|" Shouldn"t the voltage be lower with the original resistor block in the circuit? " |
It depends if the points are open or closed.
If the points are open, you will see battery voltage; less if closed (9v or so).
Chances are pretty slim that you have a bad resistor or bad coil.
Coil problems are difficult to diagnose. For starters, round coils are pretty robust & square coils arenít (because of the difference in insulation used), but neither one will hold up to a poorly done 12v conversion that allows too much current to the coil or leaving the key on (see tip # 38). Too much current creates heat which melts the insulation. Insufficient resistance in a 12v conversion will do the same thing. Rarely do coils just ďgo bad.Ē
There are a few ways to see if a coil is bad, but itís not possible to determine if a coil is good w/o some expensive testing equipment. If you detect a dead short or high resistance in the coil w/ an ohm meter, itís bad. If itís cracked, itís bad. If a sidemount coil w/ battery voltage to the primary will not jump a ľĒ gap from the secondary wire to the block, itís bad. But, here is the hard part: even if you do not detect a short, even if it will produce a spark, even if itís not cracked, that doesnít mean the coil will work when itís hot & under a load. So, itís a process of elimination. If the tractor starts & runs fine for 30 minutes or an hour then cuts off & refuses to re-start, and you checked for spark at the plugs & it had no spark at all, AND you have the correct voltage at the coil thatís a good sign that you have a bad coil. Let it cool off, restart it & if you have a good spark, odds are itís a bad coil. But, even then, you might end up w/ a spare coil on the shelf!
Bottom line.......coils do go bad, but I'll venture a guess that 75% of new N coils sold today are sold to folks who do not understand how to diagnose a poor spark problem or how a coil works. So, for those who donít know any better, in a no spark situation the first suspect is usually the coilÖÖand, more often than not, it isnít the problem.
Just pull the distributor & see where your spark went.
Assuming that the bushings & advance weights are ok, & that you have correct voltage to the coil, the most common electrical failure (no spark, weak spark) points on the frontmount are:
1. The insulator under the brass concave head screw & where the copper strip attaches. (itís fiber & will wear out; poke & prod w/ your meter leads to make sure it still works)
2. The pigtail at the bottom of the coil not making contact w/ the concave head brass screw inside the distributor. (With the coil on, the pigtail must firmly contact the brass screw. No contact = no spark.).
3. The copper strip is broken or grounded to the plate. (look very carefully for cracks & breaks)
4. The condenser wire grounding to the plate or side of the distributor.
5. The tab on the bottom of the coil not making contact w/ the brass button on the cap. (With the cap on, the tab must firmly contact the brass button. No contact = no spark.)
6. Incorrect positioning of the spring clip on the plate causing the pigtail to ground. (the open part of the clip goes between 7 & 9 oíclock on the plate. That puts the straight part of the clip opposite of the timing screw at 3 oíclock)
7. Incorrect seating of the coil on the distributor due to a loose bail or no gasket.(the coil must not move at all; if it does, replace the gasket or bail. Or stick some cardboard under the bail).
8. Water/moisture inside the cap due to gasket failure or the absence of a gasket. (the cap AND coil have gaskets)
9. Dirty/corroded/burned/incorrectly gapped or misaligned points.
10. Burned rotor, cracked/carbon tracked cap.
Unless the coil is cracked or shows a dead short, chances are it's fine; square coils rarely fail cold. Pull the distributor & do a continuity check.
First, make sure your meter/light works (don't ask....)
You can change points everyday & it will not fix bad bushings. If you are having trouble w/ points failure, check the shaft. If you detect movement, chances are it needs new bushings.
Inspect the points; if they are pitted or burned, replace them. Next, dress the points by running a piece of card stock or brown paper bag through them. New points sometimes have an anti-corrosive dielectric coating on them & old points can corrode or pick up grease from a dirty feeler gauge or excessive cam lubricant. Make sure the points align correctly. Proper alignment is also critical to longevity. Look at the points when they are closed; both sides should mate evenly. Then, check the gap at .015 on the high point of all 4 cam lobes.
Now, follow these steps:
1. Coil off, cap off, points open. One probe on the brass screw & the other on both sides of the open points. On the side closest to the cam, you should have continuity. Not on the other side! If you do, you will also have continuity everywhere because the points are grounded.
2. Coil off, cap off, points open. One probe on the brass screw & the other anywhere on the body of the distributor. You should have no continuity! Now, rotate the tang on the distributor....as the points open & close, you have continuity (closed) and lose it when they open.
3. Coil on, cap off, points open. One probe on the lead on the top of the coil, the other on the cam side of the open points. You should have continuity!
4. Coil on, cap off, points open. One probe on the lead on the top of the coil, the other anywhere on the body of the distributor. You should have no continuity!
At this point, I just put the distributor, coil & cap all back on the tractor as a unit. The reason I do this is because it is real easy to get the cap or coil mis-aligned trying to put it back together one piece at a time & the result is something gets broken or you get a Ďno sparkí problem.
It's possible to put it back on wrong & break it. Look at the slot on the end of the cam shaft. What ever angle it happens to be, turn the distributor tang to match it. Make sure you can tell the wide side from the narrow side on both the cam & distributor! (close counts) Then place the distributor on the front of the engine, gently push it in place & slowly turn the distributor body until you feel the tang slip into the slot. Rotate the distributor body until the bolt holes line up. Then, hand tighten the two bolts until the distributor body is flush w/ the timing gear cover.
Finally, double check your firing order & plug wires. Itís 1-2-4-3, counterclockwise. Itís very easy to cross 3 & 4.
I'm in Old Church, eastern Hanover Co.
Post back w/ results & any other questions.