Cleaning up engine wiring

Jeff65 · 02-04-2004, 11:33 AM

99% of the time the instrument is shorted to the back shell. The backshell is metal and its grounded. The instrument posts (screws) stick through enlarged holes in the backshell. A fiber washer (non-conductor) holds the instrumented centered once the post nuts are tightened.

If, for example, the instrument is centered, then turned in the backshell until the posts touch or hit the backshell then you have a shorting condition. One of two things happens:

If the short is to the (+) side of the instrument, it shorts the constant voltage regulator (CVR) usually frying it (toast). Replacing the without correcting the problem usually toasts the new CVR too.

If the short is to the (-) grounded side of the instrument, it causes the instrument to read full scale. Thus, for fuel, the tank would always read full; or for temperature the engine temperature would always read hot, etc.

Whenever you work on the backshell or on instruments...make absolutely sure no shorts exist between instrument posts and the backshell. An ohmmeter works fine to check between post and backshell for these kind of short circuits.

Jeff

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02-04-2004, 11:33 AM	#1
Jeff65 Registered Member Join Date: Jul 2002 Location: Oklahoma City Posts: 208	99% of the time the instrument is shorted to the back shell. The backshell is metal and its grounded. The instrument posts (screws) stick through enlarged holes in the backshell. A fiber washer (non-conductor) holds the instrumented centered once the post nuts are tightened. If, for example, the instrument is centered, then turned in the backshell until the posts touch or hit the backshell then you have a shorting condition. One of two things happens: If the short is to the (+) side of the instrument, it shorts the constant voltage regulator (CVR) usually frying it (toast). Replacing the without correcting the problem usually toasts the new CVR too. If the short is to the (-) grounded side of the instrument, it causes the instrument to read full scale. Thus, for fuel, the tank would always read full; or for temperature the engine temperature would always read hot, etc. Whenever you work on the backshell or on instruments...make absolutely sure no shorts exist between instrument posts and the backshell. An ohmmeter works fine to check between post and backshell for these kind of short circuits. Jeff