My new [ to me ] truck, a Dodge Dakota '06, 2wd 3.7L 6spd, came with an aftermarket vacuum guage, I gather so the owner could monitor intake vacuum as a means to help boost cruising mileage.........

My dilemma, is can I seek help........... to rig an led circuit to be on when there is acceleration vacuum present, and light an led when there is, and have the led go out when vacuum is nil at cruising..........Without running afoul of forum rules regarding auto ad-ons,.

since it appears to me the guage is passive as "read only" and in no way controls or advises the ECU. I want to improve what is already done.
Watching the guage can be very distracting, and I figure an led will better suit peripheral vision .

 

I assume the gauge is mechanical and thus would be difficult to add any electronics to read the vacuum value. You would thus need to add an electronic vacuum sensor or vacuum switch. With the sensor you would sense the output voltage with a voltage comparator to drive the LED.

 

Dont know if these sensors are are in America. But here in Australia certain older Ford & Holden (GMH) vehicles have Manifold Vac sensors which give a DC voltage output, which varies with vaccum. This could drive a circuit to switch on a LED at a preset value. Later types of this unit put out a varying frequency squarewave.

 

Just remove the gauge.. Its not telling you anything

 

Au Contraire Mc G.......
With a computer controlled port injection system based in part on mass airflow, and manifold vacuum, when you are cruising, and vacuum is near zero, the injection is at one level.........

Gear up and open the throttle to pass, or climb a long gentle grade, vacuum goes up, consequently injectors are told to deliver more fuel for more power...... when just plain level-road cruising, controlling your "float" can mean several MPG difference.
In time, I will install cruise control to tighten the equation up a little more, 'cuz I do most driving on highway and turnpike.
Will likely wind up moving the guage into better line of sight, though a light will be easier to track.
Ise gonna squeeze blood out of [ 3.50 a gallon ] turnips !!

 

Au Contraire Mc G.......
With a computer controlled port injection system based in part on mass airflow, and manifold vacuum, when you are cruising, and vacuum is near zero, the injection is at one level.........

Gear up and open the throttle to pass, or climb a long gentle grade, vacuum goes up, consequently injectors are told to deliver more fuel for more power...... when just plain level-road cruising, controlling your "float" can mean several MPG difference.
In time, I will install cruise control to tighten the equation up a little more, 'cuz I do most driving on highway and turnpike.
Will likely wind up moving the guage into better line of sight, though a light will be easier to track.
Ise gonna squeeze blood out of [ 3.50 a gallon ] turnips !!

Not sure on the EFI, but what you said is the opposite of carburated vacuum. With a carb the vacuum is high when the throttle plates are closed(idle) and goes lower as the throttle is opened. Never checked on an EFI engine.

 

It's the same, when the throttle is opened, more air is allowed, sensed by the MAF for EFI, but the reason the air rushes in is to fill the vacuum.

This means the vacuum drops when the pedal is all the way down due to incoming air mass, and vacuum is higher at idle.