In installing thermocouple leads for a cylinder-head temperature indicator, which is critical—the length or the resistance?

• A. Lead length
• B. Insulation type
• C. The resistance is critical, it must have the resistance specified on the indicator (usually 2 or 8 ohms)
• D. Thermocouple color code

Answer: (C)

The measurement relies on a fixed input loop resistance. The cylinder-head temperature indicator is calibrated to read the thermocouple’s small voltage accurately only when the total resistance in the circuit—sensor plus leads—matches a specified value, often 2 or 8 ohms. If the resistance differs, the instrument’s input network and any built-in temperature compensation can shift the reading, producing error or instability. Lead length is a factor because it can change that resistance, but what’s critical is that the actual resistance equals the value the indicator expects.

Insulation type and color code matter for durability and correct wiring, but they don’t by themselves determine the accuracy of the temperature reading in this setup.