Manifold Absolute Pressure Sensor Output Check Diagnosis. GMC Truck GMC C-Series Medium Duty Truck B7 1998

For 1990-2009 cars only

Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 6.0L and 7.0L 🢒 Manifold Absolute Pressure Sensor Output Check Diagnosis

Engine Controls Components
Cell 20: ECT Gauge and Sensor, MAP, O2 and TP Sensors, PCM
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 ECM 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 ECM knows the manifold pressure. A lower pressure (low voltage) output voltage will be about 1-2 volts at idle. While a higher pressure (high voltage) output voltage will be about 4.0-4.8 at Wide Open Throttle (WOT). The MAP sensor can also measure, under certain conditions, the barometric pressure. This allows the ECM to make adjustments for different altitudes. The ECM uses the MAP sensor to control the fuel delivery and the ignition timing.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

Important:

•	Be sure to use the same diagnostic test equipment for all measurements.

•	Make sure the electrical connector remains securely fastened.

Applying 34 kPa (10 in Hg) vacuum to the MAP sensor should cause the voltage to change. Subtract the second reading from the first. Voltage value should be greater than 1.5 volts. Upon a 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 a restriction. Be sure that no other vacuum devices are connected to the MAP hose.

Disconnect the sensor from bracket and twist the sensor by hand only in order to check for an intermittent connection. Output changes greater than 0.1 volt indicate a bad connector or connection. If OK, replace sensor.

Manifold Absolute Pressure (MAP) Output Check

Step	Action	Value(s)	Yes	No
1	Did you perform the On-Board Diagnostic (OBD) System Check?	--	Go to Step 2	Go to Powertrain On Board Diagnostic (OBD) System Check
2	Install a scan tool. Check for DTCs. Is DTC 33 or DTC 34 set?	--	Go to Applicable DTC Table	Go to Step 3
3	Turn the ignition ON leaving the engine OFF. Observe the MAP sensor voltage, using a scan tool. Compare the MAP sensor voltage reading with the reading of a known good vehicle. Are the voltage readings within the specified value of each other?	0.4 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 and plug the vacuum source to the MAP sensor. Connect a hand held vacuum pump to the MAP sensor. Start the engine. Observe the MAP sensor voltage. Apply the specified value of vacuum using the vacuum pump. Observe 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 a restricted vacuum source to the MAP sensor. Check to make sure that the vacuum source supplies the MAP sensor only. If you find a problem, repair as necessary. Did you find a problem?	--	Go to Step 8	Go to Step 7
7	Install a scan tool. Observe the MAP sensor voltage. Twist the MAP sensor by hand only. If the MAP sensor voltage changes more than the specified value, check for a poor connection at the sensor. If you find a problem, repair as necessary. Did you find a problem?	0 .1 V	Go to Step 8	Go to Step 4
8	Operate the vehicle within the conditions of the original symptom was caused. Does the system now operate properly?	--	System OK	Go to Step 2