Part 2 of 2

Code Chart 33:

MAP Circuit:

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 closed throttle (idle), to 4.5 volts at Wide Open Throttle (WOT) (low vacuum).

If the MAP sensor fails, the Engine Control Module (ECM) will substitute a fixed MAP value based on engine RPM and use the Throttle Position (TP) sensor to control fuel delivery.

TEST DESCRIPTION: Number(s) below refer to circled number(s) on the diagnostic chart.

1. Jumpering harness terminals "B" to "C" (5 volts to signal circuit), will determine if the sensor is at fault, or if there is a problem with the ECM or wiring.

The scan tool may not display 5 volts. The important thing is that the ECM recognizes the voltage as more than 4 volts, indicating that the ECM, CKT 432, and CKT 474 are OK.

2. This step determines if CKT 474 or CKT 432 is faulty. The scan tool will not display battery voltage, but should indicate over 4 volts.

3. The 5 volt reference Throttle Position (TP) sensor connects to the same 5 volt power supply in the ECM. If the 5 volt reference circuit to the TP sensor has a short to ground, the MAP sensor and A/C refrigerant pressure sensor 5 volt reference circuit will also have a short to ground because they connect to the same power supply in the ECM. A short to ground on either 5 volt reference circuit does not damage the ECM 5 volt power supply. When the short is repaired, the ECM 5 volt supply will return to normal.

DIAGNOSTIC AIDS:
With the ignition "ON" and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude and is referred to as BARO. Comparison of this BARO reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Reading should be within + .4 volt. If DTC 33 is intermittent, Testing and Inspection