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Motor Age Garage: Troubleshooting Traps

Warning lights, misleading trouble codes and intermittents are the spice of a technician's life.
Wednesday, October 1, 2008 - 00:00

Warning lights, misleading trouble codes and intermittents are the spice of a technician's life.

Motor Age Garage 2001 pontiac montana traction control traction control problems automotive aftermarket vehicle repair repair shop repair shops

I am teaching eight courses this semester, and live work is piling on us faster than we can work it out. The cool thing is that the students are learning to gather data and make surgical repairs.

For example, on a 2001 S10 Blazer, we had ignition related radio noise. It's not your ordinary ticket, but tremendously instructive. We narrowed it to the ignition system by killing the alternator first and then the coil while listening to the static. The noise went away the instant the coil was taken offline. We found that the little carbon button in the center of the distributor cap that rides on the rotor tab had dropped out of the cap and the spark had to jump about 1/8 inch to hit the rotor. That naked spark was producing a dandy amount of radio frequency interference. A new cap took care of that one.

A new female student of mine called to say she wasn't coming in that day because her 1995 Grand Am wouldn't start; her dad and her boyfriend said it needed a starter. She didn't live far away, so I sent a couple of other students to retrieve it with a towrope. Sure enough, the starter would not do much of anything except grunt, even though the battery was new and fully charged. Bench testing the starter, we found it healthy and strong, but a student named Hunter put a socket and a breakover bar on the crank bolt and found that he couldn't turn the engine at all. Were the bearings welded to the crankshaft? I checked the oil. It looked fine. Time to garner more info. I posed a question:

"Was the engine making noise of any kind before you parked it?" Her answer was revealing.

"No, the only noise it has been making was the air conditioning compressor, but it has been making that noise for weeks."

Bingo!

I had Hunter try to turn the engine again, this time with more muscle while I was watching the A/C compressor pulley. I saw the belt slip on the pulley. Off with the belt, and the engine spun freely. She had another compressor in the trunk that wouldn't fit the car, but it did have the same hub and pulley, and we swapped those from that unit to hers. That was a cheap fix that got the car going again.

Then there's the 1997 Grand Marquis with a wiped out rear axle that wasn't too difficult to diagnose (the driveshaft turns, but the car doesn't move), and the 2001 Explorer that came in running hot. We added 3.5 gallons of coolant mix to the Explorer's radiator, and it seems to run OK. But it's drinking water like a thirsty camel; at the very least it'll need head gaskets.

Another Explorer, a 1996 model, came in with a bad misfire and an inoperative radio. The owner told me somebody had connected a set of jumper cables backwards and she thought that had caused the problems. But after the O-scope test and a run-down of the radio circuits, all she had was a blown fuse and a carbon tracked plug and wire.

Then there was the 2001 Montana.. .

What Does 'Traction Active' Mean?

This van has quite a few miles on it and has been in the automotive shop more than once or twice. We do the brakes, oil changes, A/C work, etc. on a lot of cars that belong to faculty, staff and students, not to mention the state vehicles.

The problem with this Montana was that the message center would read "TRACTION ACTIVE" at the strangest times, usually when driving away from a stop on dry pavement.

If you've ever been stuck in the mud or in wet grass without a positive traction differential, you know that one wheel spins like crazy and the other one sits fallow during those maneuvers. Modern Traction Control Systems (TCS) provide an electro/hydraulic answer to that irritating syndrome.

The TCS on the Montana reduces front wheel slip during acceleration by first reducing engine power and then by applying the brake on the spinning wheel. The PCM supplies engine torque information to the Electronic Brake Control Module (EBCM) so that module can decide what to do about wheel slip. If a torque reduction is deemed necessary, the EBCM can't do that directly. It has to send a hat-in-hand request to the PCM, which retards ignition timing and/or shuts off some of the injectors as it sees fit. It won't kill injectors if the engine coolant is low or if the coolant temp is outside the 40 degrees Fahrenheit to +268 degrees Fahrenheit window. Engine speed less than 600 rpm also will delete the injector dropout strategy.

I repeatedly test drove the van with a freshly updated Genisys scan tool connected to it and only duplicated the problem twice. The message center would inform me that the Traction Control System was active, and I could hear the mild buzz of the EBCM doing its work, which was exactly what was described. My idea was that one of the wheel speed sensors was improperly reporting, but would that wheel be the slower wheel or the perceived spinner? It was hard to say. I couldn't pick out any unusual wheel speed sensor activity on the Genisys screen that time around, but Genisys takes a sample only twice every second.

