The Stovebolt Site Forums The Shop Area The Electrical Bay Resistor and the heater line

I have been watching the thread on wiring heaters and have a new wrinkle. I have the single wire hook up. It goes to the heater switch that goes to my fuse block. I put a resistor from Jim Carter in the line between the heater and the switch to drop the voltage from 12 to 6. I am assuming the heater is 6 volt.

This set up causes the resistor to heat up to finger blistering intensity. I didn't think you could install these things backwards to begin with plus there are no markings on the posts.

Any comments on what I am doing wrong?

MikeE,

A resistor isn't picky, any which way will work!


Stuart

I don't think you are doing anything wrong. There is no polarity to a resistor. Each end is the same. If you have hooked up the resistor in series with the heater motor(as it should be), you are splitting the 12 volts between the motor and the resistor with the same current running through both. 6 volts gets dropped across the resistor and 6 volts gets dropped across the motor. Each will dissipate the same amount of power. That's why the resistor is getting so hot. About the only mistake you could make is to hook the resistor up in parallel. This would mean that one side of the resistor would be hooked to 12 volts, the other side to ground. Now you would be dropping the full 12 volts across the resistor which would really make it get hot, and your heater motor would also still be getting 12 volts.

MikeE,

I did a search of this site on the subject of resistors in the heater blower motor circuit.

They all indicate the resistor gets VERY hot, but just like the Timex, keeps on a tickin!

It must be the Heavy Duty rated resistor, the one sold for heaters, not gauges! One member mentioned mounting his in the heater housing so the blower would help cool it, plus it was out of the way of incidental contact with a stray hand or child's finger.

I think the motor draws 6 amps + at 6 volts, so you have quite a bit of energy to dissipate in the form of heat.

While you're poking around under the dash, keep a look out for the OTHER wire...it's in there somewhere!


Stuart

The other thread about heaters is for a 55-2nd truck and I have no first-hand knowledge about that heater or wiring (the information in that thread is very hard to follow).

The Advance-Design heaters had one wire - the hot wire from the switch to the heater blower/motor. There is one power lead into the switch and one lead from the switch to the heater/blower motor. The switch produces different speeds by adjusting the output current that goes to the heater blower/motor. I think the switch can get quite hot because it "dumps" unused current (there is probably a better explanation of why it gets hot).

There is no ground lead on the original switch (into the switch or out of the switch) - the switch itself does not need to be grounded. And, there is no more than than one lead from the original switch to the heater motor/blower.

If you have two leads at your A-D heater/blower motor, either you have a replacement heater motor or someone (over the past 50 years) added a ground wire because they had a bad ground connection between the heater housing and the body.

I would put the 12v-to-6v resistor (the big ceramic-style resistor) in the power lead that goes into the switch (this resistor will get hot no matter where you put it). The motor-control switch also gets hot but I bet it gets less hot if the resistor is before the blower/motor switch)

Here is a diagram of a '51 passenger car - the wiring for the car switch and motor is very similar to the wiring for a truck.

http://chevy.tocmp.com/1951accessories/51aim40.htm

One...it needs to be on the other side of the switch for proper speed control.

Two...make sure you have the heavy resisitor.

Three...quit messing around with fire hazards and put a 12v motor in the heater.

I'm going down in flames, help.......the OTHER wire is the ground. You probably wont see it, it might be attached to the motor case/frame or more than likely it will exit the motor with the wire that goes to your switch or power.

These motors have to have a + and a -, just like the battery needs two cables to make a complete circuit.

This ground wire does not come out where it needs to be switched or fussed with, but it is there. GM didn't intend this motor to ground through some shiny place on its case..they painted the darn thing with nice black paint. That is why they put another wire to the motor...so it would make a complete circuit through the windings, so it would work!

Stuart

The resistor gets very hot indeed. I made myself a resistor out of an old space heater. I cut the wire to the right length for a resistance of 1 Ohm. The power dissipation is about 50 Watt when the generator/alternator is running. It works just fine, and it has a lot more safety margin than the resistor that they sell you.

Johannes

to add another question to this thread. when i have the resistor hooked up the motor turns very slow, without it it turns like it is going to fly away and is very noisy. is anything that i can do to get a happy medium? i only have a off and on switch hooked to it. ron

Allen,

You would want a rheostat, a variable resistor. With this device, you could run the heater motor at whatever speed you desire. I know my motor, running at full speed would scare the crap out of anyone. The squirrel cage blower wheel and the worn out motor bushings make for some strange gyrations.

Stuart

Grainger sells a 12v motor that's almost a direct drop in. Much better than farting around with resistors the size of a cigar.

MikeE: If you could stand another opinion; I'm with GMCpanel, get the correct motor. What you're attempting to do can be done, but usually it ends up being a little "backyard engineering-ish." Essentially what you are doing is putting a fixed resistance (the line dropping resistor) in series with a variable load (the motor & associated circuitry). If both had fixed resistances, then the line voltage to the second resistance (motor) would always be 6 volts as intended. However, the motor & its circuitry actually vary in resistance relative to speed & control settings. The result is that the voltage the motor circuitry sees varies depending on the resistance/current drop across the motor & the resistor. In short, the fixed resistance of the in-line resistor can not change relative to the change in the motor resistance and maintain a constant 6 volt drop. Basically that is what voltage regulators do, they maintain a specified output voltage if if the load changes.

Once again I am amazed and overwhelmed with the amount of knowledge out there on the airways, thanks much.

After my experience with the resistor getting hot, I have decided to dump it, too dangerous. Did I mention it burned through my plastic oil pressure line and spewed oil around for a few seconds. I really like it when that happens.

I am not really sure that I don't have a 12 volt heater. It is from a later model and takes up most of the space under the passenger side of the dash. It has manual levers to direct the air flow and a big round Chevy logo. There is one wire going to the switch. Sounds like what was described earlier.

I am just going to run it for the time being and come up with a 12 volt motor. Probably look at the Granger option.

Again, thanks for the help and education.

That sounds like an AD heater. Does it look like the ones on the top of this page: http://www.oldgmctrucks.com/page0151.html ?

It looks mostly like the one at the top left, but is turned 90 degrees to the left.

Hmm...if you post a pic, it would help.

I will try to do this, but don't hold yur breath. This will be new plowed ground for me.