Wiring Diagram Questions

Voltage readings for the exercise are determined using an imaginary digital voltmeter. Most readings will require the black voltmeter lead be connected to battery ground and the red lead used to probe various wires or control module/relay pins in the diagram.


Answering the questions about the wiring diagram will help you understand what voltage values are present when circuits are operating correctly. It will also demonstrate the logic behind using wiring diagrams to determine how to diagnose electrical problems. Practice reading wiring diagrams will help you when faced with real electrical problems on your truck. Remember the “Three Things”—Power, Load Device and Ground Return. The key to solving electrical problems is to figure out which of the three are missing (electrically speaking) in a faulty circuit.


Scenario 1 Questions:

The check engine light in your truck is on and after plugging in a scan tool you read a PO113 code (Intake Air Temperature Sensor). A new sensor cost $82. You have read on the TDR Forum that the sensor’s circuit should be tested before replacing it but you ignore this advice and buy a new sensor. You install it, clear the code and after a few short miles the check engine light comes back on setting a PO113 code—again. The only thing different is that your wallet is $82 lighter—guess you should have taken the TDR Forum’s advice and tested the IAT circuit.


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 2 Questions:

The intake air heater relay looks like a remote starter solenoid and operates just like the relays in the TDR article How to Read Wiring Diagrams. You suspect that the relay on your truck is bad but want to test it before replacement.


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 3 Questions:

The intake air heater on your truck will not operate even when the temperature is below 40° F. You have determined that the intake air heater relay, intake heater grid and intake manifold temperature sensor are working correctly. You suspect that the engine control module has a problem—however, a new one costs $989. A Dodge dealership would simply swap the module in the truck with a good one from their parts department to see if it was at fault. You don’t have that luxury so you have to make sure that the module is really bad before spending $982.


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?