RPM and Manifold Pressure violent fluctuations

[Jrags]

I recently had an avionics tech fix my cigarette lighter so I could plug in USB ports.  The very next flight, run up was uneventful.  Take off was fine until just shortly afterwards.  The RPM gauge and the manifold pressure gauge both began to fluctuate wildly.  The engine sounded normal and remaining in the pattern we landed without further event.  I understand the avionics guy may have inadvertently disturbed the ground wire on the RPM gauge but the manifold pressure gauge doesnt have a ground.  My A&P can't get to it for a couple of weeks.  Any idea what's going on here? 

[kortopates]

JPI EDM monitor? Make sure the monitor is well grounded directly to the engine.

[PeteMc]

You could have the Avionics shop retrace their steps and see if they can find any loose wires.  

 

[PT20J]

What kind of gauges do you have? Original mechanical? Replacement electric? An engine monitor? 

[Jrags]

The gauges are the original 1968 mechanical gauges

[kortopates]

The gauges are the original 1968 mechanical gauges

Since you posted this in the engine monitor sub-forum and mentioned concern for grounding we thought we were discussing an engine monitor issue.

With mechanical gauges there shouldn’t be anything electrical but more likely due to the proximity of the recent work done for your USB outlet; perhaps tech didn’t get them properly/fully reconnected when he did the recent work? MAP reads off an engine hose and RPM off an engine driven tachometer cable.


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[Jrags]

I'm still new here so sorry if I put it in the wrong spot.  It turns out the tach cable was vibrating and was next to the MP hose.  The tach vibrations also vibrated that hose so both gauges were going nuts.  Going to order a new cable and that should fix everything.

[PT20J]

No worries -- welcome aboard. And thanks for letting us know the outcome. It helps us all learn.