Garland CG-24, CG-36, CG-48 SERVICING continued, Thermostat, How To Replace A Thermocouple

SERVICING continued

Thermostat

Make sure the thermostat knob is correctly aligned with the dial indicator.. To align the knob with the graduated dial correctly, follow this procedure..

1.Turn the knob and shaft fully counter-clockwise.. Loosen the set screw in the knob and align the pointer with the first line of the scale at the lower left position..

NOTE : When the knob is fully rotated clockwise, the pointer should align with the last line of the scale..

2.Set the dial to 350˚F.. Wait at least three cycles before monitoring the temperature..

3.Use a surface temperature probe disc positioned on the center line with the thermostat knob and centered front to back on the griddle plate..

The thermostat will overshoot and undershoot as caused by the lag time in the thermal response of the heat source- to-plate and plate-to-sensor.. The calibration temperature will be the mid-point between the minimum and maximum readings.. Other than the alignment of the knob pointer, there is no means to calibrate the thermostat.. It has been precisely calibrated by the manufacturer..

Thermocouple

If the temperature is incorrect check the thermocouple.. The thermocouple in this unit is a Type J, which requires the use of a J-type temperature meter for checking.. For a complete understanding of the thermocouple used in the chain griddle see the GCR Service Bulletin #95-5 entitled “Thermocouples”..

A thermocouple is an accurate temperature measuring device that consists of two dissimilar conductors joined together at their ends.. It works on a simple principle, which is that the thermoelectric voltage between the two junctions is proportional to the temperature difference between them.. This principle is applied to measure the temperature at one of the junctions when the other is held at a fixed known temperature.. The technology is not new, but its use in GCR griddles is recent..

The thermocouple probe is embedded in the griddle plate and accurately registers the surface temperature.. It is a reliable device, but can cause problems when it fails for any reason.. In addition to deterioration from aging, it can fail in two ways: one from an open-circuit (that is, a break in one of its wired); the other when it short-circuits..

If a thermocouple open circuits, the thermostat will shut off and prevent the gas valve from opening, which means the burner will not function..

If a unit short circuits somewhere behind the junction, it creates a new junction and because the circuit cannot detect the existence of a short circuit fault, there is a dramatic increase in temperature.. The thermocouple detects changes in temperature, but because of the short circuit it may fail to register the set temperature at the thermostat.. This means the burner may not shut off.. At the other extreme, the short circuit may occur in a place not shielded from the flue gases, which means the burner may shut off prematurely.. These are the two conditions to look for in a shorted thermocouple..

Deterioration from aging and from welding dissimilar metals together to for a junction introduces a contaminant that causes mechanical stress.. Over time, five to ten years, metal fatigue from aging is certain.. As a result, the temperature curve will vary..

How To Replace A Thermocouple

A heat shield protects the thermocouple from the burner flames and flue gases.. Without the shield, the thermocouple would register flame heat to the thermostat instead of sensing the temperature of the griddle plate..

To replace a thermocouple (refer to Figure 3), following this procedure:

1.Open the front panel to get access to the terminal board..

2.To withdraw the thermocouple, first remove the heat shield.. It is held in place by two combination Philip and Robinson-heat size number 10 sheet metal screws.. Remove the heat shield by sliding it down the wire..

3.A bayonet connector secures the thermocouple in place.. Reach inside the hole and push and twist the bayonet connector a quarter turn..

4.Remove the connection to the terminal board and feed these wires through the heat shield.. The thermocouple is now removed..

5.Connect the wires of the replacement thermocouple, but make sure the polarity is correct.. Connect the negative red wires to terminal 7 on the thermostat and the positive white wire to terminal 8.. (See schematic diagram)..

6.Thread the thermocouple through the heat shield and into the plate cavity..

Page 10 Part #GCGTRAINING06 (03/03/08))