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Alaska TJ 11-05-2012 10:17 AM

intermittent heater blower-all speeds
The blower works sometimes at all speeds and sometimes it won't work at all. It is completely unpredictable. Help new out here, please.

Milkman 11-05-2012 10:25 AM

Mine is doing the same thing and I'm getting ready to tackle this next week. From the sounds of it it's probably the switch that you can get at your dealer for around $20.

If that doesn't work you can move to the blower resistor and then on to the blower motor itself.

RUBI 4 MY MRS 11-05-2012 01:03 PM

The first & easiest thing to check is the 3 wire connector on the back of the mode switch. Check for melting & signs of excessive heat there & make sure all 3 terminals are making good tight contact between the male & female halves. Start by wiggling the connector to see if the blower cuts out. While you are back there also check the connectors at the speed switch & the resistor for melting/excessive heat.

If all is good there & it is a bad switch then it is more likely the mode switch if it is all blower speeds of none. When the speed switch starts going bad it is usually losing 1 or 2 speeds not all of them at once.

HawaiiJosh 11-05-2012 01:14 PM

If its not the switch I'd check the resistor. Its behind the glove box, 2 nuts holding it in place, if I remember correctly the upper one is a pain.

Jerry Bransford 11-05-2012 01:54 PM

There are two primary items that would be common to a problem at all speeds, the fan speed switch and the fan motor itself. The resistor pack is not in the circuit on the fan's highes speed setting. Check the wiring between the switch and motor for being burnt, melted, etc. as suggested above. That is a very real problem, especially for older TJs, my '97 fan motor wiring looked like it had been in a forest fire & I had to replace much of it.

If the wiring is ok, I would probably lean more towards a bad fan motor or perhaps a bad connection.

Bucktaker 11-05-2012 02:23 PM


Originally Posted by Jerry Bransford (Post 2962352)
There are two primary items that would be common to a problem at all speeds, the fan speed switch and the fan motor itself. The resistor pack is not in the circuit on the fan's highes speed setting. Check the wiring between the switch and motor for being burnt, melted, etc. as suggested above. That is a very real problem, especially for older TJs, my '97 fan motor wiring looked like it had been in a forest fire & I had to replace much of it.

If the wiring is ok, I would probably lean more towards a bad fan motor or perhaps a bad connection.

Was this a big job to replace the wiring? was it available at from the dealer? looking and a 1999 with a similar problem #3 not working
Also has funny issue with the ignition sw have to be sure the radio goes off to be sure the key is in the off position and doesnt run the battery down....possible bad ignition sw?

Jerry Bransford 11-05-2012 03:10 PM

I just spliced in new 12 gauge wiring from my garage wiring stock, it just required some splicing & soldering. If just fan speed #3 is not working, I would suspect a bad resistor pack which is what gives the fan all of its lower speeds. Could be bad switch too but the resistor pack would be my #1 suspicion.

NVTJ 11-05-2012 03:34 PM

Mine was doing the exact same thing. It ended up being the plug on the heater motor itself. It was melted and only making conection some times. Pull out the battery and unbolt the ECM and its easy to get too.

Alaska TJ 11-07-2012 04:28 PM

Thanks for all of the feedback. I am just waiting for a semi-warm day to get out and check it out. It was -26 F this morning! Brrr. Luckily, the heat was working today.

I read somewhere that the cause of these burned wires, switches and relays is that the internal resistance in the motor increases with age, thus requiring increasing amounts of power to be transmitted through the wires over time. Is this true? If so, is it wise to replace the blower regardless of whether it still functions? Is it better to go with OEM, or are there other options for blower motors?

Jerry Bransford 11-07-2012 04:47 PM

Actually, the most common cause of burned wiring is the internal resistance of the fan motor decreases with age causing it to draw more current through the wiring. The fan typically draws around 15 amps which increases with age. Once it gets close to or over 20 amps, it's time to change the fan motor.

shanes04wrangler 11-07-2012 05:28 PM

when i was wiring up my fog today i noticed the plug for mine dose not clip in anymore and its loose so i loose high sometimes. can i get a new plug?

RUBI 4 MY MRS 11-07-2012 06:51 PM

Alaska TJ, Since a semi-warm day up there might be March or so you might be able to get high speed by tapping into the black/tan wire from the blower connector or the resistor connector with a jumper wire (12 gauge) & connecting it to ground. That should give you hi speed whenever you turn the key on. Add an alligator clip to it to be able to stop the blower if needed by removing it from the ground.

Alaska TJ 11-08-2012 12:44 AM

A "semi-warm" day is probably a relative term. It is supposed to get above zero degrees this weekend. Sounds semi-warm to me!

