Add some big capacitors.
Now that I have the truck back on the road, I discovered a problem with the RPM display. When the accelerator is pressed, the display goes blank. When I let off the pedal, the readout is visible.
I figured this was due to EMI from the PWM of the controller and motor. I designed a new version of the circuit using differential signaling and higher current. But, before I installed the new circuit, I wanted to observe the signal corruption.
Today, I put a 12 volt marine battery and an AC inverter in the truck to power my oscilloscope. I observed the signal sent to the display unit. I did not see any obvious signs of noise. I added another circuit to clean the signal further, but had the same result.
I pulled the display out of the dash and examined it. I found no obvious problems. I remembered that I did not have any large capacitors on the board. So, I added a 1000uF and 2200uF capacitor along the voltage regulator.
You can download the source code and some supporting information from my personal web site.