So, I'm on a weekend tour from Detroit to Battle Creek. Drive all the way there and drive around Saturday and the mag -- as usual -- performs flawlessly. Sunday morning I start the car on battery as per usual and then switch to mag -- as usual -- and the car dies. Hmmm. Start it again on batt, switch to mag, dies. Mag is dead! Finish the tour on battery.
This morning I do the usual checks: take off the contact on the hogshead, no problems there, clean it, replace it. Check the wire from that contact to the one on the dash under the hood; nice and solid. Starts on batt, dies on mag.
Take off the switch cover and find one of the screws that holds the switch onto the coil box came off and is lodged between the switch housing and the mag contact! It's shorting out! Well, that's gotta be it, right? Put the screw back in. Starts on batt dies on mag.
Am I missing something? Can a magneto simply die overnight like that?
Any advice would be appreciated.
Perhaps the screw provided a path from your battery to the mag coils, which demagnetized your magnets.
Now that the switch is fixed, try an in-car magnet recharge and see if your magneto works again.
Before you do anything else, measure the voltage output from the magneto.
I call this the Regan - Patterson Memorial Magneto test after its inventors.
To check the magneto voltage one connects a #1156 automotive bulb between the magneto post and ground. This provides a load so the magneto reading is known to be stable.
An analog (NOT digital) AC voltmeter is then connected to the same points. With the engine running at idle you should see maybe 6 volts at idle. With the throttle advanced to high engine speed a good magneto will read 20 - 30 volts. The light bulb will burn out rapidly if this is sustained.
More info here:
Royce why do you need the bulb if you have an analog AC meter? Won't the meter show the steady reading?
Description of why the bulb is used:
Thanks, I guess the bulb substitutes for the coil when you just run off the mag post.
Memorial test? Which one died?
I had a similar problem. I went thru the coil box, the coils, the mag post and the mag wire. Everything was fine. I tried using a voltmeter and could not get a reading. Since I checked everything else, I figured it must be the magneto. I pulled the engine and removed the coil. After unsoldering the copper to ground connection on the last winding, I checked continuity between the button and the frame. Sure enough, a ground. After some sleuthing I discovered the coil was grounding at the solder button. The screw holding the washer was grounding out on the rivet that held the block. Can't explain why it happened all at once when the coil had been in service for years. Unfortunately, I discovered the problem after I removed all the coils, so I ended up rebuilding the entire assembly.
RV Anderson sells new blocks that come with a rivet, screw, button and insulator for little money.
I read all of the posts - Including Mark Strange's link. Why not a digital voltmeter?
quote from above thread:
"Best to use an analog meter when testing the mag unless you're using a high-end digital meter. Most of the digital meters I've seen are subject to RF interference from the ignition and the reading will be all over the place."
Sorry but the problem with digital meters being interfered with by the RFI (Radio Frequency Interference) generated by the ignition system is a problem that expensive meters also have. I have laboratory grade FLUKE meter as well as the $5 Harbor Freight units and they both can and will be erratic reading once the engine starts. Some people use them anyway and make decisions about what to do based upon that erroneous reading. You really don't need super super accuracy here you just need to know if the magneto is putting out over 20 volts at the higher RPM and something around 6-8 volts at idle. If yes then the car should not die when you switch to mag and you can look at the wiring and switch next for what is wrong. If the amount of AC voltage seems to be in a grey area then try to borrow or get a hold of a St. Louis Magneto tester and check it with that since then you will know for sure. Many of the club chapters have that equipment for club member use. Take advantage of chapter membership if a club exists nearby since members have tools that you can borrow to help diagnose what is happening.
As a graduate back yard mechanic a light bulb could tell you everything you want to know. With a bulb attached the level of brilliance will indicate the quantity of energy provided. From dim up to blowout is a good indicator it doesn't matter what the actual voltage is. If the bulb remains dim...problem.