Code 18
THROTTLE POSITION SENSOR AND MASS AIR FLOW SENSOR - DTC 18 (HIGH THROTTLE POS. VOLTAGE; LOW AIR FLOW VOLTAGE)
The ECM looks for a combination of throttle position voltage and mass air flow voltage that cannot occur in a correctly operating system.
The throttle position sensor signal voltage (EMS track) normally varies between 0.6 volt at idle to 4.5 volts at full throttle. The mass air flow sensor load signal voltage normally varies between 1 and 4 volts; the higher the voltage, the higher the load.
The ECM will flag DTC 18 if the throttle position signal voltage shows a large throttle opening when the MAFS signal indicates a low load.
Normal Signal MAFS Versus Engine Speed Load Site 4:
Conditions Required For DTC 18 Flagging:
Throttle position sensor voltage signal to ECM - 2.25 volts or greater.
Engine load site - Load site 3 or lower.
Response time - 5 engine revolutions.
Limp Home Default
If DTC 18 is flagged, the MAFS signal is accepted as accurate. The throttle position voltage defaults to less than full throttle (1.5 volts) so that full throttle enrichment is disabled.
Possible Faults
- High resistance connection to ECM or throttle position sensor
- Restricted air intake
- Restricted exhaust
- Low fuel pressure (low engine power)
- Engine mechanical fault (low engine power)
- ignition fault (low engine power)
- Short circuit or open circuit wiring from the ECM to the mass air flow sensor
- Short circuit or open circuit wiring from the ECM to the throttle position sensor
- incorrect adjusted throttle position sensor
- Defective throttle position sensor
- Defective mass air flow sensor
Diagnostic Notes
1. The throttle position sensor and mass air flow sensor signal voltages can be monitored and intermittent faults captured using the PDU "Datalogger" function.
MAFS Test 2: Engine Load - High
This test requires two technicians.
CAUTION: The test must not last longer than 5 seconds. Always allow the engine to idle for at least two minutes between tests to allow the transmission to cool. Do not carry out more than three tests without allowing the engine to cool down for at least half an hour.
- Chock the road wheels and firmly apply both foot and hand brake.
- Run the engine at normal idle speed; select neutral; air conditioning OFF.
- Use an approved DVOM or monitor the PDU MAF signal to measure the mass air flow sensor signal voltage at the ECM. Use only an approved DVOM to measure the mass air flow sensor signal voltage at the MAFS.
- Back probe the connector pin as directed on the diagnostic flow chart.
- Select DRIVE; apply full throttle and note the mass air flow sensor signal voltage at the ECM.
The typical voltage measured should be 3.00-3.30 volts.
NOTE: The voltage readings given are typical for an engine in good condition. Ambient, elevation and/or barometric conditions will effect the readings obtained.
A known good MAFS should be used as a comparison before the MAFS being tested is condemned as defective.
Circuit Diagram
Circuit Diagram:
Flow Chart: DTC 18
JאFlow Chart: DTC 18:
