Fan blower only works on high speed....

The thermistor is the resistor like thing below the coil? And that's a high temp cut-off or something? It seemed as though, from my voltage test, that current was passing through this assembly ok regardless of where the switch was set, but does that mean the fan should be powered in these situations?

So you would recommend replacing this resistor assembly first, as it looks suspect?

murraymcleod wrote:
OK, I found the resistor assembly and removed it for inspection. It looked pretty much like the picture posted by AstroRig except the left coil resistor seemed to be tarnished...

Does the fact that the left coil looked corroded indicate it's necessarily bad?

That does not look like corrosion or tarnishing to me. That looks like thermal damage and I would almost bet that the adjacent thermistor blew and spewed hot gases on the coil giving it that appearance. Does it look like molten metal has leaked out the ends of the thermistor?

Per the Parts Catalog for your motorhome, 2003 SUNOVA ICD27C D-SERIES

The part number for the entire resistor assembly is 103930-01-710 and it is available here: Winnebago Outdoor, Victory 035-0006 RV/Auto Fan Speed Resistor (Fused)

The other alternative is to solder another thermistor inline, leaving the old one in place, as is done at the link below (on a Monaco) but life is too short for that for me.

http://www.irv2.com/forums/f115/dash-a-c-blower-speed-switch-117793.html

Maybe this?

deleted by poster!

OK, I found the resistor assembly and removed it for inspection. It looked pretty much like the picture posted by AstroRig except the left coil resistor seemed to be tarnished and it looked like some of the adjacent coils were touching (which may defeat the purpose of the coils?). (I'd post a picture except I can't figure out how to do that!)

All the contacts on the assembly had continuity as tested with a test lamp from the 12v contact, and I couldn't notice any decrease in 12 volt light intensity between the various resistor circuits.

Does the fact that the left coil looked corroded indicate it's necessarily bad?

I then removed the fan switch on the dash and tried to test it with an ohm meter, but found no rhyme or reason to which contracts provided continuity in various switch positions, so maybe it is bad? It's an INDAK off/low/med/hi switch with no part number, but a patent number of 3 159.722. I'll try to figure out how to post pics of these things....maybe below...?

For anyone interested, you download the WH chassis/engine service manual
from the Navistar web site HERE

murraymcleod wrote:
OK, but why are resistors in the airflow pathway? Are they acting like thermistors for temp regulation? Or is this too keep the resistors cool so they don't fail? But this wouldn't seem to work if he heater was blowing hot air!

It is to cool them. Even heater hot air will drop the temp a lot. They get hot enough to boil spit.

murraymcleod wrote:
Thanks...but what is the purpose of these resistors? Can they simply be bypassed/removed?Do you know if they are on a circuit board or just "naked?"

On my WH chassis they were mounted on the ac/heater plenum on the passenger side behind the hood.

Any auto parts store should have a replacement. Just have the old one in hand when you go. This is a very very common failure.