Tach won't show RPM, only when installed in panel

[4Cubbers]

Hello,
I have a 1946 J3 with a newly overhauled Tach. When I install the cable into the 90 degree fitting on the back of the instrument and use a drill to spin the cable while pushing on the end of the cable, the tach shows RPM. When installed in the plane, it will not show any RPM. Any thoughts?

Thanks,

Jake

 

[bob turner]

 

[4Cubbers]

 

[huston marlowe]

 

[JimC]

 

[huston marlowe]

 

[reileyr]

 

[bob turner]

 

[43-29080]

 

[4Cubbers]