Ensure that you have a good vacuum connection to the sensor, and that the hose is not collapsing -- no leaks !
Typical values for a MAP sensor are--
supply voltage - 4.7 to 5.3 Volts
key on engine not running - very near supply voltage (a bit lower)
engine running at idle - 0.5 to 1.5 Volts
The MAP has qty=3 terminals (A, B, & C) (yeah - the pins are "A", "B", "C" in order toward the vacuum connection - you SHOULD be able to verify the lettering on the connector or the body of the MAP itself)
Pin "A" is the ground for this sensor and is typically the terminal farthest away from the vacuum connection
Pin "B" is the output voltage
Pin "C" is the input voltage
To measure the input voltage you can unpluyg the thing, BUT it's a good-old time-trick to just measure the output on "B"
KEY-ON NOT RUNNING - IF you get anything less than 4.5 Volts (a little less if you're up here in altitude) then pull the plug to measure the real supply value. You'll wanna verify the ground as well
RUNNING AT IDLE - IF you get more than about 1.5 Volts, AND you're positive that your vacuum line is good, verify the ground and/or replace the MAP
Ensure that your ground is ground -- measure a few of em -- the signal grounds at the computer are the most relevant.
A good trick on many of the early HOs is to disconnectthe battery, then try to start the engine (that'll drain off most residual voltages) , then disconnect the computer, look for crud in the connectors, then reconnect the computer -- do that twice - it'll clean-off the oxidation/contamination on the computer connections -- same can be done at the MAP itself --
time trick -- if you run the measurements first, then disconnect the battery for a while (replacing the MAP if necessary) the code should clear-out and you can see if it re-sets.! (good time to run for a beer or ogle the neighbor's teen-aged daughter as well)
