Anyone got experience with what the output of a typical GM 3-terminal MAP sensor is? I want to experiment with using one as an electric vacuum gauge. I believe the output is a square wave, the duty cycle changing with pressure (vacuum), it's not a linear voltage out.

I also understand that they run on 5 Volts, is this correct?

If anyone is interested I can post what I learn from this experiment.

 

all you should have to do is read the voltage between the 5v refrence and computer input wires. theres a chart in the service manual that lists voltages according to pressures.

 

Your dead on with the square wave. Im curious to know how you plan to use the square wave to run a gauge. I think they work on a higher frequency being a higher vacuum.

 

With any luck I can find out this weekend just what kind of output the MAP produces. I was hoping that the frequency is constant with varying duty cycle, that's easier to work with. An integrator circuit would produce a DC value that a regular meter can work with. If it is varying frequency then I can't see an easy solution, it would require a tachometer type circuit.

 

Typical map for a non boosted application runs on 5 VDC with a variable 0~5 volts DC out based on vacuum level.

 

Most MAP sensors generate a voltage that drops as manifold pressure drops and vacuum rises. Typical readings, 0in-Hg=4.5-4.9 volts, 5in-Hg= about 3.5-3.8 volts, 20in-HG about 1 volt.

 

You guys are talking about a completly differnt type of sensor. This sensor will read only 5v or 0v and nowhere in between. If it was the good ole 2 wire sensor it would be easy to rig up a gauge.

 

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AM Racer and Danno are correct, the sensor runs on 5 volts and the output is a voltage that varies with pressure. I can see from the Haynes manual schematics that it is illustrated as a potentiometer internally, so it acts just as the potentiometer on the throttle position sensor.

I don't know where I heard the square wave theory, maybe that's a Ford thing??

 

If it is a PWM signal it's simple enough to use an integrator, like you mentioned. I can't see it being variable frequency, since that would probably require an external clock. Even so, there are frequency to voltage converter ICs available, like the LM2907.

 

Honeywell also has a whole family of silicon pressure transducers that might do what you want for less cost than a MAP sensor.

 

Yeah, it's the Ford MAP sensor that generates a square wave, from approx 90 to 170 Hz depending on manifold vacuum. That's a three terminal device too.

Apeiron, I've seen the pressure transducers you talk about. The reason I'm interested in the GM MAP sensor is because I've got 2 or 3 of them and they're readily available at the wreckers, they're rugged, and you can screw 'em to your firewall and just run them off a 3-terminal regulator. They're engineered to survive under the hood.

I didn't make it out to my folks' to pick them up so maybe next weekend I'll check 'em out and see if the three act consistently.