Other Articles - July - 2017

Faulty Brake Light Switch Has Transmission Shifting In and Out of Gear While Stopped

Technology is a double-edged sword: On one hand, today’s technology has helped create cars that are more dependable, more efficient, and just plain drive better than anything that’s come before them. These days we get more power and performance out of a tiny 4-cylinder engine than we could once have hoped for from an engine half again as big.

At the same time, though, today’s technology can cause some real difficulties when the cars aren’t working right. Back in the day, a transmission that dropped into neutral on its own probably had a transmission problem. Today? It could be anywhere in the car… even in the brake lights.

Here’s a problem that had the technician scratching his head, mostly because he didn’t understand how the system worked.

Neutrals at a Stop

A 2012 Chevy Cruze with a 6T40 came in with a weird condition: It was dropping in and out of gear at a stop. The car only had 42,000 miles on it, the ATF was clean, and there were no other problems evident.

The technician checked the computer system and there were no codes in memory. So he decided to pull the transmission out and tear it down. A thorough inspection revealed nothing wrong inside.

But, since it was already apart, he rebuilt the transmission. He installed a new TECHM and put it back in the car. And it did the same thing it was doing before: It dropped in and out of gear at a stop.

Since rebuilding the transmission didn’t work, the technician bought a low-mileage, used unit and installed that in the car. And it continued to drop in and out of gear.

Finally someone suggested that the technician connect a scan tool to the car and look for something that corresponded with the transmission dropping in and out of gear.

It didn’t take long to find it: The brake light switch was cycling on and off at a stop, right in time with the transmission dropping in and out of gear. Replacing the switch eliminated the problem.

Working the Way It’s Supposed To

Okay, let’s forget about the technician throwing parts at the car; obviously a bad choice under any circumstances. Why would a faulty brake light switch affect the transmission like this? Turns out, the computer was doing exactly what it was supposed to do, based on the information it was receiving.

That’s because this transmission is supposed to shift into neutral at a stop, if you have your foot on the brake. Dropping into neutral reduces the load on the engine, which allows it to use less gas while stopped at a light.

GM first started adding this neutral-idle strategy to a few vehicles equipped with the 6T40 in 2012, and continued adding it to the rest of their vehicles through 2014. The earlier system is called Gen 1; the later with the new control system is called Gen 2. Both generations had some form of neutral-idle system.

So, if the system isn’t universal after a specific year, how can you tell whether you’re working on a Gen 1 or Gen 2? Check the TECHM (computer). Gen 1 systems have a 1, 2, or 3 on the identifier; Gen 2 systems use a B, C, or D.

In addition, Gen 2 vehicles don’t have pressure switches, so the filters for the pressure switches aren’t there, either.

The Neutral-Idle Operating Strategy

One of the big changes to these systems is the neutral-idle operating strategy. Here’s how it works:

For the computer to turn the neutral-idle system on, the transmission has to be in drive, first gear engine braking. The transmission has to be at normal operating temperature, and the throttle position sensor and the vehicle speed have to indicate the car is at a stop. Finally, the transmission computer has to see that you have the brakes applied.

When all these conditions are met, the computer reduces pressure to the 1234 PC solenoid 5, which reduces pressure to the 1-2-3-4 clutch. This lets the clutch slip, which reduces the difference in speed between the torque converter input, and the torque converter turbine or the TCC slip speed.

The reduction in the speed difference between these components reduces the engine load while the vehicle is stopped. This, in turn, reduces the amount of gas the engine uses.

Sounds simple, right? So why did this system cause a problem for the technician? Because the brake light switch began cycling on and off randomly, even though no one was applying the brake pedal. The computer saw this signal going on and off and started applying and releasing the 1-2-3-4 clutch.

Replacing the brake light switch corrected the problem. Had the technician been aware of the neutral-idle system, he might have been more likely to look for a problem in the system before considering a transmission problem. In the end, the solution was simple: replace the brake light switch.

Today’s technology is indeed a double-edged sword. And, if you don’t understand how that technology is designed to work, you can be sure that sword will cut you to the bone. But if you take the time to learn how the system is supposed to operate, you can feel comfortable no matter what type of car you’re working on.

Thank you to Bill Anthony of Total Driveline Experts in Wood Dale, Illinois, for information provided for this article.