Diagnosing an Air Heater Issue

Posted: September 1, 2012  |  Tag: Repair

By: John Temple

One of my diesel engines is a lot harder to start when it is cold. It did not used to be that wway. I have been told it is the air heaters. How can it be fixed?
The answer to how “it” can be fixed always starts with what is broken. There are several components to the operation of air heaters. There are also other causes of difficulties with the starting of diesel engines. I will assume that the fault is with the air heaters, as you’ve indicated.

Air heaters need a large amount of current to work. When air heaters are working, the alarm tone usually lowers and the voltage drops. When you turn the ignition on the defective engine, look and listen carefully for a momentary drop in voltage. A momentary drop probably indicates there is faulty wiring that allows for voltage to drop to the cutout point. Once the engine starts, charging could overcome the drop, so that the engine heater works while running.

Start the engine with a defective air heater, and look to see if the air heaters are working when the engine is running. If they’re working, you know the engine air heaters are OK. If not, you will want to check and see if proper voltage is getting to the air heaters. If there is voltage and no heating, the heater solenoids are likely the problem. You can further check by disconnecting the power to the heaters and looking for continuity across the terminals. If the heaters themselves are good, it gets interesting. Air-heater systems have a cutoff for when the engines reach a certain rpm or temperature, a certain running time and/or if the voltage supply drops below a threshold. There are relays that operate sensing, and there is a control module that is the brains behind it all.

Check all the harnesses and related wiring connections, especially those that you might have pulled on or knocked previously. This is very important if you get that flicker when turning the key. For further testing, it helps to have two people. One person uses a meter to check all inputs and outputs as the ignition goes on and off and as the engines start. Compare tests with the good engine, and exchange parts to see if the problem travels. The other person is working the ignition and watching the boat’s voltmeter. Have wires to jump as needed.

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