How to Test a Glow Plug Relay with a Multimeter

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Glow plugs preheat the air within the combustion chamber in diesel engines, allowing them to start right away when they’re cold. Is your engine having problems starting? Maybe you notice smoke coming from your exhaust. It may be a sign that one or more of your glow plugs is malfunctioning.

Testing your glow plug on your own could help you prevent your trip to your local mechanic. 

Diesel engines can be very difficult to start in cold weather. That’s because the cylinder head and cylinder block absorb the heat of compression and stop ignition due to the greater surface-to-volume ratio. Further, inducted air temperature is low in cold engines because they rarely receive any heat from the engine cylinder walls.

It lowers the temperature as the air is compressed, and most of the heat is surrendered to the cold cylinder walls. Glow plugs function as the answer to that issue, as they activate until the combustion chamber’s temperature reaches a level where it could combust itself. 

What is a Glow Plug Relay? 

Glow plug relays are a crucial part of the ignition system in most diesel vehicles. The glow plug control unit assesses input from the ECU when you turn the key in the ignition. It determines the currents and timings needed by the plugs to start the vehicle.

The glow plug relay then converts the current for the glow plugs on and off accordingly, which causes them to heat up immediately and ignite the fuel in the engine.

Remember that these are a vital part of the glow plug ignition system. Hence, relays must be fully functioning, particularly during cold weather. 

How Does It Work?

Diesel engines are considered to be compression-ignition engines. That suggests that an added heat source is not required to ignite the mix in the cylinder. The air or diesel mixture in the cylinder is highly compressed. That leads to an extremely high temperature that the mixture ignites all by itself, creating an explosion.

Nonetheless, that process could be reset by problems if the engine isn’t hot. 

At cold temperatures, the diesel or air mixture won’t ignite right away. What does the glow plug do to protect combustion when the engine is cold? It increases the temperature within the cylinder’s combustion chamber before the engine starts.

At its peak, the glow plug’s temperature could reach approximately 1,000 degrees Celsius. That guarantees the diesel or air mixture will explode even at low temperatures. 

The time needed for preheating will differ based on the glow plug utilized. Fast glow plugs only require a preheating period of a few seconds. Several glow plugs should preheat for approximately fifteen seconds at low ambient temperatures.

Keep in mind that the glow time relay is in charge of changing the current glow plugs on and off and handling the timing. 

What Causes a Problem to Glow Plug Relay? 

Here are the problems you need to pay attention to: 

  • Wrong spray pattern of the fuel injectors

This is very difficult to diagnose but can be a problem as well. Are your fuel injectors squirting a big stream of raw fuel? That could stress the glow plugs’ tips. Visualize if you heated up the glow plugs red hot and then continuously plunged them in cold water. 

  • Glow relay not turning off

That’s not that typical, but it still does happen. A failure within the relay normally causes it. You might notice the battery not charging properly when that occurs because of a high current drain on the electrical system.

Try to start your engine and test the glow plug circuit with a multimeter after a minute of running. Do you have voltage appearing at the wiring connections on the glow plugs? It implies your relay hasn’t turned off and will need immediate replacement.

  • Glow cycle too long

That could be because of the wrong relay installed, a feedback circuit not running the relay off soon enough, or a sticking relay. In the case of a manual conversion, that could be because of continuous long pre-glow times. 

  • Too much carbon accumulation in the pre-chambers

That could be caused by overheating or shorting of the plug leading to tip burnout. We recommend that you use the proper carbon reamer every time you are changing your glow plugs. 

How Do You Test a Glow Plug Relay Using a Digital Multimeter?

Testing your glow plugs is quite a simple task. The only thing you need is a digital multimeter that can measure resistance and the tools to access the glow plug connections. 

  1. Disconnect the plastic beauty cover from the engine’s top. Pull it upwards to free the rubber cups from the mountings. Disconnect if there are any other rubber cups on the engine and reinstall them in the plastic cover hooks.
  1. The glow plugs are located at the back of the cylinder head. It’s located between the intake manifold and the common rail.
  1. Pull the connector upwards to disengage it from the glow plug. Those can be tight. 
  1. Set your digital multimeter to the lowest resistance range.
  1. Put the black test lead from the digital multimeter to a clean piece of metal on the engine to serve as an earth point.
  1. Touch the red test probe into the center pin of the glow plug.
  1. The reading on your digital multimeter must be at least 1 ohm for a good glow plug. Keep in mind that the actual reading might differ based on your engine’s temperature, the quality of the earth connection, the accuracy of the multimeter, and other factors. 

Also, any plug readings of 5 ohms and above indicate that there’s a problem with the glow plug relay and must be replaced right away. 

  1. Repeat the entire process for the other plugs. 

Take note that other engines have separate connectors for every glow plug. Hence, always follow the process above, but you might need to disconnect other components for a hassle-free and easy access to the glow plugs.

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