MAP Sensor Voltage Adjuster for the Jeep 4.0 EFI engine

By Dino Savva

If you've been tweaking more performance out of your fuel injected Jeep you might have run into a problem with running a little lean (or rich if you've installed oversize injectors like I've done). There are several options to cure this, most of which involve spending a good chunk of money. If you feel comfortable with basic electronics than you can build a MAP sensor voltage adjuster and save your money for other upgrades. That's exactly the option that I took.

MAP sensor

The Manifold Absolute Pressure (MAP) sensor is located on the back of the firewall and it senses the air pressure inside the intake manifold. The intake manifold pressure is converted into a voltage signal that's transmitted to the engine's computer which then determines how much fuel the engine needs. Under low vacuum conditions (WOT, throttle suddenly opened) the MAP is higher, the MAP output voltage is higher, and the injectors are held open for longer to inject more fuel into the cylinders. Under high vacuum conditions (throttle closed, deceleration), the reverse applies.
The MAP sensor is basically a three-wire pressure sensitive variable resistor. The three wires are the 5.0 volt input (purple/white on left), output voltage to ECU (red/green in middle), and the ground wire (black/blue on right). By increasing the input voltage we can in turn increase the output voltage to the ECU which will increase the injector duty cycle and supply more fuel. The reverse also applies. You can really fine tune your air/fuel mixture with this mod.
The 5.0 volt input wire is the only one that needs to be spliced into. The others are left alone.

Parts List

I used the following parts from Radio Shack. Total price was a mere nine US dollars:

Multi-turn Cermet Potentionmeter 1000 ohm
LM317T Adjustable Voltage Regulator
220 ohm metal film resistor
Mini Terminal Strip
Heatsink for Regulator
SPDT toggle switch
Black plastic project box

The switch is important because it allows you to toggle between the factory 5.0 volt input and the adjusted input that you dial in. Also in case something were to go wrong with this MAP adjuster you could easily switch back and not be stuck somewhere.
The equation for the adjusted MAP input voltage is:

Vinput = 1.25(1 + R2/R1)

Where R1 = 220ohm fixed resistor, R2 = 0-1000ohm variable resistor(pot).
This gives a range of adjustability from 1.25 to 6.93v


The arrangement is exactly the same as that in Aaron Young's original article. The MAP sensor voltage adjuster has five wires:

*5.0v from wiring harness to left terminal of switch
*MAP sensor input from middle terminal of switch to MAP sensor
*Adjusted voltage from voltage adjuster to right terminal of switch
*12v supply to voltage regulator
*Ground wire from variable resistor (pot) to firewall

Adjustment of the voltage from the adjuster is easy. Just hook up the positive lead from a digital multimeter to the adjusted voltage terminal (brown/yellow wire) and the negative lead to the ground. Read the voltage and adjust as required. Stock MAP sensor input voltage is 5.0v. If you turn the voltage higher than that, the air/fuel mixture will be enriched. If you reduce it to below 5.0v, the air/fuel mixture will be leaned.