Heater resistor/fan motor

[No Quarter]

I like to get the old stuff to work, I spend (probably too much) time on that before I buy new on non-wear parts. The fan motor was seized, took it apart, coals are fine, exercised it, lubricated. Now the fan motor works fine on the non-resistor circuit. But the resistor circuit can't turn it, or so it seems. I've measured 2ohm on the resistor, and it gets hot when I try to get fan motor to run, so current is there?

Resistor issue? Or fan motor issue?

[MarekH]

Well it depends where that current is going. If, after going through the resistor, it goes straight to earth, instead of through the fan, then that'd explain why the resistor is getting hot and the fan isn't turning.

If, for argument's sake, the resistor is a 2ohm resistor and the fan is a 2ohm fan, then you'd expect the voltage and current to halve across the fan when the resistor is fully in series with the fan.

Measure the voltage across both components or work out where this current is going.

EDIT:- similarly, if you find that the resistor has a low resistance but the fan side has a high resistance, then the same will also happen - the bulk of the voltage drop will be over the resistor and almost none over the fan. The symptoms will be the same.

Another approach would be to isolate the fan from battery ground. Everything should now be at 12v because there is no circuit and the components are at whatever voltage their only battery connection is at. If they are at a different voltage to battery voltage, then there has to be some path back to the battery negative (assuming your wiring has zero inline resistance).

kind regards
Marek

[No Quarter]

Both parts are out of the car, just connecting wires to battery posts, if that makes a difference

[MarekH]

That just means you can't blame the car's wiring loom for any fault. You still want to know about resistance to the chassis from every connection point and resistance of each component along the chain.

kind regards
Marek

[No Quarter]

Yes, will measure. Thanks

[No Quarter]

I get 4 ohm through the motor. So what does that tell me?

[MarekH]

For a steady current flow, you can apply Ohm's law, i.e.- V=IR

It means on full power with 12v across it, the 4ohm motor wants to draw a steady 3amps.
When the resistor is connected in series with the motor, the total resistance is 6ohms, so the motor ought to have 8v across it (with 4v across the resistor) and the current flowing would be 2amps.

What you have said is that your motor doesn't want to run when there is a voltage of just 8v across it, but does when it sees 12v.

kind regards
Marek

[No Quarter]

Marek, and others, while I appreciate the physics lesson, really, I'm nowhere nearer finding out which of the two is faulty, and if it's the motor, is it repairable?
Thanks

[MarekH]

Perhaps someone with a 1963 or 1964 car can tell you what they expect the resistances of the two components ought to be or what the voltages across the components will be when the fan is running. My car is a 1974 car so I'm not sure it'd be a good benchmark for you.

Measuring resistance values this low is notoriously difficult and often inaccurate. Multimeters tend to measure voltages and current more accurately.

kind regards
Marek

[Heuer]

The original resistor is rated at 2 ohm/50 watts.