Thermostatic (thermal bypass) valve

Wax element valve that bypasses the oil cooler until the oil is warm, keeping the airend above the condensation point.

Also called: thermal valve · oil mixing valve · thermostatic element

Purely mechanical — no electrical contact. Element rating stamped on the body or the element itself.

How it works

A wax-motor element expands with oil temperature, gradually moving the spool from full bypass to full cooler flow. Below the set temperature all oil bypasses; above it, all oil goes through the cooler.

Specifications

Common element ratings

160 °F / 71 °C, 170 °F / 77 °C, 185 °F / 85 °C

Full-flow point

Approximately element rating + 20 °F

Target running oil temp

175 – 200 °F (80 – 93 °C), and at least 15 °F above pressure dew point

Parts you can order

• AMOT

  Model G 3-way rotary thermostatic control valve

  Model G (element selected by setpoint, e.g. 160 °F / 71 °C)

  $400 – $1,800
• AMOT

  Replacement thermostatic element

  Element by setpoint (e.g. 4A xxx series)

  $90 – $350
• AMOT

  Model G / 1NF thermostatic element

  By set temperature (e.g. 160 °F, 170 °F, 180 °F)

  $90 – $380
• Aftermarket service kit

  Thermal valve seal & element kit

  By OEM body

  $60 – $260

Field test procedure

1. Feel/IR the cooler inlet and outlet: cold cooler with a hot airend = element stuck in bypass.
2. Bench test in hot water — the element should stroke smoothly by the rated temperature.
3. Running too cool (below ~160 °F) means the element is stuck open to the cooler — condensation, emulsified oil, rusted internals.

Symptoms & probable causes

• High air-end discharge temperature trip
  Element stuck in bypass, plugged cooler, low oil, or a dirty fan/cooler face.
• Milky oil, water in sump
  Machine running too cold — element stuck open or oversized cooler in a cold room.