Heater fan stuck on high speed


Asked by Sep 27, 2015 at 07:06 PM about the 2004 Mercury Grand Marquis

Question type: Maintenance & Repair

Can not change the blower speed from high ---- it will even blow when the key is in the acc prosition

8 Answers


sounds like the switch is bad...anyone else out there want to add anything>?

2 of 2 people found this helpful.

Should it not cut off the power when the key is in the acc position ?

1 of 1 people found this helpful.

Is there a relay or temp-speed secencer that may be stuck ?


There is a resistor that controls the blower motor speeds. When you turn your switch low, medium, high, the resistor regulates the amount of voltage to the blower motor to acquire the desired speeds. In the accessory position the blower motor should be off. When mine went bad the blower motor kept on running on high, even with the key out of the ignitionl

6 of 6 people found this helpful.

If you have automatic climate control it's going to be the control module in the engine compartment on the firewall. When they fail you either get constant high speed or no blower motor at all. HTH. -Jim

8 of 8 people found this helpful.

Glad to learn of the above. My fan blows high even in the Off position! Does the whole control module have to be replaced, or can resistor be replaced.


Chances are the blower motor resistor would be the culprit there Colin. Not to say the fan switch may be bad without looking at it.

1 of 1 people found this helpful.

In order to keep driving without the high fan speed irritation, I removed two fuses in box under dash. There is a rather. large relay in box under hood- warning label states only qualified technician should replace, so since there are 5 male fittings and not knowing what else might be affected, I left it alone, just pulled fuses. Owner's manual shows chart of functions and placement by number. Car ran fine without fan blowing. As long as car speed was sufficient to move air, cabin was warm. Today I pulled the resistor unit to see if there was water or rust since the problem occurred when sleet and snow hit us.. .had to pull both heater water hoses. There are rows of vanes or ribs as on air-cooled engines with some type of large button in the center, all sitting on what looked like a circuit board- all looked dry. I used a hot air blower for about 30 seconds, just in case there might be some moisture in there, hoping that might cure the problem so as.to avoid buying a replacement at this time. So, put it all back together just as darkness was falling. Will know tomorrow if my climate control is back to normal.

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