I put a space heater and a halogen light in the Jeep this evening so that I could take a look at it. Put a new switch in, but the original was in good shape. Pulled the motor wiring and verified that it was getting electrical current. Reconnected it and still no blower. So I will pick up a new blower tomorrow and install. Thanks all!

RUBI 4 MY MRS 11-08-2012 03:12 AM

It is the ground side that you need to check. The ground path goes from the motor through the resistor (high speed bypasses the resistor) through the speed switch & through the mode switch. Since it is all speeds or none it is not the resistor or the speed switch or only certain speeds would be affected. That leaves the motor (not likely intermittently working), the wiring/connectors or the mode switch. It is pretty easy to pull the motor & bench test it or test it in place as I detailed above. I wound do that before buying a new one unless it is returnable if not the problem.

The motor may in fact be drawing more amps than it used to & replacement may be warranted but first find out why it is not working. Very frustrating to put a new one in & have it still not working.

Alaska TJ 11-08-2012 01:57 PM


Originally Posted by RUBI 4 MY MRS (Post 2971789)
It is the ground side that you need to check. The ground path goes from the motor through the resistor (high speed bypasses the resistor) through the speed switch & through the mode switch.

Okay, so based upon your info and my lack of knowledge, it sounds like a bad motor is not likely. I will pull the motor and test it, but I should ask a question or two before I contort myself into that position again.

Last night I disconnected the wiring harness that connected to the motor. I put my voltmeter onto the switch side and measured 12 V. You suggest that I should check the ground side. To do this, would I check 1) the resistance between the motor and the resistor, 2) the resistance between the resistor and the fan switch and 3)???

Or can I just check the resistance between the motor ground and any other ground on the vehicle? I am not sure how to check whether the mode switch is working.

Any additional detailed info would be greatly appreciated.

Jerry Bransford 11-08-2012 02:12 PM

You can measure for resistance on the motor's ground connection directly between the motor housing and any suitable bare metal ground point.

However, a simple ohmmeter/continuity test for connection may not tell the entire story. It may indicate zero Ohms which is the same thing as good continuity but only to the low amount of voltage/current generated by the meter's tiny 1.5v battery. That same motor ground connection that shows good continuity to the meter may not be good enough for the fan motor that needs its ground connection to be able to carry the 15-20 amps at 12v that the motor draws at its highest fan peed.

Alaska TJ 11-08-2012 02:30 PM

Great. Thanks for the heads up, Jerry.

I just found this thread, which seems to walk me through most of my questions. I'll try to take a closer look this evening. Time to get out the extension cord and space heater again.


Originally Posted by RUBI 4 MY MRS (Post 2798637)
This is an old thread so you may not get the OP to answer but if you have power to the blower (fan) motor then you are losing your ground. The motor is speed controlled from the ground side of the circuit through the resistor block, the speed switch & the mode switch. Any of these 3 components may be bad but often it’s one of the connectors or the wiring that overheats & you can see melting if you check closely. Sometimes one of the terminals is “loose” fitting inside the connector & arcs causing loss of contact so careful inspection is needed. Read this thread for detailed troubleshooting of the circuit. In post #3 there is a link to a knowledge base that has FSMs & you can see the wiring diagram for the circuit there. Read the first 10 posts & I think they will help find your problem.

RUBI 4 MY MRS 11-08-2012 05:43 PM

More specific to where you are now:

You should unplug the connector to the motor. That will isolate the 2 wires. Then one will have 12v the other should have continuity to a good ground. The ground path assumes the mode switch is in any position except “off” & best to have the speed switch on the high setting. You can also test for 12v between the 2 wires. If it doesn't show 12v then there is no ground path for the meter. To test the mode switch find the 3 wire connector. Again with the switch in any “on” position the dark blue wire needs continuity with the black one while still connected to the switch. If that has no continuity then the mode switch is bad. You can then test between the dark blue/white wire & the negative motor connector. With the speed switch on high there should be continuity there & as you switch the speed switch to the other speeds the resistance should increase as you lower the speed settings. Here is a wiring diagram of the circuit that may help you understand what is supposed to be happening. Note that the motor has fused power through the relay. Then the on-off & speed is controlled by the ground side of the circuit. The mode switch sets the off-on by grounding the motor when in positions 1-6 (right dial inside the control box at the bottom of the diagram). In the top middle on the box is the speed switch with positions 7-10. That directs the ground path through the resistor or bypasses it for high (7) position. See where this gets you & post results.

