2004 Mercury Marquis Blower Motor Repair

[quote=”Durango8766″ post=60033]I don’t know if I missed something in the video, but any thought as to why that switch melted? Good video Eric, Thanks![/quote]

You did. In the closing statements of the video I talk about how a bad blower motor could pull too many amps through the circuit and as a result cause the switch to melt.

[quote=”sparky5982″ post=60102]Hey Eric,

New member, first post, long-time viewer. Thanks for another video.

So, you concluded that the motor shorted, allowing much greater amperage through the circuit, thus damaging the speed selector switch.

If the blower motor power circuit is fused at 40 amps, why aren’t all components in-line of that circuit also rated to handle 40 amps? Depending on the blower motor resistance to hold back the amperage seems like a bad idea. The speed selector switch and the variable speed resistor ought to be able to handle at least the amperage they are fused at. The wiring to the switch seems much too small for the fuse size also. You could not do this in normal industrial wiring. Thoughts?

Also, did you ohm the old motor or get an amp draw and compare to the new? I’d be curious to see if the old blower indeed had much lower resistance.

Thanks for all the videos. Carry on being awesome.[/quote]

First, you don’t ohm to get the amp draw. Ohm readings aren’t my favorite anyway because they really don’t tell you much. In the case of a motor they don’t really tell you anything at all because you’re really just testing the connection to the brushes, you won’t be able to find a bad winding unless you ohm the windings themselves. To check amps you need an inductive amp meter. It would have been nice to do a comparison between the 2 parts but I didn’t think of it at the time. For me, I saw a blower motor that wasn’t working and a melted switch. After replacing the blower motor and fixing the switch everything seemed to work normally again and has for the past 2 months.

As for the amp rating of the circuit I think they go by what the load can draw, not what each individual component in the system can draw. Since the blower was rated near 40amps I’m sure that’s where they got their fuse from. You wouldn’t want to put too small a fuse in the circuit or it may blow all the time. You need to balance between what the circuit draws normally and what would cause a fire I suppose. All the other parts become expendable. Automakers are concerned about cost above all else so they make things as cheaply as possible and hope they don’t get sued.

[quote=”bmwpont” post=60354]Good job Eric! Been there, done that on a 1976 LTD 30 years ago. The circuit is basically the same; Ford just added a relay to the circuit, probably for safety (different story, different day). The motor IS the problem, no doubt, the bearing grease is gone. As the motor runs, the bearings start to heat up, start to seize and slow the motor armature. The armature slows, the motor’s internal resistance decreases causing an increase in current as the motor attempts to run at its designed speed (E=IR).

The speed control switch contact area is very small. As the current draw of the motor increases, a point is reached where the contacts start to arc. Pitting occurs and the resistance increases between the contacts. This increases the voltage drop across the contacts and it all starts to heat up. Eventually, the plastic melts and the switch consumes itself.

In this case I would have changed the relay too. The entire circuit was stressed and the relay was subjected to high current loads also.

Always dirty,

Dan[/quote]

Thanks for that detailed explanation. I thought the same thing. Appreciate your input.

[quote=”bmwpont” post=60387]The resistor is designed to get hot. Three coils of resistance wire of varying length, hence each coil having a greater resistance. Using Ohm’s Law, (E=IR), the longer the resistance wire, the greater the voltage drop, heating the coil. The coil assembly is installed in the blower flow path to dissipate this heat. Also, notice in the video, the maximum speed contacts were the only contacts damaged. This switch setting bypasses the coil assembly and full motor current passes through these contacts. In a series circuit, this voltage drop reduces the voltage applied to the motor thereby slowing it. I’ve witnessed the coils glowing red and suffering no damage other than a slight increase in resistance.

Replacing the switch only would result in the customer returning with the original complaint.

(E=IR, E=voltage, I=current, R=resistance)

This explanation help?

Dan[/quote]

The problem occurred on the High speed circuit. High speed circuit bypasses the resistor so it’s not even worth talking about the resistor part of the circuit. You are correct about it’s operation however.