Ok guys I’m at a loss. I was plowing my driveway last week and my 1987 420 died and has no spark. I got it inside my garage where it is warm and started to test things and am pretty certain my ignition module is bad. Again. I replaced it about a year and a half ago with a gray pickup wheel. The voltage when I spin the engine over goes from .998-10.7. There are a few things I’ve found along the way as well, I replaced the coil with a known working coil and saw that the condenser was hooked up to the negative instead of the positive. Also there is a starter button on the tractor I’m guessing to bypass the delay box. Is this normal to fry a module this fast? I’m thinking no but it is bringing me back to the late 90’s when I owned a 93 Z28 with the LT1 that ate the optispark module every 30k like it was it’s job. Lol Any help is greatly appreciated.
DustinF
Joined
·
2,451 Posts
Dustin,
DOES NOT SOUND LIKE A MODULE FAILURE... If your module test shows the voltage transitioning from less than one volt to nearly 11 volts once per engine revolution when turning it by hand, then the module is switching as it should. Here is the illustration from the TM1590:
![Image]()
Do you have spark? Remember that you need to ground the other spark plug wire to observe a spark at the one you are monitoring:
![Image]()
It does sound like your tractor had some prior owner hacks to the wiring harness which may still be impacting things, so be sure to verify that all of those have been put right. The wiring is well described in the TM1590 -- let us know if you need those excerpts.
Chuck
DOES NOT SOUND LIKE A MODULE FAILURE... If your module test shows the voltage transitioning from less than one volt to nearly 11 volts once per engine revolution when turning it by hand, then the module is switching as it should. Here is the illustration from the TM1590:
Do you have spark? Remember that you need to ground the other spark plug wire to observe a spark at the one you are monitoring:
It does sound like your tractor had some prior owner hacks to the wiring harness which may still be impacting things, so be sure to verify that all of those have been put right. The wiring is well described in the TM1590 -- let us know if you need those excerpts.
Chuck
Joined
·
2,451 Posts
Dustin,
The 318 and 420 ignition is factory wired as shown below. As you noted above, the condenser is supposed to be connected to the + terminal of the coil. If you have positive voltage on the primary of the coil for at least part of the rotation of the crankshaft, you should have spark when the module switches. Also, the fact that you have voltage at the coil (and the hour meter) when the key switch is in the RUN position verifies that the TDCM is enabling the ignition correctly.
![Image]()
Sometimes intermittent symptoms are due to poor ground connections - so inspect the engine ground connection at X27 and the W1 jumper.
The ignition coil impedance is not as critical as you might imagine, and the Onan manuals show different values than the Deere literature...first the TM1590 page:
![Image]()
Then the Onan information....
![Image]()
Chuck
The 318 and 420 ignition is factory wired as shown below. As you noted above, the condenser is supposed to be connected to the + terminal of the coil. If you have positive voltage on the primary of the coil for at least part of the rotation of the crankshaft, you should have spark when the module switches. Also, the fact that you have voltage at the coil (and the hour meter) when the key switch is in the RUN position verifies that the TDCM is enabling the ignition correctly.
Sometimes intermittent symptoms are due to poor ground connections - so inspect the engine ground connection at X27 and the W1 jumper.
The ignition coil impedance is not as critical as you might imagine, and the Onan manuals show different values than the Deere literature...first the TM1590 page:
Then the Onan information....
Chuck
Joined
·
2,451 Posts
Dustin,
The measurement range you show of about a volt to nearly 12 volts indicates the module is switching successfully and is fully functional. Furthermore, the module is directly connected to the primary of the ignition coil so its positive terminal should be experiencing that same voltage swing. If the coil is good, there should be a spark generated by the coil secondary for each revolution. The spark system is pretty simple -- the spark can be inhibited by the TDCM for certain safety switch states, but if you are getting this voltage swing, the ignition is not being inhibited by the TDCM.
![Image]()
About the only unknowns remaining given the voltage tests you cite are the high tension spark plug wires and the coil itself at this point. Just a review of the proper way to test for spark presence on an Onan with wasted spark ignition where both plugs fire at the same time:
![Image]()
These illustrations are of course repeats of those shown earlier in this thread...just including them here for completeness relative to the discussion in this post.
Chuck
The measurement range you show of about a volt to nearly 12 volts indicates the module is switching successfully and is fully functional. Furthermore, the module is directly connected to the primary of the ignition coil so its positive terminal should be experiencing that same voltage swing. If the coil is good, there should be a spark generated by the coil secondary for each revolution. The spark system is pretty simple -- the spark can be inhibited by the TDCM for certain safety switch states, but if you are getting this voltage swing, the ignition is not being inhibited by the TDCM.
About the only unknowns remaining given the voltage tests you cite are the high tension spark plug wires and the coil itself at this point. Just a review of the proper way to test for spark presence on an Onan with wasted spark ignition where both plugs fire at the same time:
These illustrations are of course repeats of those shown earlier in this thread...just including them here for completeness relative to the discussion in this post.
Chuck
Joined
·
2,451 Posts
So you see a voltage switching event for each crankshaft revolution at the (-) terminal on the primary of the coil, but no spark at the secondary??? I would have to question your spark plug wires at this point...not much left in the circuit if the coil is working as it should.
![Image]()
One more test to try: set up your spark gap observation per the below illustration and then rotate the engine manually without cranking it with the starter to a place where the voltage measured at the coil (-) terminal is low -- then turn off the key switch. You should at least see a single spark at the gap when that happens, because the field in the coil is collapsing at that time. If you don't see a spark,, then the plugs are not really connected to the coil somehow, or the coil has failed.
![Image]()
Chuck
One more test to try: set up your spark gap observation per the below illustration and then rotate the engine manually without cranking it with the starter to a place where the voltage measured at the coil (-) terminal is low -- then turn off the key switch. You should at least see a single spark at the gap when that happens, because the field in the coil is collapsing at that time. If you don't see a spark,, then the plugs are not really connected to the coil somehow, or the coil has failed.
Chuck
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
