Code 34, MAP sensor

Tiny
KYLE BAUMGARDNER
  • MEMBER
  • 1990 CHEVROLET 1500
  • 1,430,000 MILES
I have the truck listed above c1500 5.0. I have replaced the MAP sensor and connector the ECM and everything that is electronic that makes the engine run. I keep getting code 34. It idles between 500 to 1,200 RPM's I have check all my wires I still don’t know what the problem is. Could you help?
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Wednesday, May 20th, 2020 AT 2:54 PM

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Tiny
ASEMASTER6371
  • EXPERT
Good evening,

I attached the flow chart for the code and a picture of the location on the right side of the motor.

https://www.2carpros.com/articles/how-to-check-wiring

Roy

CHART C-1D - MANIFOLD ABSOLUTE PRESSURE (MAP) OUTPUT CHECK

Circuit Description:

The Manifold Absolute Pressure (MAP) sensor measures manifold pressure (vacuum) and sends that signal to the ECM. The MAP sensor is mainly used for fuel calculation, when the ECM is running in the throttle body backup mode. The MAP sensor is also used to determine the barometric pressure and to help calculate fuel delivery.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Checks MAP sensor output voltage to the ECM. This voltage, without engine running, represents a barometer reading to the ECM.

2. Applying 34 kPa (10" Hg.) vacuum to the MAP sensor should cause the voltage to be 1.2 - 2.3 volts less than the voltage at Step 1. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.

3. Check vacuum hose to sensor for leaking or restriction. Be sure no other vacuum devices are connected tot he MAP hose.

CODE 34 - MANIFOLD ABSOLUTE PRESSURE SENSOR CIRCUIT (MAP) SIGNAL VOLTAGE LOW - HIGH VACUUM

Circuit Description:

The manifold absolute pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts at idle to 4-4.5 volts at wide open throttle.

If the MAP sensor fails the ECM will substitute a fixed value and use the throttle position sensor (TPS) to control fuel delivery.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 34 will set when:

When engine is less than 600 rpm.
Manifold pressure reading less than 13 kPa, conditions met for 1 second or
Engine is greater than 600 rpm.
Throttle angle over 20%.
Manifold pressure less than 13 kpa conditions met for 1 second.

2. This tests to see if the sensor is at fault for the low voltage or if there is a ECM or wiring problem.

3. This simulates a high signal voltage to check of an open in CKT 432. if the test light is bright during this test, CKT 432 is probable shorted to ground. If "Scan" reads over 4 volts at this lest CKT 416 can be checked by measuring the voltage at terminal "C". (should be 5 volts)

Diagnostic Aids:

An intermittent open in CKTs 416 will result in a Code 34.

See Diagnosis By Symptoms - No Trouble Codes Stored.

The "Altitude to Voltage" scale at the right may be used to test the MAP sensor at a specific altitude level to evaluate the possibility of a "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

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Wednesday, May 20th, 2020 AT 4:32 PM

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