Code 12
MASS AIR FLOW SENSOR CIRCUIT - DTC 12
The ECM looks for the mass air flow sensor (MAFS) load signal to be out of the normal operating range.
The mass air flow sensor load signal is used by the ECM as one of the two primary inputs for determining fuel metering and ignition timing. A 12 volt supply to the MAFS provides power for the heated sensor and circuitry. The load signal voltage responds to air flow through the sensor and normally varies between 1 and 4 volts; the higher the voltage, the higher the load.
The ECM will flag DTC 12 if the MAFS signal voltage is not between 0.2-4.5 volts in the engine speed range 400-3000 rpm.
Conditions Required For DTC 12 Flagging:
- Engine speed range: 400-3000 rpm
- MAFS voltage signal to ECM: Less than 0.2 or greater than 4.5 volts
- Response time: 5 consecutive engine revolutions.
Limp Home Default:
If DTC 12 is flagged, throttle position is substituted for engine load. Throttle position and engine speed become the primary control inputs for fuel metering. A single ignition timing value is used.
Possible Faults:
- Unmodified EMS main relay (1993 MY); Refer to Technical Bulletin 18-36
- Open or short circuit wiring from the ECM to the MAFS
- High resistance circuit wiring or connector from the ECM to the MAFS
- Open circuit wiring from main relay power (splice) to MAFS
- Defective MAFS
Diagnostic Notes
1. The MAFS signal voltage can be monitored and intermittent faults captured using the PDU "Datalogger" function.
2. 1990 model year vehicles with MAFS part no. EAC 6215 have a black ground wire connected at the stud on the MAFS body. From 1991 model year, the ground wire is not used. If the later MAFS is used on a 1990 model year vehicle, the ground lead can be eliminated.
3. If DTC 11, 14, 16, 18, 19 and/or 39 are also flagged, suspect a defective sensor ground circuit to the ECM (BY wire) or defective ECM ground circuit to the front bulkhead ground stud.
4. 1993 MY XJ6 Sedan and XJS vehicles: refer to Technical Bulletin 1836 for diagnosis of incorrect DTC 12 flagging.
MAFS Test 1: Engine Load - Idle
- Run the engine at normal idle speed; normal operating temperature; 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.
- The typical voltage measured should be 1.20-1.50 volts.
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; while firmly pressing the brake pedal, 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:
Circuit Diagram:
Flow Chart: DTC 12
���Flow Chart: DTC 12:
