The Real Deal! - May - 2022

Light Work Makes Light Work!

When a vehicle comes to your shop with a problem, we can be lulled into a state of applying automated solutions based on typical problems we’ve experienced with similar cars. Even though we are trained to follow basic diagnostic procedures, like checking the static and cranking battery voltage and charging system health, the issue at hand tempts us to bypass the basics. So let’s look at a vehicle where tending to the basics could have saved us time and effort.


I had a call that came into the ATRA Technical hotline about a 2008 Pontiac Grand Prix with a 4T65E transmission that had no lock-up command after rebuild. There were no codes nor check engine light on, and all engine and transmission scanner data seemed correct at a glance, but still, the PCM was not commanding lock-up.

A closer inspection of scanner data showed the PID for the brake switch was always on. However, when we checked the operation of the brake lights, they worked correctly; when we applied the brake, the lights were on and went off when we released the brake. So at first glance, everything was present to allow for the lock-up command, yet the PCM was still not sending a signal to the TCC solenoid.

You could feel the frustration on the other end of the phone. Like many jobs, the closer it is to being done, the more urgent the customer is to get the car back. When the tech said the scanner showed the brake switch was applied all the time, it brought up a memory of a similar job that I had worked on before.

“Does the 3rd brake light work”? I asked.

“No. There isn’t one. I mean, the light fixture is missing,” he answered.

The connector was still there, so I asked if they could temporarily wire in a bulb to complete the circuit. The technician used a test light in the circuit. Looking at the scan tool data, the Brake PID now showed ‘released’ when released and ‘applied’ when the brake pedal was depressed. At this point, I advised them to do whatever was necessary to get the light permanently functional.

On a follow-up call the next day, they informed me they found the fixture lying on the floorboard. At some point before getting the car, someone had removed the fixture for whatever reason! It was plugged in and secured in the vehicle. The tech performed a test drive and confirmed that the TCC operation was restored and the car was delivered.


So how does the 3rd brake light affect TCC function? Looking at the schematics, we see the BCM receives a signal from the brake switch, then, in turn, sends voltage out to the Center High Mounted Stop Lamp (CHMSL), also known as the 3rd brake light (figure 1). Checking the white wire at the 3rd brake light connector shows 12V when brake applied and 0V when released.

So, how does it know if the circuit is incomplete? While not only sending out voltage to illuminate the light, the BCM also senses current in the circuit. If the current cannot flow, the BCM cuts the voltage and tells the PCM that the brake is applied. If the vehicle is driven long enough with this condition, you’ll frequently see trouble codes like:

  • P0571 Cruise Control/Brake Switch A Circuit Malfunction
  • P0572 Cruise Control/Brake Switch A Circuit Low
  • P0573 Cruise Control/Brake Switch A Circuit High
  • P0703 Torque Converter/Brake Switch A Circuit Malfunction

The lack of cruise control function may be why the vehicle comes back to you with the customer saying, “ever since you worked on it…”.


If we look at GM trucks with drive-by-wire throttle systems that use a Throttle Actuator Controller (TAC), the 3rd brake light circuit is monitored for activity by that TAC module. Like the Pontiac, the Silverado will also display Brake Applied on the scan data. The TAC module is also the cruise control module. As you know, if the brake is on, the cruise control is disabled. In most cars, the 3rd brake light is inside the passenger compartment and protected from the weather, but in trucks, the light fixture is exposed to the elements and damage. The weather seal is prone to water leakage from the seal, deteriorating and corroding the connections. Corrosion means resistance, and resistance means the electrical current will be slowed. If resistance is high enough, the computer feedback for the 3rd brake light circuit will interpret this as the brake pedal applied.

The customer can cause a similar problem. Frequently, you’ll find aftermarket LED lights installed in place of OEM bulbs (figure 2). They are popular and add a unique look to vehicles. If a factory incandescent bulb was replaced with an LED direct replacement, this could also cause problems with the brake PID responding correctly, resulting in TAC module issues. In this scenario, the lights will function, and it would be easy to assume the fault is in the TAC module. However, you find the same issue after replacing the module. Remember, just because it looks like it’s working doesn’t mean it is!

You’ve undoubtedly seen the rapid blinking of turn signals replaced with LEDs, a phenomenon known as ‘hyper-flashing.’ The LED draws so little power that the circuit will behave the same as a burnt-out incandescent bulb, causing the turn signal flasher to cycle rapidly. However, there is no flasher in the 3rd brake light system, and in the case of the circuit being used as a TCC/Cruise Control Brake input, an LED is seen the same as a burnt bulb.

The circuit must be modified to rectify the situation. Therefore, a resistor must be wired in parallel with the LED in the circuit to increase the load to give it sufficient current (figure 3). If you find LED replacement bulbs in the OEM fixture, change the lights to the regular incandescent bulbs, and verify the Brake Applied PID is reading correctly. Advise your customer of their repair options. If they desire to have LEDs, recommend using a quality replacement LED fixture. A quality fixture will have the proper resistor size configured to make the system work correctly.

More frequently, a seemingly complex problem can be caused by the most simple and basic items. Overlooking fundamental checks can lead you down the wrong diagnostic path. You may be surprised by how many issues can be resolved by taking care of first things first!