroding the sensor itself. When the gap is fouled in any way, the sensor can report a higher or lower speed than actual, or send a garbled, useless signal, any of which can confuse the ABS system into working when it should not, or not work when it should.

For example, coming to a low-speed stop, three of the wheel speed sensors may be reading 50 RPM, while the contaminated sensor reports 500 RPM. The ABS braking computer compares signals and sees this situation as a vehicle out of control, one wheel turning 500 RPM and three wheels skidding at 50 RPM. Because the purpose of the ABS system is to prevent wheel lockup and skidding, it kicks in aggressively to pulse the brakes. But when ABS triggers at too-low speeds, it significantly and unsafely lengthens what should be a simple low-speed stop.

In late 2004, GM recalled 150,000 light trucks in eastern Canadian provinces (Ontario, Quebec, Newfoundland, New Brunswick, Nova Scotia, Prince Edward Island) to address the same complaint. Hubs and sensors were tested, cleaned, and treated with zinc to inhibit future corrosion.


The text of the Canadian recall:

A defect which relates to motor vehicle safety exists in certain 1999-2002 Chevrolet Silverado, 2000-2002 Chevrolet Tahoe, Suburban, 2002 Chevrolet Avalanche, 1999-2002 GMC Sierra, 2000-2002 GMC Yukon, Yukon XL vehicles located in Ontario, Quebec, Newfoundland, New Brunswick, Nova Scotia, and Prince Edward Island.

These vehicles may have a condition permitting corrosion to occur between the front hub/bearing assembly and the wheel speed sensor. If the brakes are applied while the vehicle is travelling at a speed of greater than 6 km/h (3.7 mph) but less than 16 km/h (10 mph), the corrosion may cause an unwanted anti-lock brake system (ABS) activation. If this condition occurred where stopping distance is limited, a crash could occur.

Dealers are to clean and treat the affected area, and in some cases, replace the front wheel speed sensor.


In the United States, however, GM claims the number of complaints is so small that a recall is not needed. A GM spokesman said, We are still gathering information for our response to NHTSA, but when we looked at this issue last fall, there was a significant difference in how often this issue occurred in Canada and in the U.S. We saw three incidents of this per 100,000 trucks in the U.S., versus 30 incidents per 100,000 trucks in Canada." He added, "The numbers just haven't borne out the need for a recall."

By GM's math, it would translate to 39 U.S. vehicles. Yet NHTSA's action would refute that claim. The day it opened its defect investigation, NHTSA said it had already received more than 120 complaints about the problem from U.S. owners, including 22 crashes and four injuries.

In fact, eBearing has been hearing reports about failed wheel speed sensors in GM hub assemblies for years. In the automotive aftermarket, failed wheel speed sensors were a boon to hub assembly sales because the sensors themselves were often not available separately; the consumer had to buy an entire hub and bearing assembly even if only the sensor had failed.

The current investigation covers trucks in 20 states using salt in the winter, a swath from Minnesota and Michigan, east to Main and Virginia.

The NHTSA investigation was initiated with this notification:

In November 2004, GM notified ODI that it was recalling approximately 150,000 model year 1999-2002 C/K model trucks in eastern Canada to correct a condition in the antilock brake system (ABS) that may result in unwanted ABS activation and increased stopping distances during low-speed brake applications. GM indicated that the condition can occur if corrosion forms under the wheel speed sensor and moves the sensor away from the integral speed-sensor toothed ring.

The condition does not set any ABS codes, nor does it illuminate the ABS warning lamp. GM's recall procedure involved removing the wheel speed sensors, cleaning the sensor mounting surfaces on the bearings, applying zinc-X to the cleaned surfaces, greasing the mounting surfaces, reinstalling the speed sensors, and verifying that the wheel speed signal output voltages were within specification.

GM reported a failure rate of 0.32 incidents per thousand vehicles covered by the recall in Canada. GM indicated that the corresponding rate in U.S. corrosion states was 0.03 IPTV and that it would continue to monitor that experience.

ODI has received 120 complaints involving MY 1999-2002 C/K trucks in U.S. salt belt states, which appear to be related to the defect condition addressed by GM's safety recall in Canada.

Twenty-two of the complaints allege that a crash occurred, including one that involved six vehicles and resulted in four injuries. A preliminary evaluation has been opened to assess the incident frequency and trend associated with the wheel speed sensor condition in the United States.


Based on the results of the investigation, NHTSA has the authority to force General Motors into a full recall.


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