350 Ford Cab C Motorhome fan for heater an air won't work

I have a 1992 GM van in which the heater fan cuts out when going up a steep hill making the engine work hard. I was told this is a feature to reduce alternator load on the engine and it is triggered by low engine vacuum. This seems very strange but if true, it could mean that your fan problem could be due to a damaged vacuum line or fitting.

I just looked in my Haynes Manual and it's pretty much useless for wiring diagrams.

But remember, there are two fuse panels. One under dash, one under hood. Possible that the fuse you need to check isn't in the one you looked at. Fuse panel is usually labeled. Do you have the Owner's Manual? If so, check it.

Ford used to be notorious for the wiring harness just quitting. Power seat would simply stop working one day, and the fix was to consult the wiring diagram and run a bypass wire. The auto electric shop would not dig into the harness, just wire around the problem. But I'm remembering that as an 1980's thing and haven't heard about it recently.

Does your vehicle have a BCM, Body Control Module, the problem may lie there, possible bad ground somewhere you wouldn't expect it.

Bob

I would check for another fuse someplace. There should be one for the AC clutch also.

No power coming to switch that controls with resister block. Checked with multimeter. Tried different fuse. Held motor up grounded to battery and motor worked. This has baffled everyone. Replaced all the switches, old with new, wasn't the problem. Help!

Please describe how you tested the Blower Motor to consider it Good. They can get dirt in the brush holders. Then the brush springs can't keep them pushed against the commutator segments as the brushes wear. Sometimes if you put the motor in one position it'll run and it won't run in others. I've heard of removing the motor and tapping it on a hard surface in various directions to shake the dirt loose. It may run awhile, but the dirt's still there and will usually lock the brushes again.

Have you checked for voltage at the motor terminal and the resistor block? Tried a different fuse? Is the blower motor grounded somewhere? If it's a single terminal for the hot lead, the motor needs a ground somewhere. If the plenum it's mounted to is steel, probably OK, just snug the screws. If the plenum's plastic, run a ground wire from the motor shell to a known good ground. See if anything changes.

Thanks for the info, yes we replaced item you suggested. This is why it is so baffling.

Did you check to see if it runs with the fan speed set to High, or Max, whatever the top speed is?
When you said your motor is good, I take it you hot wired it and it ran. That's High. To run slower, the switch routes the power through a resistor pack that lowers the speed by dropping the voltage to the motor. If that burns out, it gets no power till you select High.

You see the connector side (left side in this photo). It's mounted on the HVAC air plenum somewhere, so the resistor side (right side in photo) will be inside where it's cooled by air flow. The pic may not be for E-Series, it's just one I found online, but the idea's the same.
EDIT: Actually, my lookup says it DOES fit, if you chassis is 1999. And probably many other years, since I first found it as 2005. At least find the resistor pack. The connector might be loose or damaged.
Look on the ducting, maybe behind the battery, but near the blower motor, passenger side. (Suggestions from WEB, I don't have the RV nearby or a manual to look in.) Knowing what the pack looks like should help you spot it.