First, make certain the stall-vane (lift-detector) out on the left wing leading edge moves properly. (Did they paint it?)
Use your finger to lift it. Listen carefully for
a faint "click" as the vane operates the switch contained within the unit.
If that seems OK, ... then confirm the C.B. / fuse is still good (Pushed-in/not-blown)
If that is OK,... then open the inspection-cover near the lift-detector and confirm that:
1- The
short wire coming from the switch has
a proper "ground".
This is the single most-common failure point of this system. (Did your paint shop use a pressure-washer to remove paint-stripper? If so... beware! That's a real lift-detector-KILLER.) The screw which connects the ground-wire becomes corroded. Don't just look at it, ...REMOVE it, clean the area where it attaches, use
a new "star" washer, and re-attach it, and use an Volt/Ohm meter (VOM)to confirm good ground-connection. Test the system again.
If the system still does not operate:
2- Confirm electrical power to the switch by using
a VOM again.
With the meter set to the 12-volt range, and the meter''s black probe properly grounded to the airframe, check the long-wire coming from the cabin to the lift-detector for 12 volts. If that is OK, then....
3- Before proceeding further, test the lamp in the cockpit stall-warn unit. Unscrew the red lamp cover and test the lamp and/or replace it
with a known operable lamp. Test the system again and determine that the lamp actually illuminates when the master switch in ON and the lift-detector is operated. If the lamp illuminates but the horn does not blow, then it may simply be the horn that has failed. (The horn unit may or may not have
a capacitor wired across it's terminals inside the unit. Disassembly may be required to repair the horn portion of the unit. See below.*)
A- If the lamp/horn unit illuminates/sounds:
Note the correct orientation of the wires attached to the rear of the cockpit stall-warn unit (light/horn) and disconnect them (planning to re-connect them afterwards to their proper terminals....so keep them identified.) Notice that the terminals are labled (+) and (-). Apply 12-volts positive to the plus and provide
a ground for the minus terminals to again test the light/horn unit. If the unit now operates, then the fault lies in the aircraft wiring. Check that the negative terminal wire (which leads out to the wing lift-detector) is intact and provides
a "ground" when the lift detector is operated. (Do this test by using the Ohm portion of the VOM, or use
a continuity-tester.) If OK, then check that power is available at the wire which connects to the positive terminal. Make the appropriate repairs to the wiring.
B- If the lamp/horn unit does not llluminate/sound when power is applied directly to it's terminals, then you must repair or replace the unit.
*- The cockpit unit may be disassembled by using the point of
a screwdriver to "pop" the sheet-metal retainer-ring from the front of the unit. Now the "grille" which contains the lamp-holder and which has
a "circle" of "sound-holes" punched in it may be removed. Be careful. The bakelite case of the lamp/horn unit is brittle and easily damaged. The interior wiring of the unit basically has the lamp-holder and the horn-unit wired in parallel. Confirm the wiring is OK by testing the continuity from the outer-terminals to the interior contacts of those terminals. Again, be careful not to strain/stress those terminals where they penetrate the back of the bakelite case. Make any necessary internal wiring repairs and retest the unit.
The horns were usually units of two differing types. One type is
a metal-cannister about 1.25" diameter and 1" thickness,
with a 3/4" sounding-hole at it's front. This may be replaced
with a similar unit often found in electronics stores and hardware-store buzzers. The other type of horn involves
a cannister which has
a "capacitor" wired across it's terminals. That capacitor is usually the cause of the horn's failure. Any Radio Shack geek can help you replace that capacitor
with one of similar capacity. I've repaired several of these units by that simple procedure.
Good luck. (Maybe I should write
a 170 News
article and supply
pics on this subject.)
These wing lift-detectors are made by Safe Flight in While Plains NY, and they will not supply replacements directly to the end-user, but they WILL "overhaul" your unit for about $350. (Cessna will charge you over $600) Replacing the interior micro-switch can be done using electonic-store parts but usually the unit will only "get you by" the annual inspection where the mechanic/inspector tests the unit by lifting the detector while on the ground. These common store-part-switches usually require too much pressure to actually detect
a stall in the air. (The switch should activate
with only 5-grams of pressure. Use
a postal-meter-scale or ammunition-reloader powder-scale to test them.)
The cockpit units are also made by Safe Flight and I haven't asked them what they charge to repair/overhaul them, but I suspect it's about $1200...half of what Cessna charges for
a new unit, as that seems to be the going rate for the comparative lift-detector "overhaul".
These "overhaul" prices are obviously what drive the on-line-auction prices.
