I've never run into 77 and 42. 12 means the battery was disconnected recently. The rest would set if things were unplugged while the ignition switch was on.
The MAP sensors were a GM design and had a real high failure rate in the early '90s. They typically started out causing poor performance and would not set a code, but typically failed completely within a day or two. The clue was the engine would start but within a few seconds it would only stay running if you were moving the gas pedal. Didn't matter which way or how fast, as long as it was moving.
If you have reason to suspect the MAP sensor, you can unplug it, then turn on the ignition switch and try to start the engine. The Check Engine light will turn on and there will be a fault code for it, but the Engine Computer will know it can't trust its reading so it will take a pretty close guess, based on the other sensors' readings and engine operating conditions, and run off that approximate value. It won't run well, but it will run.
You can't do anything with the MAP sensor with an ohm meter because there's a lot of circuitry inside it. I did prepare a worksheet for my students to follow that involved watching the signal voltage but that was for properly-running engines. Those readings won't mean much when the engine doesn't run. When you turn on the ignition switch, that sensor measures barometric pressure. Higher pressure means more air being forced into the engine and the need to increase fuel delivery. Once the engine starts, it measures manifold vacuum as an indirect measure of engine load, and again, the need for the corresponding amount of fuel. Chrysler is the only manufacturer that has been able to make an engine run right with just that sensor. Everyone else has had to use a somewhat troublesome mass air flow sensor.
The MAP sensor is fed with 5.0 volts. Its signal voltage will always be between 0.5 and 4.5 volts, (approximately). When the voltage goes to 0 or 5.0 volts is when it sets a code related to voltage. The Engine Computer will also detect no change in voltage at engine start-up as a pneumatic problem meaning the vacuum hose is cracked or disconnected.
In the absence of a fault code, all you can do is view the data on the scanner. The signal voltage will be displayed but unless it's way off, you'll have better luck looking at the amount of vacuum. That should read "0", and the barometric pressure should be close to the actual value.
A leaking vacuum hose can cause the vacuum to appear to be low to the sensor. The Engine Computer will interpret that as the engine is under load and more fuel is needed. Beginning by the mid '90s Chrysler began plugging the sensors right into the throttle body or intake manifold to avoid leaks in the hose.
Tuesday, January 15th, 2013 AT 7:04 AM