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Wiring Diagram Exercise

As mentioned in the video, How to Read Wiring Diagrams, it takes practice using a wiring diagram to become proficient at diagnosing electrical problems. The wiring diagram for this exercise is from a 2009 Dodge RAM 2500 truck with a 5.9L turbo diesel engine. The diagram has been modified for this exercise.


The diagram is typical of examples found online for Mitchel 1, Haynes, Alldata and OEM factory service manuals. Individual components are identified and some have location references. All wires are identified using colors either by the actual color, or labels, i.e., RD=red, PK/GY=pink with grey stripe, BR/YL= brown with yellow strip etc.


A word of caution, don’t count on red wires as having power and black wires depicting grounds as there are exceptions. Grounds are depicted as a dot with a horizontal line under it and all ground symbols connect to the battery’s negative terminal.

Notes about the Diagram

The notes provide some general knowledge about how the circuits/sensor within the wiring diagram operate. This information would normally be found in a service manual and a basic understanding of how and when the intake air heater is turned on will help in reading the wiring diagram.


The “RELAY-INTAKE AIR HEATER” looks like a starter solenoid and works like the relays in the How to Read Wiring Diagrams article and even has the same terminal numbers.


The “SENSOR INTAKE AIR TEMPERATURE” sensor works as follows: The control module sends a 5-volt reference signal (aka bias voltage) on pin 47 to the Intake air temp sensor (IAT) which is grounded through the sensor return circuit on pin 24 of the control module.


When the intake air temperature is low, the sensor’s resistance is high causing the intake air temp signal voltage to be high (close to 5-volts). As the intake manifold heats up, the temperature sensor’s resistance is lowered causing the IAT signal voltage to decrease (close to 1-volt). For reference intake manifold temperature vs. IAT voltage and resistance values are listed:

IAT Temp vs Voltage-Ohms.jpg

The “HEATER-INTAKE AIR” grid operates as follows. The intake air heater element/grid is used to heat air entering the intake manifold to aid in cold engine starting and/or to improve driveability when outside temperatures are cold.

When the intake manifold temperature is above 66° F. the instrument panel wait-to-start lamp (not shown in the diagram) will not illuminate and the intake air heater will not be activated. At 32° F. the wait-to-start lamp and heater grid will operate for 10 seconds and at 0° F. operation will last for 30 seconds.

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