Carrier 19XL - Manual (Page 242)

1. Connect ohmmeter across terminals L1 and T1. Resistance reading should be greater than
50,000 ohms.
2. If reading is less than 50,000 ohms, remove connecting bus heatsink between SCR3 and
SCR6 and check anode to cathode of SCR3 and SCR6 separately to determine which
device is defective. See Figure 46 . Replace defective device and retest controller.
3. Repeat Steps 1 and 2 across terminals L2 and T2 for SCRs 2 and 5.
4. Repeat Steps 1 and 2 across terminals L3 and T3 for SCRs 1 and 4.
If the SCRs tested were not defective but the problem still persists, refer to the following
Steps 5 and 6.
5. Disconnect the SCR1 from the white gate and red cathode wires on the AK control logic
card. With an ohmmeter set on Rx1, check between white and red wires. Resistance should
normally be between 8 and 20 ohms average. Excessively high or low resistance may be
indicative of a defective device. Replace and retest.
6. Repeat Step 5 for SCR leads 2 through 6. Care should be taken to ensure that the gate and
cathode wires are replaced exactly as they were: white wire to gate (G1 through G6); red
wire to cathode (K1 through K6).
IMPORTANT: Before performing the SCR check below, remove power from the starter
and disconnect the motor terminals T1, T2, and T3.