TEST 3: Checking Continuity Of The MAF RTN Circuit
If you've reached this point, you have confirmed that the MAF signal circuit is not open. The next step is to check the MAF RTN circuit.
The TAN/LT BLU wire, of the 4 wire MAF sensor connector, is the MAF RTN circuit (and connects to pin 36 of the 104 pin PCM connector).
Now, in case you're wondering, the MAF RTN circuit feeds a Ground to the MAF sensor directly from the PCM.
IMPORTANT: Do not probe the front of the PCM 104 connector or you'll damage the terminal. This will require that you replace the entire engine wiring harness.
OK, these are the test steps:
- 1
Disconnect the battery negative cable and then unplug the PCM from its connector.
IMPORTANT: The battery negative cable will remain disconnected from the battery negative post for the remainder of the test. - 2
Connect the breakout box to the 104 pin PCM connector only. Do not connect the the Ford 104 pin breakout box to the PCM.
- 3
Place your multimeter in Ohms mode (Ω). We'll be checking the continuity of the circuit by checking its resistance.
- 4
Measure resistance between breakout box pin 36 and MAF sensor connector terminal that connects to the TAN/LT BLU wire.
The TAN/LT BLU wire is the wire that connects to terminal 36 of the PCM 104 pin connector. - 5
Your multimeter should register 5 Ohms or less if the circuit is OK and has continuity.
Let's take a look at what your test results mean:
CASE 1: Your multimeter registered 5 Ohms or less. This test result tells you that the TAN/LT BLU wire is not ‘open’ between the MAF sensor connector and the PCM 104 pin connector.
CASE 2: Your multimeter DID NOT register 5 Ohms or less. If the resistance value is in the thousands of Ohm or your multimeter registered OL (over limit), then the wire is ‘open’.
Your next step is to repair the wire since, somewhere between the PCM connector and the TPS connector, this wire is broken.
TEST 4: Checking MAF SIG Circuit For Short To Power Or Ground
So far, you have checked that:
One: The MAF signal LT BLU/RED wire is not open between the MAF sensor connector and the PCM.
Two: That the MAF RTN TAN/LT BLU wire is not open between the MAF sensor and the PCM connector.
The next step is to make sure that the MAF signal circuit is not shorted to another wire.
Specifically, we need to check that: 1) The LT BLU/RED wire IS NOT shorted to chassis Ground (PWR GND), or 2) shorted to the MAF RTN (TAN/LT BLU) wire, or 3) shorted to a VPWR wire (a VPWR wire carries 12 Volts).
OK, these are the test steps:
- 1
Disconnect the battery negative cable and then unplug the PCM from its connector.
IMPORTANT: The battery negative cable will remain disconnected from the battery negative post for the remainder of the test. This will render the power circuits without battery power and make possible the multimeter continuity tests. - 2
Connect the breakout box to the 104 pin PCM connector only. Do not connect to the Ford 104 pin breakout box to the PCM.
- 3
Place your multimeter in Ohms mode (Ω) to check for shorts to power by measuring resistance between:
Breakout box pin 88 (MAF SIG) and breakout box pin 71 (VPWR).
Breakout box pin 88 (MAF SIG) and breakout box pin 97 (VPWR).
Continuity SHOULD NOT exist between the pins indicated above. - 4
Place your multimeter in Ohms mode (Ω) to check for shorts to Ground by measuring resistance between:
Breakout box pin 88 (MAF SIG) and breakout box pin 36 (MAF RTN).
Breakout box pin 88 (MAF SIG) and breakout box pin 103 (PWR GND).
Continuity SHOULD NOT exist between the pins indicated above.
NOTE: Pin 103 (PWR GND) is a Ground circuit that connect to chassis Ground. Pin 36 (MAF RTN) is the circuit that feeds the MAF sensor with sensor Ground from the PCM. - 5
Your multimeter SHOULD NOT register continuity between the pins tested in step 3 and step 4.
To be a bit more specific: Your multimeter SHOULD NOT register a resistance of 5 Ohms or less. The resistance value should be 10,000 Ohms or more (usually OL).
Let's take a look at what your test results mean:
CASE 1: Your multimeter DID NOT register continuity in the pin tests above. This is the correct and expected test result and tells you that the LT BLU/RED wire of the MAF sensor connector is not shorted to power or Ground.
Your next step should be to verify that the TAN/LT BLU (MAF RTN) wire is not shorted to any other circuit. For this test, go to: TEST 5: Checking MAF RTN Circuit For Short to Power or Ground.
CASE 2: Your multimeter registered 5 Ohms or less in one of the pin tests above.
This test result tells you that the MAF SIG (LT BLU/RED) wire is shorted to the specific circuit (wire) that registered continuity on your multimeter.
Now, since your test result indicates a shorted circuit, your next step is to physically check the length of the engine wiring harness (between the MAF sensor connector and PCM connector) for any obvious damage.
Repairing the short between these two circuits will solve the mass air flow (MAF) sensor problem lighting up the check engine light (CEL) on your Ford vehicle.