A scan of the DTCs initially revealed only one code, i.e., C1295, which the Genisys defined as "Brake Lamp Switch Circuit Open."

So what the heck does the Brake Lamp Switch have to do with the TCS?

Well, the EBCM needs to know when the brakes are applied so it's not fiddling with the brake pressure feed to any of the wheels while somebody's trying to stop. Pin C7 on the EBCM monitors the brake switch input through a current dropping resistor, and according to GM's shop manual, the brake switch failure code can trigger the message we were getting.

Since that was the only diagnostic trouble code we had, that's the direction we headed.

Expensive Connector Shells

Step 6 in the C1295 matrix spoke of checking the ground circuit of the stop lamps for open or high resistance, and our diagnostics led us to that point. Well, the taillamp boards on this Montana were a whole lot like the ones that fail so frequently on GM pickups, and while the stop lamps typically worked with every brake pedal action, we found that we could get the right rear stop lamp to go out by gently twisting the connector.

These funky circuit boards (that's what I call them) form the electrical backbone of each light, with the wire connector plugged into the board and all the bulbs plugged into the other side. Laminated metal carries the juice from the connector terminal to each bulb. Gone are the days when each light had its own pigtail socket.

I ordered a replacement board (left and right look alike but won't interchange, by the way), but the terminal-twisting problem persisted even though the terminals in the connector looked OK. A magnified close-up look at the six-wire taillight connector revealed a ground terminal problem. It appeared to be slightly discolored and a bit loose on its pin, and after an attempt at tweaking the connector terminal, we flipped through the Delco picture chart and pursued the option of getting a replacement pigtail from a GM dealer. It was $60. That took care of the intermittent stoplight ground, and for a couple of days, the Montana performed flawlessly.

Had we fixed the Traction Control problem? The answer turned out to be a big fat "no."

Digging for Treasure

Our lack of carnal knowledge concerning this system was forcing us to dig deeper. This system isn't all that complicated, but like a lot of other electronic subsystems on different platforms, there were nuances with which we obviously weren't familiar, and there was some crucial piece of data we were missing. The false Traction Active events now were coming more frequently.

With the Genisys booted up, we yanked a fresh code: A C1226, i.e., "Right Front Excessive Wheel Speed Sensor Variation." Where had that code been the first time around? This was a favorable development but a bit annoying.

Should we throw a right front hub and sensor at it? Not on my watch! The parts house price on that hub and sensor hovers near $100 (GM's list price is about $340). We already had spent unnecessary money, and I wanted more input!

With the Genisys booted up, we built a custom data screen that included the vehicle speed and both front wheel speed sensor inputs. Graphing the wheel speed sensors, I had a student drive the van and make a recording when the concern repeated itself, which was, as the owner had said, a lot more often. The left wheel speed sensor was reporting 2 mph during the false Traction Active event, a speed that agreed with the vehicle speed sensor exactly. The right hand sensor was reporting 21 mph, and I knew the student wasn't spinning the wheels when it happened.

I still wasn't comfortable with tossing a hub and sensor at it, so I fired up the four-channel digital scope and connected it to each of the front sensors. The scope pattern showed a nice waveform of the left front sensor, but the right front sensor was as flat as a flounder, and I must have checked my probe connection a half-dozen times.

Measuring the voltage at the sensor connector with a digital voltmeter, I got a static 4.8 volts and no sensor activity on that right hand sensor with the wheel spinning. This indicates an open sensor winding, and with my meter clicked to the ohms position, I verified the problem.

Hub Time

Now I had to explain our diagnostic process to the owner, who teaches computer science and is sharp enough to understand what I was telling her. We don't typically throw parts around like buckshot in my department, but in this case we had initially gone in the wrong direction with our diagnosis.

We found a problem at the stoplight connector that we thought could have caused the concern (based on what we read in the shop manual), but the right front wheel speed sensor wasn't bad enough to throw a wheel speed sensor code during our first round of diagnostics. As it deteriorated, it turned out to be the source of the concern. A scope analysis on the first visit would probably have streamlined the diagnosis.

A technician who has seen a few of these might have found the problem sooner, but there always has to be a first time. I've seen a lot of those sensors fail, but I haven't seen them cause Traction Active messages. It was a learning experience to be sure.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at rwm19@mail.com

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