My Vacuum Pump Won’t Stop!

The brake pump in my EV has been acting up. In an EV the brake pump provides vacuum to run the power brakes, as the original source of vacuum (the internal combustion engine) has been removed. I have mine connected to a little vacuum switch (bottom of photo with screwdriver adjustment) that turns the pump on when the vacuum drops beneath a certain level. This usually makes the pump run for a few seconds when you brake. However it was turning on then staying on indefinitely. It took me a while to work out why.

I initially thought there was a leak in the power brake hoses. Or a dud vacuum switch or brake pump. One day the pump was stuck on and I measured the voltage across the pump. It was only 11.2V. Now the nominal voltage should be 13 and a bit volts. Two volts doesn’t sound like much but power is V-squared on R, so assuming the motor has a constant resistance it’s V squared equals 122 versus 169, a 50% difference in power. This might explain the inability of the vacuum pump to switch off – it just couldn’t drive the vacuum down with the reduced supply voltage.

I exposed the DC-DC converter wiring (it’s hidden under the front passenger seat), and tried to measure the output voltage at that point. The ground wire immediately fell out of it’s crimped (last year by me) connection! Oh dear, that explains it. I re-soldered the connection and immediately the vacuum pump voltage lifted to nearly 13V under load, switched off quickly, and I could hear the pump was running much faster.

This took me a few weeks to work out before I eventually fixed it. In the mean time we just put up with the pump running most of the time, and the brakes still felt fine. Like most of my EV problems it’s something due to my inexpert construction of a one-off prototype, and costs very little (just time in this case) to fix.

My daughter and I drive the EV every day. Actually it’s more precise to say she drives it every day and I use my push bike! We have now driven over 40,000 electric km in 5 years. It’s been two years since I sold my last infernal combustion car, although I still borrow/rent or travel in them occasionally.


Well that fix didn’t last long! Despite a good strong 13+ volts the brake pump is still staying stuck on at times. As a temporary measure I have wired the brake light to the relay that controls the brake pump. Press pedal, brake pump comes on. Pedal feel good so I think there is sufficient vacuum. Meanwhile I’ll try to source a new vacuum switch….

1 comment to My Vacuum Pump Won’t Stop!

  • Robert Boerner

    Hi David, As always your process for discovery and repair and your ability to communicate it are excellent. Thanks for continuing to share your experiences.