Alaska TJ 11-09-2012 12:22 PM

Okay, I spent some quality time with the Jeep last night attempting to rectify this heater situation. My first "problem" was that my heater was actually working when I started. I find it very difficult to diagnose a "faulty" heating system while it is actually working.

Anyhow, I took a look at the back of the mode switch and there was no evidence of arcs between any of the terminals. At some point, the heater blower did stop working though. I pulled the wiring harness from the blower motor and verified that there was ~12 V of current and a good ground to the disconnected wiring harness (at the connection to the blower). Reconnected the harness to the motor and it was still dead. Pulled the motor out and reconnected the harness and it was working.

I then took a closer look at the mode switch (this all occurred while the blower was functioning properly). I disconnected the 3 wires from the switch and checked for continuity between the 3 terminals on the switch in different positions. This gave me some unexpected results, I think.
In the off position - no continuity between any terminal
In most other positions - Continuous between 2 grounds
In two other positions - continuous between all 3 terminals (Isn't this odd?)

After reconnecting everything, the heater was still working, so I was unable to do any more testing.

My next plan to test the system was to run a pigtail from the ground wire at the motor to a good ground through a switch (thus keeping its existing ground connection to the mode and blower speed switches). If I keep the newly installed switch off until the blower stops working, then I flip the switch on, I would expect the blower to come on if it is indeed a bad ground. Then I think that it would have to be in the wires or in the mode switch. The wires are mostly wrapped in tape and are not immediately visible, but all of the visible wires appear to be in pretty good shape.

RUBI 4 MY MRS 11-09-2012 03:01 PM

Adding the ground wire with a switch is exactly the best thing to do. An intermittent problem is the worst to find. That will allow quick testing when it cuts out.

As for testing the mode switch, this is what should happen. The 3rd wire is for the A/C compressor. It comes on in positions 1, 2, 5, & 6 in the center lower diagram of the control box in the diagram above. The LB/OR wire (X) goes to the A/C. In those positions all 3 should have continuity (C3 A & C3 B to C3 C ground), in all except “off” (0) you should have continuity to 2 terminals (C3 B & C3 C ground).

Alaska TJ 11-10-2012 07:31 PM

Okay. We have more info, but no answers. I installed a pigtail ground with a switch to the preexisting ground at the motor. When the motor failed with my newly installed switch in the "off" position, I quickly flipped the new switch to "on" thus giving it a definitive ground connection, but the motor did not start operating. I also verified that the wires at the motor were getting power and had a good ground. Reconnected everything and the motor still did not function.

Everytime that the motor was not functioning, I have been able to verify that the motor's wiring harness had power. It seems like either the connection is faulty at the motor, or the motor has an intermittent short in it. Or maybe I should connect it to a 12 volt power source with a switch and see if it will fail.

Jerry Bransford 11-10-2012 07:38 PM

The motor doesn't have a short in it, it has an intermittent open... perhaps a bad brush. If the motor was getting +12v, and it seems like you have verified that, and it is getting a good ground which you even applied manually with a switch, I'd have to say the motor is simply bad and needs to be replaced.

The good news is the fan motor is easily replaced from the engine side of the firewall after removing the battery.

Alaska TJ 11-10-2012 08:39 PM

Excellent information. I just connected the battery directly to the motor and it didn't start until I pulled the motor out. Then it started blowing again, so I reinstalled it until I buy a new one tomorrow. Thanks all!

RUBI 4 MY MRS 11-11-2012 02:31 AM

I know that was your original instinct & it looks like you were right. Many times it is the wiring/connectors or the switch though & I believe in diagnostics before replacing parts randomly. As I stated before, intermittent problems are the worst & I hope a new motor fixes it.

Alaska TJ 11-11-2012 02:55 PM

I am also glad that I tested the entire system before concluding that it was the blower motor. I understand the overall system much better now than I did a week ago. I bought and installed a new blower motor this morning, but on the way to NAPA the blower motor wasn't working. Rather than freezing on the way to the store, I pulled over and gave the motor housing a good whack with my palm and it came on! I wish that I would have thought about trying that all of those cold mornings going to work, but knowing that something was amiss with the motor, gave me the insight to at least try hitting it into submission. Anyhow, hopefully everything is repaired now, although the fan now hums after I shut off the vehicle as the fan cage is slowing down. There was a metal clip on the squirrel cage that may have been a balancing weight or something, but I don't really know. I left it on for now, but I hope that it is balancing it rather than unbalancing the motor.

jmartin325 09-26-2016 10:21 AM

I know this is an ancient thread but did the replacement motor fix it for good? I am having same exact issue as Alaska TJ. Thanks!

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