I think I hava bad pressure sensor.
As according to the Haynes manual: "At 3.9Hg should = 3.5-4.5v"
I'm reading about 2.02v.

So 2 things: 1) Is driving with a bad sensor going to do anything harmful to the motor.
2) How much and where can I get a new/good used one?

 

...and is the N/A pressure sensor swappable with the Turbo one?

 

No.

I'm wondering how you induced "3.9Hg" of vacuum?
Is that "mmHg" or "inchesHg"?


-Ted

 

3.9inches from a hand vacuum pump.

 

You know what, it sounds like the Haynes manual is wrong.
3.9 inHg is not that much vacuum.
Your ~2.0VDC sounds about right.
You're talking about a Turbo II, right?
The FC turbo boost sensor is a 2-bar MAP, so at "0" manifold, it should read ~2.5VDC; 3.9 inHg is slightly under that, so your voltage readings are correct.

The non-turbo pressure sensor is a 1-bar MAP, and it would read slightly under 5.0VDC @ 3.9 inHg.
So those numbers you got from the Haynes manual sounds more like for a non-turbo pressure sensor.

You are talking about the sensor in the engine bay, right?
Mazda officially calls it a "boost sensor"?
The FC turbo also has a second pressure sensor located in the passenger's side kickpanel called an "atmospheric (pressure?) sensor", but you would be hard pressed to use a MityVac on that thing.
The non-turbo FC uses, what Mazda officially calls, a "pressure sensor".


-Ted

 

Usually the Haynes gives separate values for Turbo or N/A. It didn't for this so I assumed they were the same.

I've never had a problem with the Haynes but, I just checked the FSM and sure enough, Haynes is wrong!

FSM says the boost/pressure sensor(shock tower) "Voltage should be 2.0-2.5v at 100mmHg(3.9inHg)"

The atmospheric pressure sensor should be 3.5-4.5v at sea level. Which the Haynes is correct about.

hmmm...What do ya know!