GMU Hi Limit PCB Theory of Operation

System Overview

The Gas Modulation Unit (GMU) and High Limit PCB work together to control and monitor the gas heating system in each fryer vat. Their job is to:

• Control oil temperature using RTD probes

• Ensure safety by shutting off heat if oil gets too hot.

• Coordinate ignition and gas flow to maintain proper heating.

GMU (Gas Modulation Unit)

Purpose

Location and Addressing

• One GMU per vat (full or split).

• Addressed 1–8:

  • Odd numbers = Left split vat

  • Even numbers = Full vat or right split vat

Inputs

• RTD Probes (Main & Level):

  • 1000-ohm at 32°F (0°C)

  • Resistance increases with oil temperature

• Safety Circuits:

  • High Limit PCB

  • Combustion fan pressure switch

• Flame Rollout Thermocouple:

  • J-type millivolt input for flame safety

Outputs

• 24V DC to BCB: Powers combustion fan

• 24V AC to:

  • Ignition module (TH terminal)

  • Safety shutoff valve (CE only)

  • Combination pilot/main gas valve

Built in Features

• E-fuses (2): Protect against over current

• LED Indicators: Show status and errors

• CAN Bus: Communicates with control system

High Limit PCB

Purpose

Monitors oil temperature for overheat protection. If oil exceeds safe limits, it opens the safety circuit to stop heating.

Location

• One per vat

• Connected directly to the GMU

Inputs

• RTD Probe:

  • 100-ohm at 32°F (0°C)

  • Mounted on inner left fry pot heat exchanger

  • Resistance increases with temperature

Safety Trip Logic

• Trip Point: 430°F (221°C)

  • Safety circuit opens

  • Heating disabled

• Reset Point: 374°F (190°C)

  • Operator must press and hold the reset switch for 5 seconds

  • Power cycle the GMU/HL board

Output

• 5V DC to Reset Switch:

  • When pressed, sends 5V back to High Limit PCB to reset

LED Indicators

• Solid Red: High limit open or stuck reset

• Solid Yellow: High limit closed, 24V DC present

• Flashing Red: Critical error – reset required

System Interaction

• Component: Role in System

• RTD Probes: Measure oil temp & level; send resistance data to GMU and High Limit PCB

• GMU: Controls ignition, gas valves, and fan based on probe data and safety circuits

• High Limit PCB: Monitors for overheat; disables heating if temp exceeds 430°F

• BCB: Receives 24V DC from GMU to run combustion fan

• CAN Bus: Allows communication between GMU and control system

Technician Tips - "Think Like the Equipment"

• No Heat? Check:

  • High Limit LED (solid red = tripped)

  • Combustion fan pressure switch

  • RTD probe resistance (should increase with temp)

• Faulty GMU

  • Fan Not Running?

  • Confirm 24V DC from GMU to BCB

  • Check CAN bus communication

• Reset Not Working?

  • Oil must be below 374°F

  • Ensure 5V DC is returning to High Limit PCB

• Flashing Red LED?

  • Power cycle the GMU/High Limit assembly