RPM drops to zero for right mag check

[cjohngraham]

I have newly rebuilt Bendix mags on an O-360 and am using the p-lead "pick-off" method to measure RPM on a SV HDX. 30k resistors are installed near the ignition switch per the manual. This was working fine during the first two mag checks, but on the last engine ground run the RPM dropped to zero for the right mag only. The engine sounds normal and runs smoothly, with an audible RPM drop on the right mag as you would expect. I have checked continuity between the EMS and the ignition switch terminals and end to end on the right and left p-leads; everything looks good. The zero RPM problem was noticed after a brief max power runup to 2200 RPM (we're at 8000'+ density altitude with 3 blade fixed pitch prop). Would that runup have changed something? What else should I check/do?

 

[jakej]

If the resistor is ear the mag end then I’d check to see if it’s broken.

 

[cjohngraham]

Continuity check from EMS to ignition switch was good, indicated 30k ohms as expected. Still a puzzle.

 

[Dynon]

That sounds like a wiring issue, as previous posters have mentioned. Make sure that the pin isn't retracted in the D37 connector. Does the mag check pass though? IE, do you get a drop on both L and R? Does turning the key to the off position actually remove ignition entirely?

 

[swatson999]

Make sure your ears aren't fooling you on the RPM drop...do you see a concurrent EGT *rise* when turning off each mag in turn?