1984 GL1200 tach drops to zero, engine still running

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Tom_Charlton

Well-known member
Supporting Member
Joined
Jun 9, 2012
Messages
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Location
Centralia Illinois
My Bike Models
1984 GL1200I
1988 GL1500
Did an engine change because one cylinder showed 120psi, rest were over 150psi.

Now when the RPMs go below 2000, the tach drops to zero, even though the engine is still running (poorly). The tach gets power/signal from a yellow/blue wire that connects to the ICM and one ignition coil.

Does the tach drop mean that the ICM is not signalling that coil to fire below 2000 RPM? If so, should I expect an ICM failure shortly?

Guess I can change out the ICM and see what happens.
 
I now suspect the pulse generators may be weak. Is there any way to change them other than pulling the engine?

I have removed the cover and the three bolts holding the housing to the rear of the engine but do not have enough clearance to remove the housing and generators. Removing the rotor gave me a little more room, but not quite enough. I cannot break the screws loose that hold the generators in the housing without stripping out the heads (weird Phillips pattern).

Any suggestions before I loosen the back of the engine and jack there to see if that will give additional clearance?

Update: I unbolted the back of the engine and gained just enough clearance to change the pulse generators by jacking the back of the engine up about 1/2". Like she said, "That's not a lot, but its good enough." Now to reinstall the engine mount bolts, the mufflers, rear foot pegs, battery box & battery, etc. Then I'll be able to tell if the PGs were the problem.

Final Update: The problem was weak pulse generators; replaced them and the problem is cured. Did an ohm check on the generators: Both tested 1.1 - 1.2K ohms - supposedly good.
 
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