Burning smell when turning on heat/ac?

[Redsnake]

OK... this is a long shot but wanted to see if I could get some ideas where this could be coming from. My son just recently bought a 92XJ, 4.0 Auto that only had 130K miles on the clock... In really good condition or so it appears. Spent a lot of it's time being towed behind and RV.

So here's the issue. During the test drive we didn't notice any bad smells or problems other than a slightly sticking accelerator pedal which we should be able to fix. So on our drive home (separate rigs... him driving the '92 and me in my '01) ... He tells me when we stop that he smells a burning smell. It smells a bit like an electrical burning. Smell goes away w/the heater/ac is turned off but comes back strong almost immediately when turning it back ON.

I'm guessing it's a loose wire, a shorted circuit of some kind or something similar... but thought I'd ask if anyone had any ideas or if this is a common issue with a common fix. I've not experienced this w/either my sold '96 XJ or my current '01 XJ. Any advice is appreciated.

 

[Red91Laredo]

I'd check the connector for the blower motor first. I've heard of those melting before. Not sure what the cause of that would be though. Could be the motor itself on its way out I suppose. The fact that it comes & goes with the heat on/off so quickly makes me suspect it's something in or around the blower motor.

Were any other electrical accessories on when this happened? Does the Jeep have anything like aftermarket driving lights or H4 conversion headlights installed?

 

[winterbeater]

Could easily be in the blower motor wiring. Disconnect the plugs and clean all of the contacts or at lease plug and unplug them several times to polish the contacts. Bad connections can quickly generate heat and start melting plugs and wiring. Might be a good idea to check the fuses to make sure they haven't been supersized (replaced with higher amperage), which could mke stuff melt.

 

[jeepguy72987]

blower motor resistor. plug maybe melted. happens alot to silverados

 

[Redsnake]

Thanks all... Yes, there are some aftermarket driving lights mounted up front. Hopefully it's the blower motor resister or the plugin. I'll check the suggestions mentioned above over the Holiday weekend.

 

[cruiser54]

With the engine running, poke your noggin under the passenger side dash and move the mode selector on the heater control. At one point, a door will open up behind a grid and you can see into the blower motor area. It's possible some rodents took the carpet padding and made a nest in the motor area. It then travels down to the blower resistor and smolders.

 

[MoFo]

I bet Cruiser nailed it. If you have the fan speed set to anything less than full fan speed, the fan speed switch will stack one or more of the resistor elements in series with the fan. These resistor elements get hot, and since they are on the inside of the blower housing, they are cooled by the air flow inside the housing. If that airflow has been cut off by a rodent nest, the nest material gets hot. If you run the fan on high, no surrent flows through the resistor, it goes straight to the fan. Try running the fan on high for a while. If the smell goes asay, you have isolated the cause to debris on the reisistor. No need to unplug anything, or clean connectors to diaganse the problem.

 

[cruiser54]

I bet Cruiser nailed it. If you have the fan speed set to anything less than full fan speed, the fan speed switch will stack one or more of the resistor elements in series with the fan. These resistor elements get hot, and since they are on the inside of the blower housing, they are cooled by the air flow inside the housing. If that airflow has been cut off by a rodent nest, the nest material gets hot. If you run the fan on high, no surrent flows through the resistor, it goes straight to the fan. Try running the fan on high for a while. If the smell goes asay, you have isolated the cause to debris on the reisistor. No need to unplug anything, or clean connectors to diaganse the problem.

And, it's easy enough to see if you wanna take about 19 seconds out of your day to take a peek.

 

[Redsnake]

Followup:

I pulled the lower blower plastic cover off to show the blower resistor and the vacuum actuated part (not sure what it's called). I pulled off the old resistor and noticed the plastic housing it mounts to was melted at the top where it had been apparently making contact w/the resistor. Now, when it was like this the fan would run on Hi, but none of the other levels. There was also the burning smell.

So after replacing the resistor w/a new one, hooking everything back up, the blower worked for a second or two on all speeds, then there was a puff of white smoke that come out f/under the blower housing and now nothing works. I can't get the blower to run at all. I checked all the fuses in the jeep and they appear good so i'm at a loss. Any suggestions would be appreciated. I also looked for the plug-in for the blower motor and couldn't find it. Is it buried under the dash somewhere? I've attached a couple pics to show the what it all looks like. There is one plug in that was exposed and I'm not sure what it's supposed to be plugged into if anything? Perhaps it's a plug for something not used on this rig but I don't know.

Here are some pics..

Overall under the passenger side dash



The new resistor



Here's the plug I mentioned that wasn't being used..??

 

[GrimmJeeper]

pull the heater control panel in the dash, the connection to the blower speed selector likes to melt too and short all kinds of crap out.

 

[Redsnake]

pull the heater control panel in the dash, the connection to the blower speed selector likes to melt too and short all kinds of crap out.

Thanks for the tip. Pulled the control panel in the dash and the connection to the blower speed selector switch... It appears the switch is melted and the plug-in connector has a melted spot. Could this be the issue (perhaps a dumb question) and can this switch be tested somehow? I ordered a new switch that of course had to be shipped and should be here tomorrow... but it'd be nice to confirm this switch is bad somehow. Is that 'greenish' stuff some sort of grease the factory applies or is that just a lot of corrosion?

Here are some pics...





And the plug-in ... looks like one of the wires/housing is melted. Notice that it's a 5-prong switch but apparently one of those prongs doesn't have a corresponding wire that it should be connected too. Is something missing or is that normal?

 

[GrimmJeeper]

yep, melted. head to the junkyard and hack off a new connector and splice it in. they corrode with the green crap and then the resistance goes up and melts the whole deal. like I said it's pretty common. I had to replace it twice in my 95 and know 2 or 3 other buddies that we've had to replace both the connector and the switch itself. it was bad enough that I started just grabbing one every time I hit the junkyard and keeping one on the shelf.

at least you caught it now before it made fire

 

[Redsnake]

Bumping this thread back up...

Since this is my older sons jeep if he's not that motivated to work on his rig then neither am I.

So after realizing the connector for the Hi/Low switch was bad it took him months to finally hit up the 2 Jeep junkyards we have around here. He found 1 wiring harness/plug-in out of dozens of wrecked rigs he checked. This one we wired in and still no power to the actual blower motor. So after replacing the Resistor, the Switch and the Harness it's still not working.

Is it possible that we friend the 'new' resistor as it was replaced first before the switch and connecting wiring harness were replaced. I'm at a loss as to what to check next. I didn't see any exposed wiring when pulling the HVAC control panel off or see anything out of whack. Any suggestions or tips would be appreciated.

 

[md21722]

Sometimes what happens is the fan gets old and draws more current and starts melting things upstream of it in the wiring. Corrosion or bad fitting connectors are also problems. Your best bet is to trace all the wiring, switch, and resistor, and check current draw on the motor.

 

[Redsnake]

Your best bet is to trace all the wiring, switch, and resistor, and check current draw on the motor.

Thanks for the info. How does one check current draw on a motor ... especially if there is no power/voltage going to the motor?

For some reason electrical is my most challenging part of working on autos. :|