Farmall M Dimmer Resistor

Farmall65 · May 2, 2016

If i am not getting any power with a test
light when I touch it to the headlight
resistor spring, does that mean that I need
a new one?

Janicholson · May 2, 2016

Assuming the switch is turned to D, you should. If both ends of the resistor have no power, the switch is bad. Also assumes there is power to the switch. a 2.5 ohm 80 watt power resistor will replace it. Jim

gene bender · May 3, 2016

Remove it and clean the ends just finished a Cs system to get lites working as it was changed to 12v and owner wanted lites working thats all it took remember its 60 yrs old and things can get rusty ect. I havent seen a bad sw an all the yrs been playing with them restoring ect. I use a meter with pos lead going to ground at the battery because thats where the ground is sw also needs to have a good ground even the case. Not uncommon to have the sw case not grounded.

John T · May 3, 2016

With the lights on DIM, there should be with respect to frame ground, full battery voltage on the input side but less voltage on the output TO LIGHTS side. DUH that's why they burn dimmer, you drop voltage across the series to lights resistor. Is there battery voltage TO the switch on its BAT INPUT terminal??? If so but NOT on the dimmer resistors either side, the switch must be bad. If theres voltage on the resistors input when switch is on DIM but none on output to lights, resistor must be open. Get your test lamp out with one lead to frame ground and go to work!!!!!!!!!!!!!!

Bob or Jim can correct me if I'm wrong, but I believe the little small wire flat wound resistor is the light dimmer while the bigger coiled wire is the gennys field current control resistor.

John T

Bob M · May 3, 2016

Hey John T - You got the resistors reversed! The flat one wound with about 15 turns of thin wire is the generator field resistor; the open, round one - 8 or so turns of heavier wire - is the light dimmer.

Note this is for a 4-position (L-H-D-B) switch. A 3-position (O-D-B) switch omits the field resistor.

Punch below for a backside view of a 4-position switch.
4 position switch

John T · May 3, 2016

Thanks Bob, I knew you knew this and would set me straight, hey I had a 50 50 chance and blew it lol

John T

Bob M · May 3, 2016

Well John T - for a "green man" you actually do pretty good with the red stuff!

your Mech Eng'r friend, Bob M

john *.?-!.* cub owner · May 3, 2016

If you do not have lights on B or D either one, check the fuse on the back of the switch, and maybe clean the clips.

Farmall M Dimmer Resistor

Farmall65

Member

Janicholson

Well-known Member

gene bender

Well-known Member

John T

Well-known Member

Bob M

Well-known Member

John T

Well-known Member

Bob M

Well-known Member

john .?-!. cub owner

Well-known Member

Similar threads