The Transmission Killers

It reads like a classic “Who dunnit” novel: You’ve got the body (in this case, a GM 4L60E transmission) and you’ve got the crime scene (a 1996 S10 pickup), but who’s the culprit? Some would say the nut behind the wheel, but in fact it was a couple of electronic thugs – two input sensors who were breaking parole.

Daryl “Doc” Rose of Thunder Alley Transmissions in Penticton, BC explains. “It had a bad TPS and a bad Speed Sensor that ended up burning the tranny out.” A road test with a scanner and a pressure gauge revealed that when the computer lost data from the faulty sensors, everything went crazy. “The speedo was jumping, the TPS was erratic, the (shift solenoid indicator) lights were flashing everywhere, the tranny was shifting up and down, the pressures were going up and down…the computer was just basically taking a stab at it.” Without proper information, the computer supplied improper line pressures, resulting in burned out clutches, bands, and oil as black as tar.

Old transmissions used engine vacuum to tell what your foot was doing, Doc explains, and the governor on the output shaft supplied the road speed. “On new ones,” he says, “if the TPS or Speed sensors are out, then the computer has to guess at what your pressures have to be.”

Dieter Schmitz, of Mister Transmission in Oakville, Ontario (see SSGM June 2003 issue, Dieter on Drivability) agrees, noting that a TPS reading too high will give you late shifts and high line pressures. “If a TPS…is stuck at 4 volts constantly, then that’s telling the computer that you’re almost to the floor with the accelerator pedal. The transmission is going to compensate because it thinks you’re being aggressive on the accelerator, thereby scheduling the shifts later, and or raising the line pressure at the same time.”

“With these modern day electronic transmissions,” says Doc Rose, “when things like the speedometer quits, you just can’t ignore it anymore. When the speedometer goes down, it…has just lost one of the inputs it needs to decide what to do.”

Unfortunately, many drivers try to hold off on repairs. Randy Moore from the Mister Transmission national office believes that today’s consumers have a breakdown mentality. “Even if it’s limping, they’re still driving it,” he says. “They just avoid going into the repair facility because A) they’re not sure what it is, and B) they’re afraid that once they get in there, they’re going to spend some money.” But ignoring the problems will only make things worse.

A case in point was a 1997 2500 series GM truck with a chronically cold-running engine, due to a stuck thermostat. Because it never warmed up, it never went into proper closed loop. With the 4L60E transmission, closed loop is required to go into full converter lock-up, which completes the lubrication circuit. In this case, lack of lubrication burnt out the low/reverse clutch pack.

Other inputs to watch for are faulty MAP Sensors, and on Ford Diesels, a faulty tachometer will also affect transmission operation.

Sometimes transmissions get blamed for crimes they didn’t commit. “A lot of vehicles with an engine misfire are being self-diagnosed as having a torque converter lock-up problem,” says Dieter Schmitz. “Quite often people think it’s a lock-up shudder because all of a sudden things start shaking when the lock-up comes on.” In reality, when lock-up occurs the input goes from fluid to mechanical and now a subtle engine miss becomes amplified.

A trained technician can tell the difference between a lock-up problem and an engine miss, but most customers can’t. “Usually a lock-up stutter is pretty steady and rhythmic,” says Dieter, equivalent to driving over rumble strips on a highway. A misfire is more erratic, “like somebody’s bumping into your vehicle from behind.”

“We get a lot of problems that are drivability problems,” adds Randy Moore. “They’re not really transmission problems at all.” He points out that their transmission repair franchises across the country spend a lot of time diagnosing engine drivability issues.

Shop owner Karl Auerbach in Kelowna, BC mentions another problem they see at their Mister Transmission outlet — people using the wrong fluid. With the rapid growth of fast-lube outlets that heavily promote transmission flushing, Karl wonders, “Are they aware of the special fluids needed?” When a customer comes in with slippage and shudder, he first asks if they’ve had a recent flush, and often the answer is yes. “But did someone use the wrong fluid? We don’t know without doing an oil analysis,” says Karl, but they’ve had good results by just draining out the fluid and refilling it with the known proper oil. Oftentimes they don’t need to go any further.

Ford and Chrysler are the major concerns; Ford uses Mercon V and Chrysler has 7176 ATF +3 (2000 and up now requires ATF +4). Chrysler seems to be more sensitive to wrong fluid scenarios. “You can get a lockup shudder, or when the clutches are going to apply,” says Dieter Schmitz, “because the frictional characteristics of the fluid aren’t being matched to the lining.” In this regard, “re & re” units brought in for a bench rebuild are a source of concern for transmission shops. “Sometimes I have to wonder if they’re using the right fluid or not,” says Dieter. He tries to tag the unit if a special fluid is required for refill. Even if the proper oil is expensive (like $30 a litre for BMW), Dieter believes it’s worth it in the long run.

Another repeat offender in the attack on transmissions is mismatched tires on four-wheel and all-wheel drive vehicles. If the tires are not replaced in sets of four, the differing circumferences front and rear will dramatically affect the operation of the transmission/transfer case.

While this is not the whole story on external factors that can affect today’s electronically controlled transmissions, the basic theme is this: Treat the patient as a whole. It comes down to understanding how everything works together, and realizing that good technicians have to be good detectives as well.