Fan blower only works on high speed....

I had that (off/high speed only) problem with one of my Chevy cars. Dealer replaced the (fan motor) dash switch. $100.

murraymcleod wrote:
Thanks much....now I just need to find the replacement module somewhere!

You don't know necessarily know that it's resistor but it sure sounds like it might be.

In researching further, and looking at the manuals available on Winnebago's site, I found that their dash AC units are made or modified by Evans Tempcon.

This is their site, Evans Tempcon, Inc.

....and this is EXACTLY the blower motor resistor on my Winnebago.

RV212146 - Resistor

Thanks much....now I just need to find the replacement module somewhere!

The resistors cannot be bypassed, in conjunction with the blower motor switch, and blower motor relay, they regulate the amount of voltage going to the motor and thus control the motor speed. Per the Haynes manual for my Ford Expedition, which I successfully used to troubleshoot my F-53 based Winnebago blower issues,

"There are three resistors mounted on the blower motor resistor board to provide Low and Medium blower speeds. (Hi bypasses the resistors)."

(At least on an F-53 based Winnebago) The COIL type resisters mentioned are accessed from inside the engine compartment and are located against what would appear to be the firewall on the left (curb) side about midway up. The three resistors themselves are mounted so that they are inside the airflow of the AC/Heater airbox for cooling. They are on the fresh air/return side of the air box, before the heater core/evaporator, so the heat from the core does not affect them. There is also a thermal limiter mounted to the board as well. If the thermal limiter has been blown, the manual cautions against using a blower motor resistor without the limiter and using a limiter of a different rating.

Since they are mounted with the resisters to the inside of the airbox, all you will see on the outside is the bakelite-like board with a four pin connector plugged into it. Mine was rather crudely modified (I don't know if OEM or by the previous owner) and was sealed to the airbox with black RTV and secured with only two screws.

If the blower motor resistors are good, the next troubleshooting step is to check the blower motor switch and if that is good, the next step is to check the relay.

I have scanned the relevant pages from my Expedition manual, which has pictures, illustrations, and notes on continuity and resistance figures between the different pins on the resistor board, switch, and relay. I could post them, but I wonder if the OP would be better off trying to find a Workhorse manual or manual for a comparable year GM truck for troubleshooting instructions.

On edit: I found that the Winnebago dash air units are made or modified by Evans Tempcon. This is what the Evans Tempcon blower motor resistor board on my Winnebago looks like.

RV212146 - Resistor

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!

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?"

When the current goes through each resistor some of the power is consumed by the resistor so the fan runs slower. When all of the resistors are bypassed in the high position, all of the current goes to the blower.

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?"

x2 on the resistor. Older ones had a resistor type coil. I have repaired some newer styles that melted the solder strip connectors.

There is a resistor block that contains two resistors immersed in the air flow that are in circuit for medium and low fan selection. This resistor failure is very common and can typically be changed out by removing same in the passenger side foot well.