A/C Amplifier / Belt Lock Controller

A/C Amplifier / Belt Lock Controller
A/C Amplifier / Belt Lock Controller
Many import vehicles use a compressor control module commonly known as an
A/C amplifier or belt lock controller. The module controls compressor operation based
on different inputs including evaporator temperature, low-pressure switch and A/C
The module also serves as a protection device. Any problems with the A/C
system (example: excessive high side pressure) may cause compressor overheating and
the belt to slip. These conditions would lead to compressor lock up and/or the belt to
break. To prevent damage to the compressor or engine, the module compares both engine
and compressor speeds and turns the compressor off when the two speeds are not within
The compressor control module receives a signal from a revolution (speed) sensor
mounted on the compressor and an engine rpm (tach) signal. Depending on manufacturer,
the compressor speed must remain with in 5% and 30% of engine speed. If the two
speeds are out of range for 2 to 20 seconds, the module will open the compressor relay
circuit and shut the compressor off. On several vehicles, the A/C switch light will blink
or change color to alert the driver but will not set a diagnostic trouble code (DTC).
To verify a customer’s complaint of a flashing A/C light and no compressor
operation, turn the vehicle off and then restart the engine. Most modules will reset when
the ignition switch is turned off. If the compressor engages for only a few seconds and
the light starts flashing again, there are two possibilities: 1) a mechanical problem
allowing the belt to slip; 2) an electrical problem with the module sensor circuits. If the
light starts flashing before the compressor engages, it usually is an electrical problem.
One quick test that can be performed is to jump battery voltage to the compressor
with a fused jumper wire. If the compressor comes on and operates properly (including
correct pressures and temperatures) refer to manufacturers test procedures to diagnose the
control module and related circuits. While diagnosing, pay close attention to the engine
cooling fans. The ECM may not engage the fans while the compressor is jumped to
Most test procedures include performing an ohm test of the compressor speed
sensor. However, these tests do not always recommend repeating this step at the module
connector. If the two readings don’t match, then there is a problem with the wiring.
When replacing the compressor it may be necessary to use the original speed
sensor. Not all remanufactured compressors are supplied with speed sensors.
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