In a all out ditch effort to find out why my blower fuse gets so hot it melts, I have changed the blower motor 2 times, the resistor pack once and even the fan switch and mode selector panel, and the fuse still gets hot and on high it will melt. I can Ohm between all the wires and everything looks good, BUT I want to know...IF I Ohm between the hot side of the blower motor fuse and ground, I get about 55 Ohms...is this to low ? I would expect it to be open between hot and ground. maybe a wire got hot and melted and only 1 or 2 strands are touching ground causing the load. Can some one verify what I should read when I ohm hot to ground in my 1991 XJ ? I have tried everything except totally running new wires for the entire blower circuit...Thanks in advance and I hope to get to the bottom of this hot blower fuse.

 

That's a short. How many amps does the blower motor draw on high?

 

I just measured the resistance ( OHMS) between the green and black wires from a spare 89 Jeep blower motor as 6 ohms. That means the current of the motor should draw about 2 amps when running on high using the equation between voltage, current and resistance V = IR where V is the voltage of 12 volts, R is the resistance of the motor of 6 ohms and I is the current it should draw in amps. At lower speeds it will probably run less than 2 amps.

I'd agree with Turbo that there is a short and I'd suggest carefully looking at the wiring after the fuse that blows.

Using the equation you can calculate the current from the the point you measured to the 55 ohm: I = V/R as 12/55 = .218 amps or 218 milliamps. I'm sure that's not blowing the fuse.

Hope this helpful.

 

I hope an electrical guru will chime in but I don't think you can calculate amp draw of an operating motor by applying Ohm's law to static motor parameters.

 

general rule, if its hot there is a lack of resistance somewhere and needs further inspection / testing / replacement.

 

I was thinking the same thing, that ohm'ing between hot and ground I should have very high resistance. I went out today, and with everything hooked up and the A/C running on max, the fuse heats up right away...Sooooo, I disconnected the blower motor completely...and tried it again thinking that if I had a problem with the wiring it should still heat up if it is indeed shorted....right ??? ( or am I way off base here ) anyway, with the mode selector in max, and the fan switch set to High, and the A/C running...the fuse didn't even get warm...!@#!@#%@!#% Now, I am really in a jam, it seems like the blower ( load ) motor is the culprit...when I hook it directly to the battery it run with no problem and the wires do NOT get hot,,,,so am I correct in assuming that with the blower motor disconnected, and everything else hooked up normal, and it doesn't get hot that the motor is STILL the cause ? man this will make me crazy hehehe any ideas ? thanks guys, OH and the motor pulls almost 2 amps on high, and when I ohm between the 2 wires on the motor is is almost 2 ohms...pretty low I think, wish I knew what a good one measures ......

 

Which fuse is it? 25 amp fuse #5 in the fuse block?

 

You may want to check for a short to ground in the power wire circuit from the blower switch to the blower motor.

Disconnect the battery negative post connector.

Remove the connector from the blower switch, blower resistor, and the blower motor.

Using a digtal Ohmmeter set on the 200 Ohm scale, touch the meter probe (+) the blower connector pin cavity with the Dark Green wire and attaching the meter (-) probe to chassis (not engine) ground. You should see an open. If you see high resistance or continuity, the power wire is chafted/shorted to ground. If you do see an open, reattach the blower resistor connector only and read the blower connector again. Again, you should see an open. If you see continuity then the short is in the wiring from the blower resistor to the blower switch.

 

No if it's hot there is a LACK of resistance. Resistance resists current flow and does not increase it.

Unplug the motor and if the fuse no longer gets hot the motor winding is the problem.

And no you cannot calculate via ohms law except on static numbers... So you would need to know the rpm and specific resistance of the winding at that speed...

 

well today, I went out and disconnected the blower motor, the resistor pack, the selector switch and the blower speed switch and pulled out the blower fuse. all points ohm short point to point and open to ground...I didn't like the way the wire between the fuse block under the dash ohmed between the load side of the fuse and the selector switch, some times it had 55kOhms to ground depending how hard I pushed on the wire...I ran a new wire and now I can run full speed and the only thing that gets hot is the slider selector switch. the fuse and the wire I ran gets warm but nothing like it was and it does not melt the fuse that I installed in the wire I ran. I took apart the slider selector switch and cleaned it real well, it is a simple switch with heavy copper wipers inside it, maybe they are getting hot and that is why the switch it self gets real hot. a pretty simple circuit but still needs more investigating as to why things heat up. BUT at least now with the new wire run between the fuse block and the slider switch I can use the system. Has anyone solved the problem with the fan circuit heating up ? I have noticed it heats up a lot faster when in a cool situation where the compressor is running, the slider switch supplies both compressor run command and also the blower circuit, so it is a pretty busy switch...what are your thoughts ? thanks