I have had a repop switch on my 1927 Model T Roadster Pickup for well over 10 years now. Last week whilst driving down the road on magneto my engine started bucking and snorting, backfiring and then just quit. I was able to switch to battery and restart to drive home without problem.
Today I dug into the problem and in short order I found that my mag was making 2 VAC at idle and a whopping 4 VAC at high rpm. Switching over to mag would produce one anemic pop and then the engine would die.
I did a quick in car charge of the magneto using my arc welder (50 VDC delivered at about 250 amperes... be careful and do it right!). The mag then delivered 18 VAC/24 VAC (idle/fast rpm) as usual. Switch the engine over to mag and buck, snort dead.
I tested the mag wire for voltage and didn't find any, the switch must be bad. I once had a nut in my switch on my 1913 come loose and kill the mag so I know that the engine behavior is what you would expect with a bad switch.
Here is what the switch looks like when dropped out of the dash. There are two types: tab retained and screw retained. Unfortunately this one is tab retained. You have to bend the tabs open to remove the guts. You can only do this a couple of times before the tabs break off and you need a whole new assembly!
Terry, if you anneal those tabs each time you bend them, they will last much more longer. I don't think they will last forever with repeated bending, but it will help. Dave
You had a bad contact in your switch, the material around one of the points is burned out and the contact point has changed colour.
I think you need a rebuild kit.
I had the same problem with my 1926 touring a few year ago. A short in the switch and the magneto died. Tried to fix the switch but didn't arrived.
I bridge it with a +0+ switch since no problem.
I just put in a repop switch a couple weeks back.Works ok now.BUT IF I have any more switch trouble I am done with the factory style switches.Cost to much, limited amount of repairs due to design and etc.
That toggle switch is what I did on the TT for awhile and it worked great. Somethings did improve with time and switch design is 1 of them.
I noticed low battery voltage going to the magneto contact caused by tracing of material on a repro switch back, pin type. I used a cutting wheel on a dremmel tool to break the lines created by the contact wiping. Solved the voltage leak. unfortunately I didn't take an after picture.
Buy an original switch to replace yours that uses pins as posted above, and you won't have to worry about tabs any more!
A question: Would a non conductive lubricant, such as dielectric grease, help reduce the erosion of the contacts, formation of conductive trails, and reduce wear on the plastic plate? This grease will also reduce the corrosion that might occur on all of the contact points.
I actually thought about trying the dielectric grease but if you think about it,it would be a great free ride for the copper flakes to travel back and forth across the contacts.
With some lubrication, how much chance is there of copper flaking off? I sure understand the potential for copper flakes to travel with the grease. The grease being non conductive, there would have to be a continuous line up of flakes, contacting each other, between the terminals, to short out.
As I said, just thinking out loud.
I just did this exercise on my Racer switch - used all original internal parts except the back connection piece - purchased the "Best" quality one available AND I did lube with dielectric grease - turns as smooth a butter !
I agree with Larry's suggestion on eliminating the tab problem. If you have a tab type switch, keep an eye open at swap meets for a pin type. New back plates of both types are available. Both types also come in a good version (more expensive) and a lower quality version. The good one is a phenolic laminate material and the cheaper one is plastic.
BUT before you spend money on either one, you may be able to repair the switch without it. The original back plate is a thick cardboard type of material that may have become warped, causing some of the contacts to not contact. The cure may be as simple as rubbing the plate against sandpaper to flatten it.
The bad contact did cause some cavitation of the plastic but that is a separate problem. The copper tracking in the plastic from abrasion of the contact caused the major problem: demagnetizing the flywheel magnets.
Most folks want to avoid that occurrence, that is why I posted this.
I've rebuilt numerous switches, and all of them have been pin backs. Many times the layers have begun to separate, and I've had good luck using Crazy Glue to put them back to normal. I clamp each pin separately to the original thickness. It takes three operations to accomplish this, then I sand the entire thing as much as possible, as Steve has shown above, until it's relatively flat.
I don't believe any of the tab back parts will interchange with the pin backs, but don't know this for sure. Ben Martin would know.