Usage:
3606 (8RB),
3608 (6MC),
3612 (9RC),
3616 (1PD),
3618 (2NW) Engines
This article will attempt to answer some questions that have been raised about thermocouple wiring color codes.
The wiring between the Engine Mounted Terminal Box (EMTB) and the MMS panel is obtained locally by the shipyard/contractor/dealer and many standards for type K thermocouple wire colors exist. The polarity, not the color, of the connections is critical to accurate temperature measurements. Therefore, it is important to know which color wire is positive and which color wire is negative. The table lists some of the known standards for type K thermocouples. The thermocouple wiring in the MMS Marine Engine Control Panel uses the US and Canada Standard shown in the table.
The table can help determine polarity in most cases. However, if you want to be positive of the polarity, use the following procedure. Strip the insulation from each end of both wires in a short piece of the thermocouple cable. Short one end by twisting the two bare thermocouple wires together. Heat the twisted wires while measuring the millivolts across the other ends of the two thermocouple wires. If the millivolt reading is positive and increases with a temperature increase on the heated end, the positive lead of the meter is connected to the positive lead of the thermocouple cable.
Polarity is important. If the interconnect wiring is crossed on one end, the temperature reading will be low and the measured temperature will decrease when the actual temperature increases. This condition is easy to detect. If both ends of the interconnect wiring are crossed, the temperature reading may or may not be accurate. The error would be approximately equal to the difference in ambient temperatures at the two ends of the interconnect wiring. The error would also be somewhat affected by the actual thermocouple temperature. This condition is much harder to detect.
Three different color codes have been used on the 124-4591 Exhaust Sensor Cable that goes between the engine mounted thermocouple rail and the EMTB. These wires are marked with labels showing polarity in the EMTB. The color codes that have been used in the 124-4591 Exhaust Sensor Cable are Yellow (+) and Brown (-), Green (+) and White (-), or Aqua (+) and White (-).
Although you have to destroy a thermocouple to see it, the 124-4596 and 124-4598 Thermocouples have used at least two different wire color codes. These color codes are Green (+) and White (-) or Yellow (+) and Purple (-).