Installing Mitchell Gauges

Contributed by Gil Alexander

Mitchell Instrument's STC'd gauge package requires a logbook entry by an A&P and a Form 337 submitted.

Items Needed But Not Supplied

1. Number 8 wire gauge ring terminals with 1/4-inch hole. The instructions say to use existing wiring, but Gil Alexander's '77 Tiger used 3/16-inch terminals on the power wiring to the old ammeter, whereas the Mitchell shunt has 1/4-inch threaded terminals.
2. New hoses to senders. Gil used new AN-4 firesleeved hoses: 10.5 inches for fuel pressure and 18 inches for oil pressure. But a 16-inch hose would probably be better for oil pressure (see the photos).
3. Clamps to mount the senders, 5/8 for the engine mount side and 1 9/16 for the two senders. Instructions call for all clamps to be cushioned MS21919, however the oil pressure sender does not have a ground connection, and cushioned clamps do a good job of electrical isolation. Since my supply house did not have 1 9/16 plain clamps, I found that a 1 7/16 cushioned clamp with the rubber removed worked well. The AN3 bolt that is used to connect the small and large clamps can be used as a ground point for this sensor. The installation instructions ignore this....
4. Usual assortment of #20 aircraft wire and good quality ring terminals and splices.
5. You need to splice 3 or 4 wires into a single wire in four places, I found that soldering and a good quality heat shrink worked well. Check with your A&P on this and do what he/she prefers.
6. Rubber "booties" to protect the exposed hot wires (when the Master is ON) on the ammeter shunt - not essential, but a nice safety feature to have - MS25171-1 (2).
7. AN929-4D caps (2)- to seal off the old fittings that penetrated the firewall.
8. AN910-1D couplings (2) and AN816-4D (2) pipe thread to flare fitting to connect the sensors to the hoses.

Wiring the gauges

The gauges need quite a bit of work wiring before any installation occurs. Since each gauge is a separate module, all of the common wiring (power, grounds, lighting) needs to be connected together. This is how I did it.... top of module is on the left.

When this package wiring is completed, you should have ends for - two red wires to the ammeter shunt - two ground wires, meter ground and lights ground - a lighting power connection - a hot 12 volt connection (direct to bus, not through an Avionics switch), fused or breakered - and finally three sensor wires.

I chose to get power from a new 1 Amp breaker, since I had to add one for the Horizon tachometer I am adding at the same time. It was easy to just make it two new breakers. I placed them side by side under the new instrument stack. The STC says to use the power wire that was connected to the original instruments - via the fuse shared with the strobes.

(More to come.)