Voltage-drop testing of AC switch is critical. Many "failed MCC Thermostats" are actually voltage drop problems.
Typically illuminated switch with LED indicator.
LED brightness indicates switch condition. Dim LED = voltage drop = corrosion. Even slightly dim LED indicates problem.
When pressed, completes circuit to MCC (pressure switch is there as well).
Moisture causes corrosion on contacts. Creates voltage drop that prevents proper MCC operation. Most common electrical failure.
Coil creates magnetic field when energized. Pulls clutch plate into engagement with pulley. The ohm meter is not your friend. Substitute test. 3 amp current.
Provides power to clutch coil. Check for corrosion, proper seating, and secure connection.
Monitors evaporator temperature, system current draw, and voltage. Protects system from damage.
Disengages clutch when evaporator approaches freezing. Re-engages when temperature rises. Prevents ice. Generally is designed to disengage the compressor clutch at 30-32°F (0-1°C) and re-engage at 35-37°F (2-3°C).
Monitors clutch current draw. High current = short-to-ground. Low current = open circuit. Protects electrical system.
Measure voltage drop across closed switch with clutch engaged. More than 0.3-0.5V drop = replace switch. Critical test for electrical diagnosis.
Use high-current test light to bypass clutch. If test light operates at 3 amps, compressor clutch is bad - replace compressor.
Use jumper to bypass switch. If system operates, install gauge set and diag system pressure. If pressure is good, replace pressure switch.
Compare LED brightness to known good switch. Dim LED indicates internal corrosion and voltage drop. Replace if dim.
| Symptom | Cause | Diagnosis |
|---|---|---|
| Clutch not engaging and MCC LED off | Low gas charge | Bypass pressure switch or install gauge set |
| R-Series open circuit fault | Tailigate is open | No air flow across the condencer causes high pressure which opens pressure switch. Close the tailgate. |
| Clutch not engaging and MCC LED solid | Voltage drop in AC switch | Voltage drop at MCC. Excessive voltage drop is usually from corosion in the AC switch. |
| MCC flashing fault codes | Clutch circuit problem | Read MCC codes, diag circuit from MCC to clutch or potential temp prob problem |
AC switch internal corrosion is the most common electrical failure. Even a slightly dim LED indicates excessive voltage drop. Always perform this test before replacing the MCC