ignition coil driver

Summary

In this episode of Diagnose Dan, the team tackles a BMW E36 3 Series that’s misfiring on one cylinder. Despite previous attempts to fix the issue, including replacing the coil and checking wiring, the problem persists. Using a thermal imager and oscilloscope, they diagnose a faulty ignition driver in the PCM, specifically a broken MOSFET. After exploring the role of MOSFETs in ignition systems, Dan demonstrates a test setup showing how a functional MOSFET should behave. With a new (used) PCM installed, the issue is resolved, proving the importance of understanding ignition drivers in engine diagnostics.

Takeaways

• 😀 The BMW 3 Series has a misfire issue, running on only 5 out of 6 cylinders.
• 😀 A thermal imager was used to diagnose the problem, revealing that cylinder number 6's exhaust manifold was colder than the others.
• 😀 The previous shop had replaced the spark plug and ignition coil, but the problem persisted. The PCM (Powertrain Control Module) was also checked and found to be fine.
• 😀 The issue was traced to the PCM's ignition coil driver on cylinder number 6, which failed to provide ignition control.
• 😀 A back-probing technique was used on the ignition coil to measure control signals from the PCM, revealing the lack of control for cylinder 6.
• 😀 To rule out wiring problems, measurements were taken directly at the PCM, confirming that the issue was within the PCM itself.
• 😀 The PCM's ignition coil driver on cylinder 6 was faulty and required replacement or repair, as the driver inside the PCM failed to switch on the ignition coil.
• 😀 A MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) is used to protect the CPU in the PCM and handle the high current needed to control ignition coils.
• 😀 The MOSFET switches the ignition coil to ground by using a small voltage on the gate, creating a current flow that induces a spark in the ignition coil.
• 😀 The damaged PCM was found to have a short in the MOSFET for cylinder 6, which caused current flow into the CPU, damaging the PCM beyond repair, leading to the replacement of the PCM.

Q & A

What is the main issue with the BMW E36 that is being diagnosed?

The main issue with the BMW E36 is that it's only running on 5 out of 6 cylinders, with no ignition on one of the cylinders.

What diagnostic tool did the technician use to determine which cylinder was misfiring?

The technician used a thermal imager to identify that cylinder number six had a colder exhaust manifold compared to the others, indicating no ignition.

What was the previous shop's diagnosis before the car was brought to the technician?

The previous shop determined that the ignition coil and the clock had been replaced, and they had checked all the wiring. They also sent the PCM to a specialized company for testing, but it came back without any issues.

What is the purpose of using a scope during the diagnostic process?

The scope is used to analyze the electrical signals from the ignition coils, specifically to determine whether the PCM is properly controlling the ignition coil on cylinder number six.

What did the technician find when testing the ignition coil control signal for cylinder number six?

The technician found that while power was reaching the ignition coil, the PCM was not pulling the power to ground, which meant it was not controlling the ignition coil properly.

What is the role of a MOSFET in controlling the ignition coils?

A MOSFET is an electronic switch that protects the CPU in the PCM by carrying the current needed to activate the ignition coils, preventing the CPU from handling too much current directly.

What causes the failure of the ignition driver in the PCM, according to the technician?

The failure of the ignition driver was caused by a short in the ignition coil, which damaged the MOSFET and the pin inside the CPU that controls the MOSFET's gate.

How did the technician demonstrate the function of the MOSFET during the repair process?

The technician used a test setup where a MOSFET from a working ignition coil was tested by applying a small voltage to the gate, which closed the switch and lit a light bulb representing the ignition coil.

Why was the old PCM deemed beyond repair?

The old PCM was beyond repair because the MOSFET and the CPU's pin controlling it were completely damaged due to the short in the ignition coil.

What solution did the technician use to replace the damaged PCM?

The technician used a used PCM, but since it had an immobilizer code, it needed to be reprogrammed to work with the car.