1998 Dodge Truck Fan works intermitently

  • 5 POSTS
  • 1998 DODGE TRUCK

Heater problem
1998 Dodge Truck 6 cyl Four Wheel Drive Automatic 100000 miles

Heater Fan works when it wants to. Seems to work better when it is warmer outside.

Do you
have the same problem?
Monday, February 8th, 2010 AT 7:08 PM

1 Reply

  • 29,768 POSTS

The goal should be to make it work when YOU want it to! :)

The diagram in the service manual is confusing. They show a simple, straight forward circuit with reliable mechanical switches, but there are other unexplained wires in the speed-switching circuit.

First of all, can you make the fan run in any speed? If you can, the motor and relay are ok. If it only runs in the highest setting, the most likely suspect is the resistor block.

If there is nothing you can do to make the fan run in any speed, measure the voltage on the resistor block next to the fan motor on the passenger side of the heater box. The ignition switch must be on. You must catch this while the problem is occuring. There should be battery voltage on the black wire with tan stripe. If it's missing, unplug the motor connector and check for voltage on the dark green wire.

If there is voltage on the dark green wire but not on the black / tan wire, the motor has worn brushes and must be replaced, or there is a break in the black / tan wire between the motor and resistor block.

If voltage is missing or vey low on the dark green wire, the relay has burned / corroded contacts or is not turning on. Try switching it with one of the other relays. It is in the Power Distribution Center under the hood.

If you do find battery voltage on the black / tan wire at the resistor, the fan should run on the highest speed even if the resistor is defective. Check for voltage on the tan, light green, and light blue wires at the resistor. Under normal operation, each of these should have considerably less than 12 volts, but in the defective state, all three should have 12 volts.

If 12 volts is present on all four of those wires, the problem is with the switches in the control assembly or its ground wire. An easy clue is to run the engine, turn the mode switch to any of the ac settings, and watch if the ac compressor clutch engages. If it does, the ground wire for the control assembly is ok. Replace the control assembly. If the compressor clutch does not engage, measure the voltage on the black / orange wire on the control assembly. It must be at or very near 0 volts. If it's higher, the ground wire is broken or it has a bad connection on the body.

The engine must be running to check the ac clutch. It is turned on by the engine computer, but only when the engine is running.


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Thursday, February 11th, 2010 AT 4:18 PM

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