magneto problems

   / magneto problems #1  

Eric_Phillips

Platinum Member
Joined
May 16, 2005
Messages
706
Location
Rochester, NY
Tractor
FarmTrac 270DTC
I have a '65 Gravely L8 with no spark. I tested the cutoff switch and when not pressed it has continuity to ground. I started tracing the short back into the magneto. From the points I disconnected the wire to the cutoff switch, the condenser and the wire to the coil. The terminal on the points has continuity to ground. There is no continuity from the condenser body to its wire lead. There is continuity between the insulated wire off the coil and ground, the coil ground wire was still connected to ground. There is no continuity between either the ground or positive wire of the coil and the spark plug connection on the coil. I did the testing with a digital multimeter. I either got no tone and no reading or it toned and gave a reading of 0 ohms. I am assuming the points should not have continuity to ground since this is the connection that gets shorted by the cutoff switch. I think the condenser is good. What about the coil? This thing has me baffled since 5 years ago it was running. It has sat in the barn since then because of a fuel system problem and I fixed that. The points look clean and in good shape so I can't figure out what could have gone wrong.
 
   / magneto problems #2  
Five years is a long time for tungsten contact points to sit idle, they probably have formed a coating of oxide on the surface, try filing them then recheck the gap.

Anther possibility is that the impulse drive has a stuck spring, are you able to hear the magneto give a sharp clicking sound when turning the engine over slowly.

If the points were closed during your test they would have shown as grounded.

 
   / magneto problems
  • Thread Starter
#3  
Yes the impulse drive sounds like it is working.

Are you saying the only time the terminal of the cutoff switch is not grounded is when the points are open?

The points looked clean but I can dress them a little to ensure they are clean.
 
   / magneto problems #4  
Yes the impulse drive sounds like it is working.

Are you saying the only time the terminal of the cutoff switch is not grounded is when the points are open?

The points looked clean but I can dress them a little to ensure they are clean.

Technically you do not require any external wires connected to the mag, the brass screw nut terminal top right hand side when grounded kills the mag by shorting out the condenser.

If dressing the points wont bring life to the magneto, I would first replace the condenser then if the magneto still refuses to make spark with the cap removed use a thin screw driver to probe the magnets to see if the screwdriver is attracted'

It's possible your magneto magnets have lost their magnetism and need to be recharged.

Here's a very well written article that explains the problem and the fix.

The Why and How of Recharging Magneto Magnets - Restoration - Gas Engine Magazine

mag.png
 
   / magneto problems #5  
I would suspect that condenser as the most likely culprit, that is after eliminating obvious things as shorts, rusted points broken wires etc.
 
   / magneto problems #6  
I would suspect that condenser as the most likely culprit, that is after eliminating obvious things as shorts, rusted points broken wires etc.

My money is on lost magnetism.
 
   / magneto problems #7  
My money is on lost magnetism.

4 sure a possibility but also the most costly component so eliminating all other causes first is a must IMO.

As an aviation mechanic I worked an my fair share of mags as every engine had 2 mags.
At that we always replaced the points and condenser as a pair but here we are talking a tractor and not an A/C.
FYI many an aircraft used basic tractor mags with parts actually interchangeable but prices 'sky high'. (pun intended)
 
   / magneto problems #8  
4 sure a possibility but also the most costly component so eliminating all other causes first is a must IMO.

As an aviation mechanic I worked an my fair share of mags as every engine had 2 mags.
At that we always replaced the points and condenser as a pair but here we are talking a tractor and not an A/C.
FYI many an aircraft used basic tractor mags with parts actually interchangeable but prices 'sky high'. (pun intended)

The difference between a tractor magneto and the ones used in aircraft it that the later had to be very reliable, they used platinum contact points and some mags had two sets installed for reliability and safety.

Also you;ll find platinum contact sets were used on very old stationary hit and miss engines, the platinum alloy used in the contacts was 80% platinum with 20% iridium. The iridium was used to harden up the platinum making it more durable.

Spark Plug Inspection and Maintenance – Breaker Point Inspection
 
   / magneto problems
  • Thread Starter
#9  
Thank you everyone for the information. After doing more research in how an ignition system works I realized I had the functioning backwards. I always thought it was the closing of the points that made the spark fire not the opening of the points. I also learned that just blindly testing the cutoff post for continuity will most likely not tell you anything since the engine is most likely to stop with the points closed. Hopefully I will be able to get out and do some additional testing this week.

The magnetism thing sounds interesting. I am hoping that is not the issue since I didn't find the link very clear on how to re-magnetize the magnet.
 
   / magneto problems
  • Thread Starter
#10  
I checked the points and when open the cutoff post has no continuity to ground so they do break. I checked the gap and it seemed a little tight so I recapped them. I then hooked everything back up and tested the cutoff post and sure enough there was continuity to ground again. I disconnected the coil and the wire to the points and the ground wire is shorted, no resistance. From the spark plug terminal to either the ground or points wire is open, no continuity at all. It is a Zico X5700B coil. Am I correct that this is a bad coil?
 
 
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