Constant 12V during crank?

Tom Kelly

Active Member
Jun 23, 2016
I finally replaced the crappy Speedhut boost gauge with a new one. It calibrates to actual barometric pressure every time the engine starts. The instructions say it has to be wired to a 12V source that doesn't cut out during cranking in order to calibrate properly. Sure enough if you just wire it to a normal 12V that does cut out it gives weird readings. The tech rep told me that this would be the case. What circuit will do the trick in my 86 GN?
Got the gauge working now. It's the Autometer vac/boost digital gauge. It is now producing reasonable numbers. However, I noticed that it seems to be consistently different than the numbers on my scanmaster. For example, the gauge shows a vacuum reading 3 less than the scanmaster. I suspect that the difference might lie in how each calculates the reading. It might have something to do with ABSOLUTE vs RELATIVE? I've been reading up on this difference but can't seem to get my head around why they are so different. I live generally close to sea level so it's not that. I think I just need a knowledgeable person to help me get it. I have not had the chance to compare boost numbers yet.