Rule303

Hey All,
I have a '97 Sport 4.0 and last year the blower for the AC started to work intermittently and then just stopped working.
The motor itself seems OK, if I wire it to 12v directly it runs fine.
I get 12 V at the connector but when I wired in a light bulb and turned the rheostat up and down the lamp was steady (didn't dim or brighten).
At first I thought this would mean the resistor was bad but now I'm not so sure.
Any help?

Mark

MtnHermit

Does it run at the highest speed setting? If so, then you need a new resistor. If not, then it's the wiring, some great threads on diagnosing the wiring.

Rule303

No. It doesn't run at all.
But if it's the wiring why am I getting a steady 12 volts with, apparently, steady current?
In any case, thanks for the tip. I'll check the wiring threads too!

Mark

RTorrez1

It's possible that you have a bad or loose wire in the blower. Try giving the wires a wiggle and see if it come on since you say it comes on when you run a direct wire. Also make sure the the connections are nice and clean.

The_ocho

Most peoples problem is either the resistor in the switch like said before or the wires on the back of the switch. I know with mine I have to tap on the switch to make it work but now days its gotten so bad Iv been thinking of just bypassing the factory switch and putting in a toggle for a simple fan on and off function.

CCKen

How are you measuring for voltage at the blower connector? Are you going from connector pin cavity A to chassis ground or are you going from connector pin cavity A to pin cavity B. Pin cavity A is power from the blower relay and pin cavity B is ground through the blower resistor block and the blower speed selector switch, or in the case of low speed, direct to the selector switch.

You need to go from blower connector pin cavity A (the one on your left as you look at the connector) to pin cavity B, and turn the blower speed selector to 1, 2, 3, and 4. If you get power on 1 only the resistor block is burned out. If you don't get voltage at all, you have an open ground circuit.

Testing using power probe light in this instance is useless, you need a voltmeter.

Rule303

Thanks for the input folks but, my apologies, I guess I wasn't describing things well.
I'll try again:
The blower motor for my AC does not run at all (any switch position).
I have tested the blower motor by running battery voltage directly to it and it runs fine, so probably not the motor itself.
I have tested with a Fluke DMM between the pins on the motor's plug and with the AC on and varying the setting on the switch and get the expected 12 volts at each setting.
Because a resistor is used to vary the speed of the motor I would expect current to vary with each switch position but not the voltage (or do I have that backwards?).
With that in mind connecting the lamp to the power in plug and varying the switch position I would expect the lamp to brighten or darken as the switch is turned up or down. I suppose I could also connect an amp meter in series with the lamp but I assumed the light would be enough to tell me if current is in fact changing with the switch position. Based on the lamp it's not.
So, I'm either making false assumptions or there is some other component of the system that I need to test.
Am I helping here or just dirtying the waters more?

Mark

Rule303

So, given what I wrote in the post above, what am I missing?

CCKen

Here's some circuit diagrams, one for the 97XJ and one that I put together that covers 99 and up. There's also an explanation how the blower works in the '97 circuit.

Look at the '97 circuit diagram and compare it to my circuit diagram. Note that the '97 diagram fails to show the ground connection splice between mode selector and the illumination lamps to ground G108.

You may want to check the ground circuit for high resistance. Using your Ohmmeter (battery (-) cable disconnected) measure the resistance in the ground circuit from pin B of the blower connector to chassis ground. Run the fan speed selector though all the settngs. Mode selector must be in any setting but off.

I suppose if the illumination lights work the ground from them to G108 is intact.

Also, visually examine the blower resistor block connector, it has been known to burn up.

Danno25nh

The schematic shows the resistors as voltage limiters. Your voltage should change based on the position of the switch.

Based on the drawings provided by CCKen, the problem would likely be from the return path of the motor, through the relay (good point to check, I'd jump it as #2 check) and up to fuse 6 (first place to check)

Rule303

Ah, gentlemen that is flip'n brilliant! Exactly what I was hoping for (not that all the replies didn't help along the path to figuring this out).
Thanks to all of you for the guidance! I'll give it a go and let you know what I find.

Mark