A while back I got a rush from rewiring the power brakes' vacuum pump: it had been directly connected to the battery, allowing it to turn on at odd times, even when the car was off. I rewired it to be part of the ignition circuit.
The rush came with a nasty let-down. It turned out that the pump blew the fuse regularly. So I increased the fuse rating (which everyone knows is a bad idea). The big problem turned up a little later: sometimes the car would "stall", refusing to turn on unless the vacuum pump was running.
At least I could just pump the brakes to restart. But it was still a safety issue, and not just from a driving standpoint. My wires weren't rated for the pump's current, and they were getting warm.
This weekend, I bought a 40A relay off the shelf in my local auto parts store. I left the vacuum switch wired to the ignition circuit, but instead of running the pump, it runs the relay. The pump is connected to the battery through the 40A side of the relay.
It works! Of course, there is a tradeoff.
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