4.11 THERMISTOR TEMPERATURE SENSORS TESTING - CONTINUED.
Lethal voltage levels are used in operating the FDECU. Be sure power has been
disconnected prior to servicing the unit. Injury or death can occur if personnel
contact any electrical terminal while the unit is connected to power source.
a. Disconnect Leads.
(1) Tag and disconnect outside air sensor wire lead (1) from TP4 terminal OAS.
(2) Tag and disconnect outside coil sensor wire lead (2) from TP4 terminal OCS.
(3) Disconnect remote control box connector P2 (3) from pc board.
(4) Disconnect inside air sensor (volute) connector P1 (4).
(1) Using a multimeter set to measure resistance, connect test leads to the various contact points as follows to
test each thermistor. Note resistance reading for each one tested.
Inside air sensor (volute) at P1 (4) pins 1 and 2.
Inside air sensor (remote) at P2 (3) pins 5 and 12.
Outside coil sensor wire lead (2) and TP4 terminal COM (5).
Outside air sensor wire lead (1) and TP4 terminal COM (5).
(2) A thermistors' resistance will change in response to various temperatures. Compare the resistance
readings measured with the following list of temperatures and associated resistance. Use this list to
estimate the resistance based on the current ambient condition. For more accurate results, you may
remove the thermistor being tested and immerse it in a bath of water and crushed ice to establish a
reasonably accurate 32 F reference. A thermistor should be considered defective if it tests more than 10%
above or below the estimated resistance or 5% above or below the ice water (32 F) reference.
32 F --------- 32,654Ω
55 F ---------- 17,439Ω
77 F ---------- 10,000Ω
100 F ---------- 5,827Ω
(3) Replace any defective thermistor. Note that if a thermistor shows a short or open condition the
interconnecting wiring or connectors could be damaged and not the thermistor. The volute thermistor(s) are
epoxied in place and cannot be removed. If the volute thermistor tests defective, be sure to check the one
not in use and connect to it if not defective. If both volute thermistors are defective, the entire volute
assembly must be replaced.
c. Connect Leads.
(1) Connect inside air sensor (volute) connector P1 (4).
(2) Connect remote control box connector P2 (3) onto pc board.
(3) Connect outside coil sensor wire lead (2) to TP4 terminal OCS per tag and wiring diagram figure 4-14.