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Ford Truck Club Forum > GARAGE TALK > Garage Talk: Shop Class 101



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  #1  
Old July 15th, 2011, 09:19 AM
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Thumbs up How to Test a Manifold's Absolute Pressure Map Sensor

How to Test a Manifold's Absolute Pressure Map Sensor

Test a Manifold's Absolute Pressure Map Sensor The manifold absolute pressure sensor, or MAP sensor, is generally used in place of the mass airflow sensor in an automobile. It preceded the mass airflow sensor and was much less efficient. The MAP sensor measures the pressure in the intake manifold through a vacuum. The more pressure, the lower the vacuum and the lower the voltage output. The lower the pressure, the higher the vacuum and the higher the voltage output signal. The mass airflow measures the volume of air moving into the manifold.


Things You'll Need
Voltmeter



1
Connect the voltmeter to the MAP sensor by connecting the black lead to a good ground and connecting the red lead to the center of the three wires in the MAP sensor's electrical connector.

2
Turn the ignition key on with the engine off. The voltage displayed on the voltmeter should be between 4.5 to 5 volts.

3
Start the engine and the voltage displayed should be .5 to 1.5 volts at sea level or close. The voltage will decrease as altitude increases. If the voltage differs greatly on either test, the MAP sensor is fried. Replace it.




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  #2  
Old February 14th, 2018, 03:16 PM
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Hi JSoko,
I found your procedure on testing a MAP sensor.
I used this procedure on my 1994, F250 XLT 4X4 7.5l truck. The voltages are VREF is 5 volts, BP/MAP 2.5 volts and Signal Return is 11.7 volts.
The truck starts just fine, fires right off and runs, then after engine temp comes up rpm fluctuates and after a period of time it dies.
What I'm trying to diagnose is- when the engine comes up to temp the engine rpm fluctuates between 900-1200 rpm. It does this up and down until it stalls. I do see black smoke out the tailpipe.
I checked using a frequency setting on my DVOM at KOEF and I have 159hz and voltage at BP/MAP (center wire) is 2.5 volts. With engine on an up to temp (before rpm hunting) it registered 104 hz still 2.5 volts. What I didn't check was vacuum to frequency using a vacuum pump.
Have you had any experience with this type of symptom? Any direction is helpful.
Bulldog74
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Old February 14th, 2018, 11:49 PM
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Have you checked for OBD1/EEC codes?

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS
  #4  
Old February 15th, 2018, 02:27 PM
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I haven't picked up an OBD-1 scanner yet.
After the last write up I checked the MAP sensor again for voltage at BP/MAP center position again. With KOEF it still has 2.5 volts connected (by back probing the connector) but when disconnected from the MAP sensor I do read 5 volts. Also, there's no check engine light. What I'm thinking is that whatever electrical component is in the map sensor is failing after vacuum build up takes place.
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Old February 15th, 2018, 10:43 PM
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Pick up a used map sensor from a junkyard cheap and throw it in there, see what happens.

2003 F-150 XL RCSB 2wd 4.2 5 speed 3.55 LS


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absolute , manifold , map , pressure , sensor , test

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