Weak Coil Question

SealClubber

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Can a coil be failing/failed and still read 40 ohms? The engine lock saucer eject on my Corvette gets weaker as you play it until it can no longer eject the ball. Only takes a few times. It reads 40 ohms and doesn't feel any hotter than the other coils. The plunger is not binding in the sleeve at all. This is more like a kickback than your typical saucer eject with a cam. Not sure what else it could be if it isn't the coil. The switch which activates it is a normal switch and not an opto and it is working fine. It just starts losing its ooomph to the point it can't lift the ball the 1/2" onto the playfield. Any ideas?
 
Can a coil be failing/failed and still read 40 ohms? The engine lock saucer eject on my Corvette gets weaker as you play it until it can no longer eject the ball. Only takes a few times. It reads 40 ohms and doesn't feel any hotter than the other coils. The plunger is not binding in the sleeve at all. This is more like a kickback than your typical saucer eject with a cam. Not sure what else it could be if it isn't the coil. The switch which activates it is a normal switch and not an opto and it is working fine. It just starts losing its ooomph to the point it can't lift the ball the 1/2" onto the playfield. Any ideas?

I don't know anything about pins but is your voltage correct?
 
As far as I can tell. It is the same as the other coils and the voltage doesn't drop after the coil gets weaker.
 
When the coil gets weak, try grounding the tab of the coil, if it works good there, try the metal tab of the drive transistor. Could be the transistor is weakening or a bad connection.
 
As far as testing things in circuit (transistors) they can test good but still be bad. If the game in question has another identical coil, swap them out and see what the outcome is before you go removing board components. Otherwise you need to remove the associated transistor/s and test them OUT of circuit.

FWIW - I had a game that had a coil that acted weak. Everything checked out normal (coil sleeve was clean, proper voltage) so I replaced the coil and it worked fine. I always wondered about this since most people claim a coil never goes bad. Then I read a repair that the pinball Ninja wrote about and he had a similar situation and replacing the coil fixed it. So clearly if the Ninja will replace a coil because it acts weak then they can grow weak. .
 
When the coil gets weak, try grounding the tab of the coil, if it works good there, try the metal tab of the drive transistor. Could be the transistor is weakening or a bad connection.

That's what I would do but I would probably go for the driver transistor first. If it works manually from the driver transistor but not when the transistor is actually driving it I would replace that driver transistor. If it's still weak when manually energizing it at the transistor you know it's either a connector or coil problem.

It could be the coil but it's pretty unlikely. A coil itself won't "go weak". Assuming it's not damaged in any way. That much is a fact. However, it could fail in a way that makes it weak but still appear fine. Like windings shorting internally. That's a whole different ballgame than "going weak" over time which is a common misconception (I believed this myself at one point). A transistor on the other hand I've definitely have seen "go weak". I wouldn't have though that would happen until I actually saw it happen.
 
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