Case of the smoking heater

:helpme:
It started on my way home with the fan only running on the highest setting. A hour later I smelled something burning and a bit of smoke coming from the dash area. I immediately turned the heater off and continued home with no negative effects except a cold cabin.
Today I pulled the dash and rounded up the usual suspects. Inspected the wiring for melted insulation. Found nothing. The switches were next on the list. They are as perfect as when I installed them in 2013. I tested all the connections at the resistor end and they all had good voltage at the proper terminals. Lastly, I did the sniff test. There was no giveaway odors there. My only clue is the one speed (high speed) only fan.
How would solve this case? Toss the 18 year old fan for a new/er unit or put every thing back together and let her smoke herself into submission? :flame:
:repair:

almost certainly the blower motor resistor and it's plug.
under the dash in the heater case on the passenger side.

The thermal fuse in the speed resistors likely burned out leaving you with high only. That is what should have happened. It almost sounds like yours did not fail in the correct mode. Sometimes the resistor elements fail in a manner where they touch each other. They are almost like toaster elements. This resistor part is held into the heater (hvac) box with two screws. Right under the dash on the passenger side. Pull it out and take a look.

winterbeater said:

The thermal fuse in the speed resistors likely burned out leaving you with high only. That is what should have happened. It almost sounds like yours did not fail in the correct mode. Sometimes the resistor elements fail in a manner where they touch each other. They are almost like toaster elements. This resistor part is held into the heater (hvac) box with two screws. Right under the dash on the passenger side. Pull it out and take a look.

Click to expand...

The resistors look good but I did not test them. Is there a test for them?

I took the radio out and went for a ride today,...well ran an errand. After about 20 minutes the yellow and brown wire that get its power from the heater control switch got hot at the blower motor switch. That is the wire that go directly across the resistors to power the fan on high speed. It do not appear the switch is bad neither does it show signs of being overheated. Both the switch terminals and the harness connectors need a good cleaning followed by an attempt to tighten the connectors. That should breathe new life into the that part of the blower.

With the resistor pack out of the air box I reached into it and turned the motor via the fan. The unit is bad,...it is hard to turn. That explains why it would not work on the low settings where the amperage is lower. However, on the high speed mode, the draw of a hard to turn motor seem too much for connectors to handle. Now, time for a low cost high quality replacement blower motor. :eyes:

Is there a test procedure for the resistor block? I cannot find one any place.

I found a write up on installing relays for the blower fan. It may be a worth while exercise if it reduce he load on the switch and connectors.

With it out, you just check across the terminals with ohmeter on ohms resistance. You are just checking for continuity, just like touching the ohmeter probes together. Normally, the little thermal fuse is blown. If the resistors (look like toaster coils) are bad, you will see the break. The thermal fuse part can be replaced if you can solder.

winterbeater said:

With it out, you just check across the terminals with ohmeter on ohms resistance. You are just checking for continuity, just like touching the ohmeter probes together. Normally, the little thermal fuse is blown. If the resistors (look like toaster coils) are bad, you will see the break. The thermal fuse part can be replaced if you can solder.

Click to expand...

Than you Winterbeater.

Visually, everything look good,..no broken terminals or coils that I can see. I will put the OHM meter across the coils/terminals to verify a connection do exist. That little round thing that look like a diode I think is the fuse you mentioned. I never would have thought a fuse would be in that part of the circuit.

winterbeater said:

With it out, you just check across the terminals with ohmeter on ohms resistance. You are just checking for continuity, just like touching the ohmeter probes together. Normally, the little thermal fuse is blown. If the resistors (look like toaster coils) are bad, you will see the break. The thermal fuse part can be replaced if you can solder.

Click to expand...

Well, I finally got around to doing some work on the XJ. It turns out that nothing was wrong with the blower motor. I removed the motor a few weeks ago to look for the source of a water leak that showed up on the passenger floor. When I replaced the motor the rubber seal flopped inside and touched the fan causing the friction I felt through the resistor opening. Because of this, the thermal fuse went bad resulting in the fan running on high speed only. This external friction on the fan blade must have put more load on the fan switch in the dash resulting in heated wires behind the radio. I have replaced the switch, cleaned and tightened the wires in the connector and soldered a fuse holder with a 15AMP fuse on the resistor pack to replace the thermal fuse. What is/was the value of the thermal fuse? Anyway, all is well now. It work like it is supposed to work i.e. all four speeds blows air just fine without the smell of burning wire/s.
Thank you Winterbeater for your tip. atriot: