2002 Mercury Marquis overheats

Tiny
GMAN747
  • MEMBER
  • 2002 MERCURY MARQUIS
  • 2WD
  • AUTOMATIC
I changed the thermostat and water pump and it still overheats while stopped at traffic lights it is ok when on the open road
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Monday, June 2nd, 2014 AT 2:37 PM

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Tiny
CARADIODOC
  • EXPERT
Check the fan operation. It isn't needed at highway speeds.
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Monday, June 2nd, 2014 AT 2:42 PM
Tiny
GMAN747
  • MEMBER
Ok, I checked the fan and replaced it but the fan doesnt turn could it be the fuses or relay swiches if so how do you check them
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Tuesday, June 3rd, 2014 AT 1:24 PM
Tiny
CARADIODOC
  • EXPERT
There's entire pages in the service manual on how to test the system. Those steps should be done before replacing parts, especially an expensive fan motor. An easy place to start is by substituting the fan relay with one of the other ones like it, but then you have to wait to see if that works. Instead, remove the relay, pop its cover off, reinstall it that way, then squeeze the movable contact with your fingers. The fan should run even with the ignition switch off.

There's two totally independent circuits involved, and the results of squeezing the contact will tell us which one has the problem. The low-current circuit is the one controlled by the Engine Computer. It activates the magnetic coil inside the relay. You're doing its job by squeezing the contact. At that point the contact turns on the high current for the fan motor.

The most common problem is the old fan motor was shorted or tight. Either condition will cause it to draw unusually high current and blow a fuse. If that's what happened, you already replaced the cause of the problem, the fan motor, but not the result, ... The fuse. That would make the motor not run now when you squeeze the relay.

If you have a standard fuse for the radiator fan, you check them by pulling them out and looking at them. You can also check them in the fuse box with a voltmeter. A good fuse will have 12 volts on both terminals. A blown fuse will have 12 volts on just one terminal. Plug-in fuses have two little test points on top for taking voltage readings.

Some cars still use fuse link wires that can't be visually inspected. These are used only in high-current circuits. They are dull green, gray, white, black, or orange wires that are spliced into the other wires they protect. You test those by tugging on them. Good ones act like a wire. Burned-open ones act like a rubber band.
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Tuesday, June 3rd, 2014 AT 9:27 PM

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