Mechanics

BLOWER/RESISTOR PROBLEM

1997 Isuzu Rodeo • 18,500 miles

Hi, I have a 1997 Isuzu Rodeo that is not blowing air. If I recall correctly, it didn't work that well on the lower settings for a while, and now it's not working at all (perhaps this indicates a problem with the blower motor switch?- I'm having trouble finding a diagram or detailed description of the switch part itself.).
First, I checked the fuse in the box underneath the hood. It was blown. Replaced it, and that fuse blew when I cranked it up. Is this most likely a wiring problem? I have gotten to the blower motor, behind the glove compartment- took a meter down there and tried to check out the resistor. Caveat. This was my first time using an ohm-meter, but it seems like it might be pretty hard to screw up, no? So, nothing seems to be reading (took readings between pins 1-2, 2-6, etc. Based on another poster's suggestion). It's not zero, like when you touch the two probes together- it's at the other end of the spectrum. Just to clarify, I had the car off (but radio on). Everything was also still in place (should I have removed the resistor from the motor and tested it? I can't really figure that it would make a difference, but my electrical/wiring knowledge is fairly limited. ).

So, at this point, I have some information, but I'm not sure what it means. Maybe my resistor is bad? Or maybe it's a wiring problem? I read in another post to check the blue/white and blue/black wires, but I had trouble following the thread. There are the wires, one of which is blue AND white and the other is blue AND black. The post I read said to test the blue and white wires- so maybe I'm misunderstanding something here.

Any suggestions? Thanks! - Hannah
Avatar
Hanboo28
December 26, 2011.



So does the fuse blow as soon as you turn the blower fan on?If so unplug the blower motor replace the fuse and turn the fan on thru all the speeds. DOes the fuse blow then?

Saturntech9
Dec 26, 2011.
Here's a totally different way to approach this. Remove the blown fuse, assuming it's for the heater fan, plug in a pair of spade terminals, then use a pair of jumper wires to connect them to a 12 volt light bulb. When the short is present and the circuit is powered up, the bulb will just be full brightness but it will limit current to a safe level. Now unplug the fan motor. If the bulb goes out, the motor is shorted and it likely damaged the resistor. If the light is still bright, unplug the resistor, then the switch, until the light goes out. If it is still bright, something in the wiring is shorted before the switch. Wiggle and unplug things in the circuit until the light goes out. That will allow you to zero in on the cause of the short.

Caradiodoc
Dec 26, 2011.
I didn't notice when the fuse blew, but I can try to figure that out next time I crank it (when I get some more fuses, haha!).

Thanks, cardiodoc- I will try this! It sounds like it might narrow down the problem.

Also, I tried testing the ohms using a different setting, and am now getting a reading between pins 1 and 2- about 3 ohms (posting I read earlier said I should get 2.4). Between all other pins, it's an open circuit- the needle is on the far left side of the meter.

Thanks for the tips!
-Hannah

Tiny
Hanboo28
Dec 26, 2011.
If the test light trick doesnt work then the blower fan motor is just drawing too many amps and needs to be replaced.

Saturntech9
Dec 26, 2011.
Yup. And without a diagram to look at, it sounds like the resistor is bad too if there's a terminal that reads open circuit to all of the others. They usually have a thermal fuse built in that burns open when the motor draws too much current. That makes the resistor get hotter than normal and it won't get sufficient air flow to cool it.

I haven't replaced many fan motors but those that were blowing fuses were typically never shorted; they were just real tight. If you connect it directly to a battery to test it, you'll see an unusually big spark when you connect the last wire, and when you disconnect it the motor will stop spinning right away rather than coasting to a stop over a period of a few seconds.

Caradiodoc
Dec 26, 2011.
So, I didn't try the trick with the light bulb- but I did get my sister to hold the meter on the fuse (or, rather, where the fuse goes) and check the volts on different settings. Once I cranked it up, it held at 15 volts through all settings. However, when we replaced the fuse and tried the same thing, the fuse didn't blow until the highest setting (4).
So. To recap. Blower isn't working at all. Fuse is blowing when you get to the highest setting. Pins 1-2 have a reading (3 ohms), all other pins on the resistor read open circuit. I tried to test the wiring harness, but I think I'm having trouble grounding the meter and I don't trust the readings I got.
So, I guess I might try and take the motor out and plug it in somewhere and see if it works. Any other suggestions? I know the motor switch was indicated when the blower was working ONLY on high settings, but I'm not sure blowing the fuse on the highest setting is an analogous situation?
Thanks!
-Hannah

Tiny
Hanboo28
Dec 26, 2011.
You should do the test light test on the highest setting and see if the test light is lit on the highest setting to elminate the possibilites of the blowing fuse issue.

Saturntech9
Dec 26, 2011.
So. Update: The motor appears to be working. I pulled it this morning and hooked it up to the battery, and it spun good (winding down slowly once we broke the circuit). It was a little sticky when we first pulled it out, and had some pine straw and dirt in it.
Tested the relay (the blue and white wire running to the blower motor) and it had power, so I think that means the relay is good.
My money now is on the resistor (maybe the sticky motor overheated it?) But I still have some suspicions about the fan motor switch.
Does anyone know a way to test the fan motor switch (maybe one that doesn't require me to take apart my whole dashboard, haha.)? I read on another post that the black wire to the fan switch should have 2 ohms or less- I tried to trace the only black wire leaving the fan switch harness, and lost it over near the passenger side door.
Any suggestions would be appreciated- and thanks for the tips so far!
-Hannah

Tiny
Hanboo28
Dec 27, 2011.
Does the fuse still blow?If so why dont you try what cardiodoc suggested to eliminate a short to ground and then try to isolate it to the motor. Its most likely the motor and possible the resisitor as well. You want other test methods but wont even try the ones you were already given. Makes no sense to me.

Saturntech9
Dec 27, 2011.