Continuing in my do it yourself series, and sensor testing, will be MAP sensors. MAP sensors are used by the ECM to help determine how much air is entering the engine.
You will need a multi-meter, some alligator clips, vacuum pump, and a 6-volt power source. If you aren't familiar with multi-meters, take a look at my Multi-Meter Basics. For the power source, I'm using a flashlight battery, but you could use anything that is 6 volts. If nothing else get a helper and use 4 AA batteries stacked together to get to 6 volts.
At the end of this you'll probably need an extra set of hands, as you end up with wires everywhere.
Here is a YJ (both 2.5 and 4.0) MAP sensor setup:
Here is a TJ (98-02 2.5 / 98-03 4.0 others maybe similar) MAP sensor setup:
Using the pin configurations above:
A - Ground side of the 6-volt battery.
B - Signal, positive probe of the multimeter goes here. The negative lead of the meter will go to the ground side of the 6-volt battery.
C - Positive side of the 6-volt battery.
At this point with no vacuum on the MAP sensor the voltage should be roughly 5-6 volts. The YJ one I'm testing showed 5.47 volts at 0 just for reference. Now place 15 pounds of vacuum on the MAP sensor, the voltage should drop, my YJ one dropped to 2.07 volts. You need a voltage drop of at least 1 volt. The voltage may vary some, but it should change nice and smoothly as vacuum is increased or decreased.