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Monday, October 13, 2008

2004 Chevy Silverado A/C Fan Stays On

Robert asked: My 2004 Silverado heater-a/c fan stays ON when the ignition is OFF. It sounds to me like a connector problem at the resistor module. I have replaced the module twice and the blower once. I have not replaced the connector. Is 12V applied to the module at all times? If so, then could a possible short directly to the blower motor cause this condition? I know of no other possibility, do you?


Sparky said...

I would assume that the blower is staying on in the high position and that is probably due to a faulty relay inside the blower resistor assembly or the red and orange wires are melted together. You do have manual a/c don't you? The only wire that should be hot when the key is off is the red wire. There are tests voltages listed at the following blog.

If there is any damage at the harness connector, the connector and resistor have to be replaced at the same time.

Anonymous said...

The problem is Intermittent and sometimes returns immediately after all Lamp Checks are completed following restart. I have checked the voltages at the connector all positions of Fan selector function normally yielding 12+ Volts to the Module Connector and Zero Volts for Position Zero. The Module must have the equivalent of a Solid State Relay for the Highest Speed. The Common (Negative) lead might be the problem. The Harness Connector is not in any way damaged, discolored or showing any signs of having been overheated. When it is functioning normally all 5 Blower Speeds are present.. When it Malfunctions it immediately is running at maximum speed and does NOT turn off when the Ignition Switch is turned to OFF, Accessory or LOCK positions. When it malfunctions, disconnecting the Resistor Module connector Stops the Blower. Reconnecting the Resistor Module immediately and the Blower DOES NOT come back on.

Previously, I had the dealer replace the Ignition Switch and Fan Module. I suspect that the accessory terminal in the Ignition Switch was defective.

The mechanic stated that the Ignition Switch was definitely defective and even though the Resistor Module was at that time missing 2 speeds that the Ignition Switch caused the intermittent operation of the Blower. He then replaced the Resistor Module to get the missing speeds back. Subsequently, (about 2 years) the Blower Motor seized up and fried the Resistor Module. I purchased a New Blower Motor and Resistor Module and installed them myself.

The vehicle has 120,9XX miles and previously was driven about 125 miles daily to and from work with the Blower usually on the Highest Speed in Winter (Heater) and Summer (A/C).

Do you think that I may have another defective Ignition Switch? If not, is there an Electro-Mechanical Relay somewhere that may have welded contacts in the circuit? If so, where is it located? If not, what is my next step in diagnosing the problem? Unfortunately I do not have an electrical diagram or service manual for this vehicle. I am approaching my wits end at this time as I am sure you can appreciate..

Thank you for the previous information as it was extremely helpful.

Sparky said...

In order for high blower to be achieved there must be power (12 volts +) on the orange "F" and red "G" wires at the same time.

The orange wire energizes the pull in coil in the relay and the red wire then supplies power to the blower motor. The relay and blower motor share a common ground through the black wire "E".

In your particular case I would monitor power on the orange wire and ground on the black wire with the blower switch in any and all positions other than high. If at any time you have a completed circuit, then you will have to determine if your blower switch or wiring harness is shorting out.

Also I hope that you will be using a digital multimeter that will read volts regardless of polarity. If you were to notice a completed voltage circuit on the orange and black wires at any time be sure you check what polarity is displayed. In some very rare cases a permanent magnet fielded motor will generate a current in a coast position and will back feed the controls.

The cheap test would be to have the blower motor connector partially released and when the condition is induced, unplug the motor and see if the relay releases (clicks). If the high blower condition only occurs after the blower switch has been placed in the high blower position then I would suspect that the contacts inside the relay that is part of the blower resistor are sticking.

You could also try adding a temporary ground to the black wire. I do not really think an extra ground would do much good though because usually a weak ground for a relay will cause it to buzz and melt and the relays cannot recover from this. Also the blower could not operate at high speed if the ground was weak.