Blower Motor Blowing Fuses

[ctholl]

I’m tracking down an electrical gremlin on the wife’s 66 New Yorker Original A/C car.

So I’m finding the the fuse for the blower motor starts burning as soon as it’s placed in any position but off. (Marked with red arrow)

I disconnected the new motor tested resistance and it measured at .6-.7 no good so I ordered another one this one measures at .3-.4 again no bueno. These were both four season motors is there a better brand? I have another one on the way but I’m not holding my breath.
The blower blows strong when it works.

The blower switch, resistor and AC switch were bench tested before install and measured good.

Any where else I should be looking to fix this?

 

[65Port]

How is the ground for the blower motor? With the car off, has the wiring been checked for short to ground? Can you check for a short on the fan (from power input to fan case)? You can also put your meter in place of the fan to see what the voltage is and if the fuse is still getting warm when you turn it on.

 

[Big_John]

I disconnected the new motor tested resistance and it measured at .6-.7 no good so I ordered another one this one measures at .3-.4 again no bueno.

Do you have a spec for that? I've never seen one.

That aside, that is actually lower resistance than I would expect.. And I think has nothing to do with your problem. Fuses "blow" from excessive current draw (amperage). Anything under 20 amps draw won't cause a problem.... But you are having excessive draw (over 20 amps obviously). That could be a bad motor or a problem with the wiring or switches.

In this case, with a heater motor, usually the problem is either the commutator/brushes are worn or dirty, or the bushings in the motor are dry and causing excessive friction in the motor. If the problem isn't the motor, then it's in the wiring (short to ground) or in the switch.

Checking motor resistance (I assume across the two contacts) would tell you if there was a bad winding, commutator etc... But only if the resistance was much higher or showed the circuit to be open. Again though, your readings don't show that. In fact, I could make a good case for the difference in resistance between the two motors is negligible and could be just the difference in contact of your test leads to the motor. Give either motor a spin and I'll bet there's a difference just from where the brushes land on the commutator.

Testing the switch without a load can tell you if it's not operational, but that's about it. A dirty switch can check just fine for resistance. I give this analogy... Take a piece of #12 wire. Check the resistance, and it shows zero. Hook a motor to it and it runs fine. Now strip away the insulation and wire to where you just have 1 strand of wire. Check the resistance on that strand and it will also show zero. Now hook up a motor to the one strand and what does it do? It burns out. What really needs to happen is what's called a voltage drop test that's done under load. Look for a You Tube video on that subject.

From your post, I can't understand if this happened (fuse blew) with an older motor or with a replacement motor.

 

[ctholl]

This is a new motor currently in the car. I moved the ground to where my TCU is grounded so I know the ground is good. No voltage drop on the positive or negative sides. Spun the motor in the car multiple times getting a resistance value of .3-.4

The fuse is now only burning on the high blower setting.

It’s 10.28 volts at the motor while it’s on high, at .3 ohms that’s 34 amps.

 

[volksworld]

connections at the fusebox itself? rust between the clips in the box and the fuse? (steel connectors were a great idea)...there may be a male spade rivited to the backside of the fuse clip with a female spade crimped to the wire plugged onto that...any corrosion there will cause issues too...blower fuse was absolutely melted on my 68 20 yrs ago...jumped it out with a blade style fuse holder...I just replaced the box with a non melted one and all new connectors...but left the blade fuse there cause i still don't trust it...