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Thermo King Questions- Thermo King Alarm Codes- Troubleshooting Tip. loaded unit get emergency help- TK manuals and file

Updated on December 4, 2015

Tech Tips: Troubleshooting Common Thermo King Alarm Codes

I have been a Diesel Truck Mechanic since 1978 and have worked on both Thermo King and Carrier since 1985. I know how hard it is to get good information or help.

Use the following Tech Tips to troubleshoot common alarm code problems on Thermo King Trailer units

If you need more immediate help contact me at the link below.

Good Luck and Thanks for using my site.

Tech Tip: Thermo King Alarm Code 10

High Discharge Pressure

This Thermo King Shutdown alarm code can be difficult to troubleshoot. Here are a few tips to make the process a little easier.

1. Check for broken fan belts.

2. Check for defective 12 volt battery.

3. Check for defective High Pressure Cutout Switch.

4. Check wiring harness on the 8D circuit.

Tip1: Broken fan belts

1. This is the easiest item in the list. Open up the lower and upper front doors and check for missing or broken belts. Replace as needed. Be sure to check the idler gear box for frozen or worn bearings.

Tip 2: Battery has a dead short.

1. This is more common than you would expect. When you turn on the unit it losses power before engaging the starter, the screen may go blank or blink on and off. Use a voltmeter and verify battery voltage. Jump the starter and observe the battery voltage. If the starter fails to engage and the volt reading falls off to 1 or 0 volts then you have a bad battery. Replace the battery.

Warning: Any attempt to jump start the battery may cause the battery to explode.

Tip 3: HPCO High Pressure Cutout Switch defective.

1. The HPCO is located on the top of the compressor. Disconnect the harness and bypass the HPCO switch. If the unit starts then the HPCO switch is defective.

Tip4: 8D wiring harness problem

1. If the unit still won't start then verify voltage to HPCO switch. Do a volt drop test to check for an open circuit. You will need a wiring schematic for the Thermo King unit you are working on. Trace the wiring circuit and check any fuses or switches connected in the circuit.

These tips should solve your problem, but there is always the exception. Please feel free to leave any other tips in my comment section and I will consider adding them to this site.

Tech Tip: Thermo King Alarm Code 20

Engine failed to start.

Troubleshooting List:

1. Yanmar 486 Fuel solenoid failure

2. Plugged fuel system

3. Air in fuel system

Thermo King Alarm Code 20 is one of the most common alarm codes you will encounter. With few exceptions you have a fuel system problem. The following tips will solve this problem quickly.

Tip 1: Checking the Thermo King fuel solenoid.

1. Clear any alarms.

2. Position yourself in front of the Thermo King Trailer unit so that you can touch the side or back of the Yanmar engine's fuel solenoid. The fuel solenoid is located on the back of the injection pump.Turn the unit on and keep a finger on the solenoid. The unit will buzz indicating a start up. The fuel solenoid will clunk and the starter will engage. If the solenoid clunks then it has energized and is ok.

Tip 2: Dirty inline fuel filter

1. Check the inline fuel filter. The filter is located in the transfer pump inlet hose fitting. Loosen and remove the bolt and washers. Remove the filter from inside the bolt and check for debris plugging up the screen. Clean out the bolt and the screen and the banjo fitting. Reinstall the bolt and washers. (You can change the spin on fuel filter if needed now).

Tip 3: Prime theThermo King fuel system.

1. Loosen the hand primer on the transfer pump.

2. Pressurize the fuel tank. (I use an old Freon bottle with compressed air and one of my Freon hoses for this). The fuel tank needs to have at least ¾ of a tank of fuel. If not you will need to add fuel to the tank. Locate the breather fitting on top of the tank next to the supply and return fuel lines. Then using your tank pressurize the fuel tank. Watch the front of the tank to ensure you don't over pressurize it.

3. Bleed the air out of the fuel lines. Locate the bleeder bolt on the top of the injection pump and open it to allow air in the fuel system out. (On Yanmar Tier II engine there is a banjo fitting that will have to be loosened).

4. Turn the unit on. It should start up. If not then repeat steps 2 and 3 until you get all the air out.

Tip 4: Confirm that return fuel is getting back to the fuel tank.

1. Locate the return fuel line. Loosen the hose fitting and carefully pull out the line and see if fuel is coming out. If there is no fuel then you still have air somewhere in the system. A good steady stream indicates that the fuel system is functioning properly.

If you check the return fuel line every time you work on a fuel system, you can stop expensive warranty claims



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