by Jim Mais                                                                                                                 © 1999

This model heater uses a glow plug to ignite the fuel. There is no spark plug.
Two relays are combined in one package and mounted on the top of the heater.
They are labeled CR1 and CR2 on the diagram. There are several safety circuits. Battery voltage (12V) is available at all times to the heater. When heat is desired, the ON/OFF switch is closed and relay CR1 operates. Assuming that the heater is in the cold state, the thermo-switch is closed to the NO terminal. The path from B thru CR1 and the thermo-switch energizes the glow plug. The fan motor also starts at this time. There are two fans. One pulls in combustion air and the other blows room air over the heat exchanger.

While the glow plug is heating, the flame safety heater also is energized. If combustion does not take place in a reasonable time (2 to 3 minutes) the heater will be shut down and no more fuel will flow. After waiting for the safety switch to cool down (1 minute) the reset lever may be pressed to restart the heater.

The ballast resistor (hidden inside the heater casing) reduces the glow plug voltage to approx 6 volts. While the glow plug is heating up, the fuel pump begins to deliver fuel which is sprayed directly on the glow plug. The fuel pump is a solenoid operated device. A set of points on the fan shaft open and close (thru a reduction gear) and trigger the fuel pump solenoid on and off. Each pulse of the solenoid delivers a tiny quantity of fuel to the heater.

(Note that the solenoid is polarity sensitive; if the wires are reversed, it will not pump.) Fuel quantity is adjustable by turning the threaded fitting on the pump discharge side. Screwing it in will decrease fuel quantity.

When the glow plug reaches operating temperature, the fuel should begin igniting.
The thermo-switch senses the increased temperature in the heat exchanger and its contacts switch over. This opens the path to the glow plug and also to the flame safety heater. Combustion continues without the glow plug.

In order to keep the fuel pump energized, a holding path is established for relay CR2 (the fuel pump relay).     This holding path is thru the now closed contact on the thermo-switch, the closed CR1 contact and CR2's own contact.
CR2 became energized originally thru the diode when the thermo-switch was in the COLD position. The heater will continue to operate until the operator switches off the power or the Temp Regulating control (Temp Reg) opens the circuit.

In either event, both CR1 and CR2 will drop out, stopping fuel flow. The fan continues to run, however, to purge the heat exchanger. The fan obtains power directly from the thermo-switch. When the heat exchanger cools down, the fan stops as the thermo-switch changes over to the Cold position. Now the heater is set to re-start if the ON/OFF switch is closed. The heater will cycle on and off automatically under the control of the Temp Reg control. Each time, the fan will continue to run until purge has been completed and then the glow plug will re-energize in preparation for another ignition cycle.

The safety fuse is built into the heater. In case of control failure and subsequent over heating, the Over-Heat thermal switch closes. This short circuits the Safety Fuse directly to ground. The fuse blows and the fuel pump can not receive power.

  Click here for a simplified schematic diagram of the heater.

  Pic of the fuel pump.

  Click here for a series of photos of the heater internals and disassembly.