The spinning stops here

Jan. 1, 2020
“Limited Slip” and “Positrac” differentials have a set of clutches between the axle gears and the carrier and a heavy S spring between the axle gears to keep spinning to a minimum.

Any time a vehicle alters its path of travel and bends its path into a curve, there are things happening that most drivers simply don’t think about. To begin with, the front wheels have to toe out on turns; the inner wheel has to make a tighter circle than the outer one, and this has to happen no matter which way the vehicle is turning.

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We should offer a hearty salute to the gifted engineers that set up this complex arrangement to begin with. But the challenge of making a turn isn’t limited to the wheels that steer unless the car is front wheel drive. The differential at the heart of the driving axle has to allow the inside wheel in a turn to rotate at a different speed than the outside wheel, and the spider gears in the differential do a slick job of allowing the wheels to turn at different speeds when necessary. These spur gears at the heart of the final drive don’t interact with one another unless one wheel needs to spin faster than the one on the opposite end of that axle. Some toy remote control cars even have differentials in their tiny little drive axles; it’s that important.
All that having been established, everybody who has tried to drive uphill in wet grass, ice or mud knows how annoying the action of these same gears can be when they deliver generous amounts of torque to the wheel with the least amount of traction, all the while letting the opposite wheel remain more or less asleep.

When I’m cutting my grass on my John Deere F525, it obviously is equipped with a differential, because when I get on one of the hillside portions of my yard and the weight of the mower shifts to the left wheel, the right one spins like crazy and the mower just stops moving. I have to lean my 230 pounds of body weight hard to the spinning wheel side of the mower in order to get traction on the spinner and get moving again.

This kind of thing also is an issue for drag racers. Some of the low budget hot rodders in the South will borrow Billy Bob’s Lincoln cracker box and weld the spider gears to the axle gears on cars they take to the drag strip so as to have both wheels shoving their favorite toy through the quarter mile. But driving these cars around town with the axle gears welded together is an extremely unpleasant experience.

When we have a vehicle on the lift (let’s say it’s a pickup truck) and we’re operating the engine and the driveline in hopes of finding a noisy axle bearing, we’d really like both wheels to spin because we want to check both axle bearings. But unless the differential is one of the limited slip variety, the passenger side wheel tends to spool up to speed while the one on the opposite side may remain stationary. In that case, the way to divert power to the driver side wheel is to grab the passenger-side park brake cable and pull on it so as to slow the wheel that’s spinning, and the gears naturally begin to throw torque to the previously fallow side of the axle.Differentials known as  “Limited Slip” and “Positrac” differentials have a set of clutches between the axle gears and the carrier and a heavy S spring in between the axle gears so as to keep spinning to
a minimum and traction at the max. Mechanical systems like the one just described work to force both drive wheels to move the car on slippery surfaces yet still let them slip when necessary – and that’s one of the functions traction control systems handle electronically, but that’s only part of what they do.

Autopilot
Traction control has been around since the 1980s. According to the sources I found, BMW birthed the concept of traction control as we now know it. When the Antilock Brake System (ABS) electronic control unit (using the wheel speed sensors) detects a spinning wheel, it fires up the ABS fluid pump, and then it machine-guns high pressure brake fluid pressure pulses to the brakes on the spinning wheel using the solenoids in the ABS valve body to isolate the brakes on the fallow wheel while directing fluid to the other one, thus letting the differential gears do the rest of the work. 

On many (not all) Traction Control System (TSC) equipped vehicles, the Powertrain Control Module (PCM) gets into the act as well, electronically de-torquing the engine (with spark timing and injector pulse) to mitigate wheel spin, particularly if both wheels are spinning. The PCM takes similar action if the engine is over revving or the vehicle is over the safe speed for the factory tires, so the programming already is in place, you see. On some early systems this engine de-torquing was done with a secondary throttle plate that was situated above the foot-driven plate. If the driver’s foot gets heavy enough that wheel spinning gets totally out of hand, the PCM salvages the situation by partially closing the upper plate, forcibly reducing the airflow on its own – it’s remotely akin to the choke on a carburetor, but it doesn’t enrich the mixture.

The ABS/TCS control unit receives inputs from the wheel speed sensors and the brake switch, and the TCS system also is influenced by the traction control disable switch. A couple of decades ago at the dealership where I worked, police cars that were equipped with traction control had the TCS disabled (the TCS OFF switch was bypassed with a jumper), because TCS is, more often than not, a problem for cops.

TCS Basics
When one wheel begins to spin much faster than its counterpart, the ABS unit enters traction control mode and uses the very powerful ABS pump to crank up brake pressure. As stated earlier, it closes the isolation solenoid in the non-spinning wheel’s brake circuit to block fluid flow to that wheel and sends pressure to the brakes on the spinning wheel. The TCS system doesn’t want to completely stop the spinning wheel; it only wants to slow it enough to divert some of the torque. 

So if the spinner begins to slow down too much (these decisions are made at Uzi speed, so if multiple modules are involved, it’s on the high speed Controller Area Network (CAN) bus), it isolates the wheel it is slowing and returns that fluid to the master cylinder to be recycled. Obviously, if the brake is applied, this whole process goes offline. Of course, normal ABS function is still enabled during stops, particularly if the wheels begin to slide.

