Good idea. After I wrote the post and kept looking online I was reading about oil pressure switches. Then I remember that I actually have a low pressure light that is activated by a low pressure switch, which means that I will need a NC relay that opens when the pressure is low. Then the relay would have a positive signal from the ignition as is now (but connected to the NC terminal), and grounded through the low pressure switch.
Sounds like it should work. Take a 5 pin relay and run the low pressure switch through 85 to 86. This will energize it with key on - engine off. Run your vacuum pump 30 to 87A. This will turn the pump on when engine pressure build and your light goes off. Leave 87 blank. You will need to get power from the key on/switch on to go to either 30 or 87A. Hope this makes sense to you.
Exactly. I actually have had the vacuum pump wired for 3 years on key on so I just need to add the pressure switch to the ground of 85-86. For those that can follow all this electric stuff I also have a vacuum activated solenoid connected to a vacuum reservoir so the vacuum pump comes on at 17ish and goes off at 22ish. The vac reservoir is also connected to the manifold vacuum through a one-way valve to help with vacuum.
