(c)
Repeat steps (a) and (b) using receptacle pin B in upper shroud.
(d)
Remove ohmmeter from receptacle in upper shroud.
(2)
Test Insulation Resistance of APU Engine Leads
(a)
Connect one side of ohmmeter to one socket of engine lead plug and other side of ohmmeter to ground.
(b)
Test engine leads for shorts to ground. Resistance must be more than 10,000 ohms.
(c)
Remove ohmmeter from engine lead plug.
D. If balancing resistor test for the EGT is not necessary, restore airplane to normal configuration (AMM 49-71-0/501).
3. Exhaust Gas Temperature Balancing Resistor Test
A. Equipment and Materials
(1)
Wheatstone Bridge - approved to 13.50 (+0.01) ohms minimum
(2)
Multimeter - measurement range of 13.50 (+0.01) ohms, Model 8012A-01, Fluke, P.O. Box 9090, Everett, WA 98206-9090 (this is an alternative to the wheatstone bridge)
B. Prepare for EGT Balancing Resistor Test
(1)
Prepare same as for Insulation Resistor Test (Ref 2.B.).
(2)
Install jumper across pins A and B on receptacle in upper shroud. The jumper must be of copper with resistance value not to exceed 0.05 ohm.
C. Test Exhaust Gas Temperature Balancing Resistor
(1)
Connect wheatstone bridge or the multimeter to receptacles 1 and 2 of plug disconnected from EGT indicator.
(2)
Measure circuit resistance. The resistance value shall be as shown in Fig. 501.
NOTE: If resistance is not within tolerance, check circuit for loose, corroded or shorted connections. Dissimilar materials at connections (nuts, screws, washers, etc.) can also cause incorrect circuit resistance.
(3)
If circuit resistance value is met, disconnect wheatstone bridge or the multimeter from circuit and proceed to step D.
(4)
If circuit resistance value is not met, proceed as follows:
(a)
Remove resistor cover (Fig. 502).
(b)
When circuit resistance is in the range of coarse and vernier adjustments, adjust resistor as described in step (5).
(c)
When circuit resistance is above the range of coarse and vernier adjustments, adjust resistor as described in step (6).
(d)
When circuit resistance is below the range of coarse and vernier adjustments, adjust resistor as described in step (7).
(5)
Adjust resistor using coarse and vernier adjustments.
(a)
Reposition airplane lead from one coarse adjustment terminal to other coarse adjustment terminals and one vernier adjustment terminal to other vernier adjustment terminals until circuit resistance value shown in Fig. 501 is obtained.
(b)
Disconnect wheatstone bridge or the multimeter from circuit.
(6)
Decrease circuit resistance using resistor spool.
(a)
Disconnect wheatstone bridge or the multimeter from circuit.
(b)
Install jumper across pins 1 and 2 on EGT plug that is disconnected from indicator. The jumper must be of copper with resistance value not to exceed 0.05 ohm.
(c)
Connect one airplane lead to coarse adjustment terminal A3 and other airplane lead to vernier adjustment terminal B2 (Fig.\502).
(d)
Using soldering iron, disconnect resistance spool wire end from vernier adjustment terminal B3.
(e)
Connect one lead of wheatstone bridge or the multimeter to vernier adjustment terminal B3.
(f)
On the basis of approximately 0.7 ohm resistance per foot of wire, unwind amount of wire necessary to give approximately correct resistance.
(g)
Remove enough insulation from resistance wire to make a good contact with wheatstone bridge or the multimeter lead; then connect other lead of wheatstone bridge or the multimeter to wire. Move lead along wire, removing wire insulation as required, until resistance value shown in Fig. 501 is obtained.
(h)
Cut wire at this point and remove wheatstone bridge or the multimeter from circuit.
(i)
Thoroughly scrape insulation from end of wire.
(j)
Wind excess wire back on spool and once around center post, leaving sufficient length of wire to reach vernier adjustment terminal B3 for soldering.
(k)
Solder end of wire to terminal.
(l)
Remove jumper from EGT indicator plug.
(m)
Check circuit resistance per steps (2) and (3).
(7)
Increase circuit resistance using resistance spool.
(a)
Disconnect wheatstone bridge or the multimeter from circuit.
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