If both wheels begin to spin, the ABS detects the spread in wheel speed between the traction and non-traction wheels, that’s when the PCM de-torques the engine just enough to stop the spinning. The purpose of applying a lot of throttle (if you’re a normal driver) isn’t to spin the wheels, it’s to get the vehicle moving, because that’s presumably why you’re behind the wheel.    

If you’ve ever wanted to get moving in a hurry and lost traction on a slippery surface, you can appreciate the PCM’s help. I threw a rod out of my dad’s 1966 Chevy pickup when I was a young teenager, because the grass was wet and I was trying to get off the shoulder and onto the road. At the time, like most teens, I wasn’t as savvy about what a worn out Chevy straight six could endure. When the truck started spinning, I just applied a little more throttle, and it was a bad call. Had the truck been equipped with TCS (not to mention a rev limiter) I wouldn’t have found myself on a creeper pulling the oil pan later that day.

What Can Go Wrong?
The wheel speed sensors (the front ones particularly) are in a fairly hostile environment, thus they fail more often than other ABS/TCS components. That being said, it takes a particular type of failure to cause TCS problems. It has to be an in-range failure that happens in such a way to fool the TCS system into thinking a spinning situation has happened when in reality it hasn’t.

The sensors that are built into the hub bearing would seem fairly robust, but they usually fail when the bearing starts making noise – presumably the trauma of bearing vibration compromises the sensor. Also, some cars have fairly fragile terminals in their wheel speed sensor harness junctions, and pin fit is a big issue. That particularly is the case on GM cars, where simply disconnecting and reconnecting them a few times can compromise the connection. The male pins typically are on the
sensor side, and these are replaced with the sensor. The female pins are the most trouble prone and unless you replace the connector, you’re stuck with those, and they’re the most wear-prone terminals. From that perspective, it would make more sense to put the female terminals on the part being replaced, but for some reason manufacturers don’t think that way.

The engine de-torquing part of this system seems to be the least trouble prone part of the system – on the vehicles with the choke-like throttle plate, wide open is default, and it is spring-loaded that way. But “perfect storm” type wire harness issues can cause that plate to close when it shouldn’t, at least in theory.

On the sensors that aren’t built in – the ones that simply mount to the hub assembly, you have to make sure there isn’t any rust or other obstruction that is preventing the sensor from seating the way it should. Variable Reluctance wheel speed sensors (the old fashioned ones) should, on the average, produce about 0.350 volts AC when you spin the wheel at one revolution per second. If in doubt, compare the opposite wheel’s readings. If you aren’t getting the same reading on both wheels, the sensor may not be properly mounted. That can cause too much clearance between the sensor tip and its pulse ring, which can change the amplitude of the signal and trigger false TCS activation. Check the mounting area for rust and burrs in those cases.

Broken wiring on the harness leading out to the wheel speed sensor on high mileage cars is not uncommon, especially on the front wheels. In a turn, if speed sensor wiring has an intermittent open, the ABS/TCS module tends to be fooled and can do unexpected things.

Most customers won’t complain about their traction control system unless it starts activating falsely, throwing warning lights or triggering message center flags. If TCS goes offline without letting them know about it, they might not even realize it’s inoperative, which isn’t a big deal.

These systems are typically scan-tool friendly and can throw quite a few codes, but not as many as you might expect. A peek at the trouble code charts on most ABS/TCS systems isn’t too scary – there just isn’t that much that can go wrong with these systems related to the Traction Control function. That’s not to say they can’t kick us around sometimes, especially when the problem is intermittent.

The 2001 Pontiac Montana I wrote about in 2008 was one of those. The TCS would activate periodically when the van was accelerating gently on dry pavement. Initially the only code we got after duplicating this concern was a C1295. Well, GM’s test matrix for that code led us around Robin Hood’s barn. We replaced a tail light board and a connector because you could twist one of the stoplight connectors ever so slightly and the stop light would go out. So we replaced the taillight board and its connector, but that turned out to be a rabbit trail. It was a forest and the trees situation, you know, one of those where you get in so deep that you lose track of reason? As we reflected on the situation, we did the forehead palm smack. Why the heck would a stoplight problem cause false TCS activation? It might cause the TCS NOT to activate, but false activation?  Not likely.

Anyway, our test drive went fine after that silly stop light repair, but the owner drove the vehicle that next day and the concern was still there. This time when we pulled the codes, we got two codes instead of one – the stop light code was back (which was particularly disgusting) but we also had a 

C1226, which speaks of wheel speed variation. It was interesting that we hadn’t seen this code before. On our next test run, we had the wheel speed sensors graphing on the display and when weexperienced the concern, the right front wheel speed sensor was reporting spikes in wheel speed, and while it’s hard to figure how that’s possible, replacing the hub on that side took care of the whole deal. We didn’t even readdress the silly stoplight DTC, because she wasn’t having any other concerns.

Conclusion
Since controlling engines and transmissions were the first really complicated electronics endeavors in automobiles, ABS/TCS concerns aren’t usually too bad if you have a description of the system, a good scan tool and schematics, etc. But in cases where things get really nasty, it helps to have access to online services like Identifix and iATN. But since TCS problems aren’t all that common, each new entanglement with one of these can be a learning experience. Technology keeps changing, and learning is what we do.              

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