Vacuum Gauge always 6-8 inHG wrong?

[z284pwr]

I've been wondering this ever since I had the Haltech/Autometer Gauge installed....I've asked a few different boards and no one seems to know anything about it, so I figured it wouldn't hurt to ask.....
Anyways, heres the thing, I've got the Autometer Electric Boost/Vac 30 inHG/20psi gauge, and a Haltech E6K computer, and the Gauge has ALWAYS been ~6-8 inHG low, as in, the Haltech reads ~22 inHG at idle and the Autometer reads ~14. Then at WOT the Haltech reads ~6 and the Autometer 0. Then when I let off and get full vacuum, the Autometer will always peg at 20 inHG and has never ever gone higher than 20. From what I've talked to people with, the Haltech is correct and the Autometer is wrong. Anyone have any ideas on what could cause this? Bad Autometer MAP sensor?
Thanks for any ideas.

[Guest]

With the gauge only going as high (or is that as low? ) as 20", I woould suspect the gauge or sender, but I'd also look at the Haltech as well, at WOT it should be a lot closer to 0, usually at 0, but all depends on the entire combination, not just would should be.

What does the Haltech show, when you turn the ign on, but not start?

Also elevation may have an effect on the readings. But that's quite a difference.

[z284pwr]

With the gauge only going as high (or is that as low? ) as 20", I woould suspect the gauge or sender, but I'd also look at the Haltech as well, at WOT it should be a lot closer to 0, usually at 0, but all depends on the entire combination, not just would should be.

What does the Haltech show, when you turn the ign on, but not start?

Also elevation may have an effect on the readings. But that's quite a difference.

When I say 20, it won't go any higher of number *Towards 30*, more vacuum. The Haltech shows ~6.8-7.5 at WOT and shows ~5.3 inHG with key on, and not running, this is at 4400 feet elevation, and its ~862 Millibars.

[Guest]

Yeah that elevation may have something to do with the readings, I used to have a chart that would show pressure changes for elevation, but I don't remember where to find it.

Just out of curiosity have to tried other MAP sensors or guages to see if there was any change in the readings?

Yes, I knew what you meant by "no higher than 20", but we are discussing vacuum, so is a higher number higher or lower? hehe

[z284pwr]

I haven't tried another MAP sensor for the gauge itself since Autometer's MAP sensor plug is a special plug and I didn't really want to jack up the plug just to test somethig, even though I may end up doing so.

[RacerX11]

Get a good mechaincial boost/vacuum gauges if you are worried abiut accuracy. The GM MAP sensors are not known for being terribly accurate. I have also heard similar complaints about the electronic boost/vacuum gauges. As long as they are consistent, you can tune the Haltech properly and not have to worry about it.

Here's a link for baro pressure vs. altitude: http://www.sablesys.com/baro-altitude.html

At 4400 feet, you are at ~86 kPa, and ~25.5 inHg. If the Haltech is reading 862 Millibars (86.2 kPa), then I would say it is right on. Have you performed the baro lock feature on the Haltech? It sounds like the Autmeter is calculating gauge pressureand compensating for altitude, while the Haltech/GM sensor is giving an absolute reading.

Another way to check out the sensors is to get a hold of a hand-held vacuum pump. This will allow you to apply different vacuum levels to each sensor, and compare te gauge read-out to the electronic dislayed value.

Marty