Installing Mitchell Gauges
From Bondline
Contributed by Gil Alexander
Mitchell Instrument's STC'd gauge package requires a logbook entry by an A&P and a Form 337 submitted.
Contents |
Items Needed But Not Supplied
- 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.
- 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).
- 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....
- Usual assortment of #20 aircraft wire and good quality ring terminals and splices.
- 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.
- 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).
- AN929-4D caps (2)- to seal off the old fittings that penetrated the firewall.
- 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 stall warning horn.
Mounting the stack
I elected to mount the Mitchell stack with #6 flush head screws on each side, and a round head #6 on the top. The round head screw will pass through the overlay - plastic or future metal - and help hold it down. I centered it in the existing gap, and made the top flush with the factory hole. This gave me room under it for the two breakers mentioned earlier. You can also see the 1/8 aluminum plate, tapped for 2 #6 flush mounting screws that holds the ammeter shunt - it's mounted behind the triangular factory cut-out - this makes easy routing for the #8 power wires from the battery that connected to the back of the old ammeter.
Looks good with the overlay. I used 1/16 black rubber tape as a temporary measure to fill the gap in the old plastic overlay... a metal one is coming in the future to match a future Fred K. metal overlay on the left side. The new metal overlay can be flat since does not need to hold the plexi gauge cover in place any more. Yes - the two breakers did get labelled.
Mounting the sensors
The oil pressure sensor mounts on the upper right motor mount tube, just ahead of the voltage regulator. This picture shows the mount with 18 inches of -4 hose... a few inches less probably would be better. Note the non-insulated clamp to pick up the sensor ground return. A separate ground wire is connected to a ring terminal on the screw connecting the two clamps.
The fuel pressure sensor is mounted on the upper left motor mount near the electric fuel pump. The 10.5 inch hose works here. The STC calls for a mount on the lower engine mount tube, but the hose routed better this way. Note the AN929-4D cap on the AN fitting that was the original pass through the fire wall.
(More to come.)
