Thermo King MD-MT, Operator'S Manual

Page: 72 / 192

Electrical Maintenance
72
Fan Speed Control
The S-2 and S-3 evaporators are equipped with a
fan speed control circuit. The speed control circuit
is connected to the +12 Fan circuit, the Fan SC
circuit, and ground. The voltage on the Fan SC
circuit controls the evaporator fan speed. The
speed control circuit limits the voltage on the Fan
SC circuit to 7.5 volts, and the fans run at
approximately 1900 RPM.
Circuit Boards
The printed circuit boards are generally fail safe
and should require no repair. Some things to note
are:
• The current paths on the circuit boards can be
damaged (scratched, etc.) and cause an open
circuit.
• Do not use flat washers between the circuit
breakers and the circuit board. Flat washers at
this location can contact a current path and
cause a malfunction.
• A feed thru hole may burn out between the top
and bottom surfaces.
• A circuit board cannot be repaired. It must be
replaced if it is damaged.
CYCLE-SENTRY Option
Refer to the CYCLE-SENTRY IV Operation and
Diagnosis Manual for all operation and diagnosis
procedures. The MD-MT is not presently included
in that manual but the system is similar.
Option Board Jumpers
The option board has three 7.5 amp fuses that are
used as jumpers to configure the board for
different applications. If these jumpers are not in
the correct positions, the unit may not function
properly. The correct jumper positions for the
MD-MT are: J1—Position A, J2—Position A,
J3—Position A.
1. Correct Jumper Positions
Figure 26: Option Board