Manifold Absolute Pressure Sensor Output Check Diagnosis. GMC Truck GMC C-Series Medium Duty Truck B7 1999

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 7.4L 🢒 Manifold Absolute Pressure Sensor Output Check Diagnosis

Engine Controls Components
Cell 20: Engine Controls, Exhaust Gas Recirculation Valve
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The manifold absolute pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load (intake manifold vacuum) and RPM changes; and converts these into a voltage output. The powertrain control module (PCM) sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changed, the output voltage of the sensor also changes. By monitoring the sensor output voltage, the powertrain control module (PCM) knows the manifold pressure. A lower pressure (low voltage) output voltage will be about 1-2 volts at idle. While higher pressure (high voltage) output voltage will be about 4.0-4.8 volts at wide open throttle (WOT). The MAP sensor is also used under certain conditions, to measure barometric pressure, allowing the PCM to make adjustments for different altitudes. The PCM uses the MAP sensor to control fuel delivery and ignition timing.

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

When comparing scan readings to a known good vehicle, it is important to compare vehicles that use a MAP to vehicles having the same MAP sensor.

Applying 34 kPa (10 in Hg) vacuum to the MAP sensor should cause the voltage to change. Subract the second reading from the first. Voltage value should be greater than 1.5 volts. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.

Check the vacuum hose to the sensor for leaking or restriction. Be sure that no other vacuum devices are connected to the MAP hose.

Remove the sensor from the bracket. Hand twist the sensor to check for an intermittent connection. Output changes greater than 0.1 volt indicate a bad connector or connection. If the connection is OK, replace the sensor.

Step	Action	Value(s)	Yes	No
1	Was the On-Board Diagnostic (OBD) System Check performed?	--	Go to Step 2	Go to Powertrain On Board Diagnostic (OBD) System Check
2	Install a scan tool. Check for DTCs. Is DTC P0107, DTC P0108, or DTC P1107 set?	--	Go to The Applicable DTC Table	Go to Step 3
3	Turn the ignition ON. The engine is OFF. Using a scan tool, observe the MAP sensor voltage. Compare the MAP sensor voltage reading with the reading of a known good vehicle. Are the voltage readings within the specified range of each other?	0.4-0.10 V	Go to Step 5	Go to Step 4
4	Replace the MAP sensor. Refer to Manifold Absolute Pressure Sensor Replacement . Is the action complete?	--	Go to Step 8	--
5	Disconnect the MAP sensor. Plug the vacuum source to the MAP sensor. Connect a hand held vacuum pump to the MAP sensor. Start the engine. Note the MAP sensor voltage. Apply the specified value of vacuum using the vacuum pump. Note the MAP sensor voltage change. Is the difference in the voltage readings greater than the specified value?	34 kPa (10 in Hg) 1.5 V	Go to Step 6	Go to Step 7
6	Check for leaking or restricted vacuum source to the MAP sensor. Check to make sure that the vacuum source supplies the MAP sensor only. If a problem is found repair as necessary. Was a problem found?	--	Go to Step 8	Go to Step 7
7	Install a scan tool. Observe the MAP sensor voltage. Hand twist the MAP sensor. If the MAP sensor voltage changes more than the specified value check for a poor connection at the sensor. If a problem is found, repair as necessary. Was a problem found?	0.1 V	Go to Step 8	Go to Step 4
8	Operate the vehicle within the conditions under which the original symptom was noted. Does the system now operate properly?	--	System OK	Go to Step 2