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Answers to Questions:


Scenario 1 Questions for reference:

1A) How can you isolate the IAT from the engine control module; test the IAT circuit and the wires going to the module?

1B) Using a digital voltmeter how can you test the IAT sensor?

1C) Can you measure resistance of the IAT sensor? Is this a better test than measuring voltage?

1D) How can you use a hair dryer (or heat gun) to test the IAT sensor?

Scenario 1 Answers

1A) Unplug the connector at the IAT sensor and turn the ignition key to the run position. Connect the red lead of the voltmeter to the DB/VT (dark blue/violet) wire and the black lead to the DB/DG (dark blue/dark green) wire. A reading around 4.8-volts indicates that the engine control module is sending the 5-volt reference signal to the IAT and that the ground return wire is connected to the control module. This test verifies that the wires from the engine control module to the IAT are good and that the control module has power (if it didn’t there would be no 5-volt signal present).


1B) With the IAT connected to the control module, back probe the DB/VT wire (back probing the signal wire allows you to read voltage with the sensor plugged in—you can use a safety pin inserted into the connector). Estimate the ambient temperature and take a voltage reading at the IAT signal wire. If the ambient temperature is between 60° F to 80° F signal wire voltage should be 3.60 to 3.08 volts. This test verifies that the IAT and control module are working.


1C) Yes, you can measure resistance of the IAT sensor. Unplug the sensor and turn your voltmeter into an ohmmeter. Connect the leads to the IAT sensor (doesn’t matter which lead goes where). Estimate the ambient temperature and read the resistance. This procedure is not a test for accuracy of the sensor—it’s more of go/no-go test. Reading IAT signal voltage with the sensor plugged into the control module is a better test as voltage is what the control module reads in operation—not resistance. The sensor could have the correct resistance at a particular temperature but a PO113 code could still be set if there is something wrong with the IAT circuit.


1D) Heating the sensor will decrease the IAT voltage at pin 47 of the engine control module. If voltage decreases as the sensor heats up it is working correctly. This is not a test for accuracy of the sensor—it’s more of go/no-go test. An alternative method to determine if the sensor is working is the use of a scan tool to view IAT temperature. Heating the sensor should cause the reading to increase. CAUTION: Be careful heating the sensor as hair dryers and heat guns get really hot—don’t melt the sensor!


Scenario 2 Questions for reference:

2A) What are the wire colors and terminal numbers for the intake air heater relay? With the intake air heater off, what voltage values should be on each of the relay’s four terminals with all wires connected?

2B) What should the relay voltage values be if the intake heater in operation?

3B) How can you use a jumper wire (short length of wire with alligator clips at each end) to test the relay and the wire connecting it to the control module?


Scenario 2 Answers

2A) Relay terminal 30 (RD/YL) should have 12-volts, terminal 87 (BK) should show 0-volts (ground through the heater), terminals 85 (BR/YL) and 86 (BK) should have 0-volts.


2B) Terminals 30, 87 and 85 should be 12-volts, Terminal 86 should be 0-volts.


2B) To bypass the engine control module, test the relay and the wire connecting the control module to the relay (BR/YL, terminal 85), unplug the wiring harness connector from the module. Connect one end of a jumper wire to battery positive and touch the other end to the BR/YL wire at the control module wiring harness connector (not the control module). The relay should click and there should be power on relay terminal 87 (black wire) going to the intake air heater. CAUTION: Don’t leave the jumper wire connected too long as the intake air heater uses up to 200 amps and will drain the battery quickly.


Scenario 3 Questions for reference:

3A) If the intake air heater doesn’t work how can you “test” the engine control module?

3B) Before replacing the control module, what values should each of the module pins have with the ignition key on?

Scenario 3 Answers

3A) A bi-directional scan tool is required to test the engine control module. These scan tools have functional tests that check operation of solid-state controllers. For example, within the scan tool’s menu there is a test for the intake heater relay. When selected, the scan tool commands the engine control module to turn on the intake air heater relay. If the module can’t perform this test it should be replaced. Bi-directional scan tools are available for as little as $100 with the more expensive scan tools able to perform more functional tests.


If you don’t have a scan tool there is an alternative test for the control module. The test is deductive and works as follows: If the module has the correct input/output voltage values, and all the wires are intact, it should be capable of operating the intake air heater. This also assumes that the ambient temperature is below 66° F. If the heater will not operate, then the module is probably bad and needs to be replaced. If any voltage values at the module are missing, fix these first BEFORE purchasing a new module.


3B) With the control module plugged in, turn the ignition key to the on position; back probe the wires at the module with the red lead of the voltmeter and connect the black lead to ground. Here are the voltage values for each module terminal/pin:


A “real” engine control module will have way more pins that the ten used in this exercise (more like 50) however the logic for testing remains the same. If all the module inputs are correct, and the wires going to the various actuators/controls are intact, then the only thing left is a bad module.


Independent repair shops have used this logic for testing solid state components before bi-directional scan tools were available. A competent repair shop will always test, and not guess before replacing black boxes (control modules) because like you, they can’t return them to an parts store if a new module didn’t fix the problem.


Here are some additional tricks for testing solid-state/black boxes/control modules. With a voltmeter connected to suspect pins, try lightly tapping on the module with a screwdriver handle. If the vibration changes voltage readings the module is bad. Also, try gently heating the module with a hair dryer/heat gun. If voltage readings change when heat is applied the module is bad. Don’t overheat the module!

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