912 ULS with 460 hours built in 2008 1000 RPM drop during Mag test

Michael,

The one constant you seem to have is that no matter what you do, you have a big RPM drop on the LEFT Mag SWITCH, regardless of what ignition module that switch is connected to.  This suggests to me that the RIGHT switch or it’s wiring may have an intermittent ground or wiring problem that is foiling the ignition module it’s connected to.  So, if the right switch is partially grounding it’s module, then you would get a big drop when you fully ground the other module with the LEFT switch. Reversing the wiring connectors would make no difference, the problem would still appear when using the left switch.

Are you using shielded cables (with the shields grounded)  from the ignition modules to the mag switches?  The installation manual states that if this is not done then the ignition modules can be shut off by EMI.  I would test the right mag switch circuit to make sure it’s completely clear of grounds, and that the wire shield is grounded well.  

Hey Jeff.

Thanks for the info.   I did disconnect the "mag" switch wires from their switches and then I ran a ground wire to the ends of those switch wires.  The drop was the same.  So the switches are probably good.  I also tested the switches for continuity/no continuity too.

That being said, the two lines running from the 2 6-pin plugs are not shielded and are run together.   So I guess that I will run the two mag switch wire apart from each other and try grounding them to the engine block.   I wonder how two mag wires running next to each other would exhibit this behavior on only one side.

I will try this out tomorrow.  Thank you for the suggestion.

Try changing mag switches.  I have had many issues with the same symptoms that just turned out to be a faulty mag switch or bad earthing on manifold.  

If you read the response before you will see that I completely bypassed the switches and just touched each mag wire to the engine block.  The same problem where one dropped off by 750-1000 rpm still occurred.

Thanks.

michael Vadney wrote:

Hey Jeff.

Thanks for the info.   I did disconnect the "mag" switch wires from their switches and then I ran a ground wire to the ends of those switch wires.  The drop was the same.  So the switches are probably good.  I also tested the switches for continuity/no continuity too.

That being said, the two lines running from the 2 6-pin plugs are not shielded and are run together.   So I guess that I will run the two mag switch wire apart from each other and try grounding them to the engine block.   I wonder how two mag wires running next to each other would exhibit this behavior on only one side.

I will try this out tomorrow.  Thank you for the suggestion.

Michael,

The way you did the test, you used the two existing unshielded wires from the 6-pin connectors to the switch.  It’s my understanding that these wires, if not the shielded type, can pickup electromagnetic interference which can cause trouble with the ignition modules.  

I realize this is an unlikely cause of your problem, but you might as well completely rule out a problem with these wires by removing them from the circuit during a test, or just replace them with the specified wire type.  The two wires should be individually shielded, not a shielded pair. The install manual is very specific about using 18 awg wire with a braided shield, with the shield grounded at both ends. Even if this is not the problem it’s a cheap thing to make right. 

See attached pages from install manual. Good luck with this, I know these things are frustrating.