The heater fan is only blowing one speed. (I think it is high speed, but am not 100% certain about that.) Can the blower resistor be tested to make certain it is bad? Is so, what is the test procedure?

The vehicle is a 1984 full size truck.

in my '85 FSM, it really doesn't go into anything relating to testing the resistor other than inspecting it for burnt, or touching coils. there are, however, a couple of flow charts to diagnose the cause for a malfunctioning heater/ac fan. if you want to pm me your e-mail addy, i'll scan and send those charts in a PDF format.

Yes you can do a ohm test on each of the speed resisters, but its just easier to take it out and look at it or measure it to tell if its bad. It should be apparent if it is bad.

But, I'm going to say as old as the truck is, its no doubt the resister, or/and the connecter itself as well may be fried . Get it apart and look at it, its probably already been changed before, maybe even a couple of times.

Typically if high only its the resistor.But they did have lots of problems with the switchs those years.Rocky

once again, my old mind forgot about those crappy switches in those years. hey rocky, we need to hook up so you can refresh my memory ! i'm really getting bad these days !

moparx dont feel bad days i forget my name it seems.Just remember the switchs because I changed tons at work and on my own.The resistors for the most part in older cars lasted forever.But i have seen them go bad.pretty easy to test just use the old Ohm meter.Rocky

Like you stated, if high only, most of the time its the resister.

But it could be the switch as well. The resister takes most of the abuse though. But the motor , resister , switch and its connecters all work together and can be effected.

If the motor starts to draw to much current it can overload both the switch and the resister. The weakest link goes first. Typically the resister takes most of the abuse. A lot of times when it goes, it can take out the connecter as well from the heat that occurs, but not always.

If you lose a resister twice in short order, and maybe a switch as well, start looking at the motor. Its next, but for the OP, I'm betting the resister, hope he replies back to what he finds.

UPDATE:

I removed the resistor and examined the wiring. There are no obvious problems. The wiring harness at the blower resistor connection looks good.

There are 4 positions on the fan switch. With the blower resistor removed, the fan will still run on high speed. There are 3 wires that go from the fan switch to the blower resistor. With the switch on low speed, there is 9.6 volts in one wire. The next faster speed has 9.6 volts on 2 wires. On the 3rd fastest position, there is 9.6 volts on all three wires. Is the 9.6 volts the problem?

On the resistor, I used an ohm meter to test between the various terminals where the wires connect to the resistor. There was virtually no resistance on any of the circuits. Several of the circuits had 0 ohms. A couple read less than 1 ohm. In looking at the resistor, I don't understand how this outcome can occur. It seems like it should either have some resistance or else be an open circuit if the resistor is burned out. Maybe I' do not understand how the resistor functions or I'm not testing it correctly?

Any advice?

The batter charger was on the battery when I took the voltage readings at the blower resistor. There was 14.2 volts measured at the battery.

You have verified you are getting power at all switch positions.
You have found no resistance at the resistor.
The fan is running on high at all switch positions.
As multiple other folks have said, everything points to a bad resistor.

Edit, You are Correct Stumpy, I gave info on Wrong year.

Sport440, it's an 84 not 94.

One reason for trying to make certain it is the resistor is that I have NOT found a new resistor for this truck yet. Anywhere at any price. The resistor for a 1985 is readily available and cheap.


If I'm reading it correctly, the wiring diagram indicates the current flows from the fuse panel to the switch, then to the resistor and on to the motor. I'm NOT very confident that this diagram is correct. Some of the things I see on the truck do NOT match the wiring diagram. Perhaps the wiring diagram is for trucks without air conditioning. Apparently there were some significant differences in the HVAC systems of air conditioned and non air conditioned trucks.

Just read supposedlt 87 Ram from NAPA can be used/I know I replaced one in my 83 Ram years ago.Do a google search for the year and resistor.From what i remember they used pretty much the same things for years. Rocky

Quick I did post the wrong year info 94, vs your 84. You are correct it is a positive fed from switch to motor through resister. Those years had some Very complicated wiring set ups including being fed through the AC switch as well. Some just by passed the the trucks wire diagram and rewired their own setup in using the trucks fan switch to operate the blower. I know, not what you want to hear. But in can be fixed the right way as well. But checking a diagram out there appears to be a lot of possible failure spots. For a 84 truck, I would rewire it to make it as easy as I could.

the wiring schematics are very similar between at least 83-87, and there are minute differences between ac & non ac diagrams, but as mentioned, these are very simple systems to work on. [compared to today's vehicles] offer still stands on the diagnostic flow charts i have.