Fan motor fuse blowing

[lush90]

When I got my '91 MM 4x4 a few years back, the previous owner had hacked into the wiring for a few accessories. The fuse panel was melted where the blower motor fuse was held. I cleaned up the other wiring issues and checked over the blower motor wiring but couldn't see anything wrong. I by-passed the the panel by putting an in-line 30 amp fuse for the blower and it has worked perfectly for about a year and a half. About a month ago, I could smell burning plastic and it was the in-line fuse holder melting slightly. I shut off the blower till I got home, checked the wiring again, nothing. Turned the blower on, works fine. Yesterday, smelled burning plastic and saw smoke before I could turn it off. Fuse holder melted shut with apparent blown fuse. Removed fuse, touched the two wires momentarily and the fan kicks on. What would cause such a random problem? Any help is greatly appreciated!

[camoit]

The motor is going bad. if you keep using it with a heavy fuse it will catch fire. There is too much load.

[lush90]

That makes sense. I ordered a new one, should have it by the middle of the week so hopefully I'll be back in business. The only other issue with this truck besides the horn not working is that about one out of twenty or twenty five starts, it acts like it has a low battery, barely cranking. Jump it and it starts perfectly. The battery tests strong and the starter is relatively new. The ignition switch is worn enough that I can remove the key while it's running, so I am guessing it might be responsible, but then why does it start easily with a jump?

[mllrtm79]

The only other issue with this truck besides the horn not working is that about one out of twenty or twenty five starts, it acts like it has a low battery, barely cranking. Jump it and it starts perfectly. The battery tests strong and the starter is relatively new.

I had a similar issue, finally replaced the battery and found a dead cell that wasn't holding a charge. Also it turned out that the battery cables were just slightly loose, replaced the bolts in them when I changed out the battery, and haven't had the problem since.

[Acuta73]

Put a multimeter on the body, cab and bed, and the chassis. See if you get a positive current anywhere, might have a low-voltage short/leak somewhere in the wiring. If so, first place to check is behind the heater controls if you have the kind that slide in that truck (dunno bout 2nd Gen too much).

Do the multimeter test with the engine off, then again with it on. Won't help you find the break if there is one, but will help you figure out if there is one to begin with.

Other possibility is a ground fault, which I am personally terrible at diagnosing.


Edit: Come to think on it, replacing that fan motor might do the trick by itself....bad motor can cause a V-leak

[pennyman1]

check and clean the ground connections from the battery to the tray to the frame and to the block - they corrode and will cause all kind of weird problems. The starter may also have a dead spot, or a bad spot on the solenoid contacts that cause a bad connection.

[lush90]

Okay, I finally got around to replacing the blower motor. I popped out the glove box for access, and removed the one nut and two bolts to drop the assembly and then the three nuts to remove the blower motor. If I can post the picture, you will see that my heater resistor has been damaged. After I replace it and the motor, do you think it was the resistor that as the problem and that the motor was okay?

[pennyman1]

If the resistor was shorted to the motor, then yes; but the resistor may have failed due to the motor drawing excessive current and overheating the resistor.

[lush90]

Hopefully the picture is here! heater resistor_opt.jpg

[pennyman1]

Thats not good - it should never get that hot to melt things no matter how old it is.

[lush90]

Got everything back together, had a 50/50 chance to get the blower motor wired to rotate in the correct direction (couldn't understand the diagrams included with the motor) and was wrong. Much easier the second time. Soldered and heat shrunk a new inline fuse for the blower and it runs perfectly. Drove it for about 20 minutes with the fan on the third highest setting and the new fuse holder was warm, not hot, to the touch. Should it be? I pried open the old melted fuse holder and the plastic from the 20 amp blade fuse was just black and crumbly.

[camoit]

warm is OK but not melting hot. 20 amps is a lot of power. Thats how much a table saw runs, it's also your standard house circuit breaker. If you think about it you are running a table saw under there. Thats why we use AC power in homes and not DC. AC can go longer distances then DC. Nickolai Tesla won the race with AC over Alexander Gram Bells DC so here we are today. Or was it the other way around? The alternator only puts out 35 Amp. Thats why the truck stops when you turn on the blower and head lights. Where ever there is resistance heat will be generated. So that tells me that there is some kind of resistance at the fuse.

[pennyman1]

That was Edison, not Bell with DC power. But now with solar panels, that all may change - people are going off the grid and using DC for their appliances. That is until they find out (again) why AC is better.

[Acuta73]

AC was better for transmission. Didn't degrade over distance like DC does. There was a way to make DC do it, but forgot what it was.

[pennyman1]

It also had to do with shock and electrocution hazards - DC is much deadlier than AC at high current - it stops the heart and frys the body quickly.

[lush90]

Drove for about an hour today with no heating of the blower circuit! Thanks to all for the advice. Just for a learning experience, is there an easy way to test the old blower motor before I pitch it?

[camoit]

You would need an amp meter. Then use some jumpers off a battery and see what it draws.
Somthing like this. http://www.homedepot.com/p/t/2028543...ff&R=202854398

Keep in mind it will draw more with the blower on the shaft then with out it.

[Acuta73]

Curious.

Did replacing the motor cure the fails-to-start issue? Or too early to tell?

[mitsumopar]

You could also check the blower housing to make sure there is nothing binding up the fan blades. Also check the ground lead from the motor.

[lush90]

Yes, the new blower seems to have taken care of the no start problem. The old blower motor must have really created a heavy draw.