Heineken's Garage

Diagnostic - CPU Issues
A CCU with a non-working CPU won't show any signs of life. All display elements, LEDs, buttons and knobs are non-working. When plugged into the vehicle, no initialization motor movements can be recognized. One way to confirm a CPU problem is to check pin 30 of the CPU with an oscilloscope. That pin carries the clock signal and should show a 11.9 MHz wave of 3 V peak-to-peak:

If the CPU is unable to start, the following pattern can be observed. It repeats with a frequency of about 28.6 Hz and indicates the processor is stuck in a restart/reset-loop:

If the CPU refuses to start after capacitor removal, trace cleaning, trace repair and capacitor installation, it's might be damaged beyond repair. Installing the CPU on a known working CCU (using a 64 pin socket) is a proven way to confirm a broken CPU.