I think I can make (repair) my heater motor resistor by buying a couple of resistors and soldering them on in place of the original coils.

Voltage = Current / Resistance

I know the voltage = 12V, so to determine the resistance I'll need to know the current that the blower motor needs to run on for the LOW and HIGH speed settings.

Anyone out there with an electrical background that would know what current draws I should assume for this undertaking?

Any help or direction is much appreciated.

 

Is your existing resistor good or bad? What car?

I took current measurements on my 72 a while back; cannot find the data right now. I THOUGHT I posted it.... I kinda remember 4A - low, 6A - med lo, 8A - med hi. I did not do HI, as it uses the relay under the hood and I did not feel like getting dirty then.
Do you have a DC ammeter? Most go up to 10A, about the normal continuous draw of a good fan on high.
If the resistor is good, you can use the ammeter clipped in in place of the fan fuse for the low settings (how I did it).
If your resistor is good, you can also measure the voltage drop across each one at different speeds also.

Once you find the drops and currents for each speed, you can calculate the approximate resistance and power rating needed for your resistors.
R = V / I
P = V2 / R = VI = I2R

Of course the measurements need to be all done with engine at idle.
These will be ballpark values and may vary due to inductance of the motor.
I have not done this but I might later...

Take your specs and go to digikey.com, search resistor, plug in your requirements, and see what the physical sizes are.
Those coils of wire the factory used were very compact but required airflow for cooling.

 

I'm assuming that this is for an unusual '50's model or something similar, because anything from the '60's or '70's should have an exact replacement in the corner auto parts store (or something you could easily modify).

The easiest thing to do would be to find another one and measure the resistance.

Failing that, you can get a big-a$$ rheostat (like 10A or something) and run the motor through it, decide what speeds "sound good," then measure those resistances and use them.

Finally, you should run it through an ammeter on full, check the current draw, add a bit for a safety margin, and use that as your value for the current capacity of the resistors (which may get hot after long use and need to be cooled).

You will probably find that you need heavy duty porcelain-encased resistors to handle the current comfortably (an ignition ballast resistor or 2 from a MoPar might work).

Expected resistances would probably be in the 4-20 Ω range, as a wild guess, and 10-20A current.

- Eric

 

Sorry - it's a 55 S-88 - only low and high settings to begin with.

The resistor is bad - if I jump the terminals (i.e. bypass the resistor), it works - plug in the resistor and no go. The resistors aren't available so I was going to try to repair in lieu of having only "High" all the time.

Because of where it mounts and the length of the pigtails on the car, I really can only use the correct resistor.

I have a Fluke multi-meter - pretty sure that has an ammeter in it?

 

Keep it original if possible. Leave Mickey to Disney. < ( I understand the test, that's good research. I mean as far as replacing origional with other parts.)
Just for info: HEATER RESISTER GM Group# 8.852 Part# 3131533 is for ALL 1954-56
this may help you find one. Somebody has it on a shelf. I'll still be looking.
Here's a link for Part Numbers and list of Parts on hand from Collectors Auto Supply for your 1955:
http://www.collectorsautosupply.com/query.asp
They have your part listed and show no inventory, but they may be able to help finding one.
I found 2 new Resisters for my 63 Olds 88 on e-bay ( he even lived nearby)

 

I agree - and thanks for the info - to clarify, my "mickeying" was only to temporarily get the fan blowing while I search.

Great info and link too

 

Flukes are pretty good - ignore the name...

Turn to the 10A setting, put the probes in the the sockets for the 10A setting, then jump across the bad resistor and record the HI speed current.
Of course there is a risk here of blowing the Fluke's internal fuse, as the motor may draw over 10A on startup.
An old analog meter is a bit more forgiving.

When i get home, i can measure the resistor voltage drop on mine at low, to at least give you a starting point. (Besides, i am curious now...)

 

Alright hold on to your butts:

Since you have a good multi-meter, be sure to verify that the resistor is truly shot and its not just dirt/corrosion on contacts and plugs. I would measure the voltage that the motor is seeing when power is applied through the resistor. If the resistor is totally shot/burnt up you will get 0 volts. But its also possible corrosion or oxidation just slowly increased the circuits resistance to the point that there is not enough current for the motor to start. If thats the case there will be some voltage, but probably not much. Cheapo brass bristled gun barrel cleaning brushes (sporting goods section at walmart) work wonders on electrical contacts and plugs followed by q-tips and isopropyl alcohol.

If you just want a new one, be sure to measure the current by completely severing the circuit and then insert the meter inline to complete the circuit. Connecting the meter in parallel with anything is asking for inaccuracies.

I would guess you are probably gonna see about 6Amps flowing when set to full blast high. The 12-14 guage wire used in cars gets hot with anything more than that and ~1ohm per leg of copper wire is about right too.

For a resistor you would probably need a 2-3 Ohm resistor. That would allow a flow the current right around ~3 amps. And though your motor speed will not be perfectly linear to the drop in current its gonna be roughly spinning at half the speed.

**IMPORTANT** Electronics resistors are pretty much all rated at how many Watts they can handle. The coil of resistance wire used originally is much more resilient wattage wise than your average circuit board resistor. You will not be able to use small through hole board resistors. They are usually rated for AT MOST .5 Watts (one half). A rough way to calculate the resistors power dissipation is Watts = Volts * Amps so since 3 Amps will flow through the circuit and the resistor will be seeing 12 volts. That would mean you need a resistor rated for more than 36 watts. If you are using a single resistor the bare minimum would be a 40W resistor (don't look to be very common), but 50W (or more) would be safer anyways. So a single 2Ohm 50W resistor. They are around, but still not terribly common and as a result a bit pricey (for a resistor).

Now since more common ratings are always cheaper. A nifty trick of electronics is you can spread the Power dissipation out across multiple resistors by connecting them in parallel. You just have to be sure that the resistance is maintained at 2 ohms. Its also easier if they are all the same value. You can google how to calculate parallel resistance, but I'll tell you if you are going to use 4 resistors they all need to be 8ohms that way when connected in parallel the cumulative resistance the circuit will see is going to be 2ohms. Since the voltage and current doesn't change, that 36 watts is going to be spread out across all four of the resistors and 4 resistors generally have more surface area which makes them more efficient at dissipating that 36W of heat to boot. So you can probably get away with 10W resistors. (10W x 4 resistors sharing the heat load = combined 40W rating). And just like that you went from probably a $10 resistor to a (4 x $0.50 =) $2 resistor.

LOL, Any questions?

 

Wow...need to digest-thanks...i'll post when i get to the next step..

 

So...I think I solved my woes (see pics)

Bought a 2 speed fan resistor for a similar era cadillac on ebay for $10

cut a new fiberboard for mine using the caddy one and used the resistors off of that one...my dad always says "If you can't fix it with 'Pop' rivets, then you can't fix it"

I'll post if (when...optimism) it works - tests good with ohm meter.

Thanks to all for your input and guidance...

 

Sounds (and looks) like a good fix!

- Eric

 

...and the verdict is in

Works like a charm - installed under the cover of 20 degree darkness last night and I was soon being warmed by two - yes two - distinct fan speeds.

 

Great! I knew this would work better than a regular resistor.
Do you recall the Ohm reading when you checked them?

 

I want to say it was around 1.5 OHMS on "High" (i.e. one resistor) and close to 4 OHMS on the "Low" (through both resistors).

I have no idea if this is what the original was but I have to assume that for a similar era cadillac (the source of the new resistor I used for parts) with a 2 speed heater it's got to be in the same neighborhood.

Hope this helps & Merry Christmas to all