Winter is coming, and the starting problems. If you find difficulty when starting your equipment, the glow plug may go wrong. So, how to test it? This guide is your answer.
We’ll introduce the warning signs of a failing plug and three simple DIY methods to test them.
What Are the Signs of a Bad Glow Plug?
Before you grab your tools, let’s confirm you’re on the right track. A failing glow plug will give you several clear clues:
- Hard Starting: This is the #1 symptom. The engine cranks for a long time before it finally catches, or it won’t start at all in the cold.
- Rough Idling on Startup: For the first few minutes, the engine shakes, sputters, or feels uneven. This happens because the cylinders with faulty plugs do not fire correctly until the engine warms up.
- White Smoke from the Exhaust: That puff of white smoke is unburnt diesel fuel. A faulty glow plug means the cylinder isn’t hot enough for proper combustion, so raw fuel gets pushed out the exhaust.
- Check Engine Light: On modern vehicles, the engine’s computer can detect a problem with the glow plug system and activate the check engine light.
Why You Should Always Test Before Replacing
Testing is a crucial diagnostic step that saves you time and money. Here’s why:
- Pinpoint the Real Problem: As forum member “Jeff-C-IL” pointed out, hard starting can also be caused by low compression or fuel issues. A quick test confirms the glow plugs are the actual problem before you spend money.
- Find the Specific Faulty Plug: Your engine has one plug per cylinder. It’s rare for all of them to fail at once. Testing lets you find the single faulty plug and replace only what’s necessary.
How to Test Your Glow Plugs: 3 Easy Methods
Here are the best ways to test your glow plugs, from the most common method to a simple trick for older tractors.
Before You Begin: A Quick Checklist
- Safety First: Always work on a cool engine and disconnect the negative battery cable.
- Get Your Tools: You’ll need a digital multimeter, a basic socket/ratchet set, and maybe a test light.
- Check Your Battery: Glow plugs are power-hungry. Use your multimeter (on the DC Volts setting) to test your battery. A healthy battery should read around 12.5 volts when the engine is off.
Method 1: The Multimeter Resistance Test
This is the most reliable and safest method. Best of all, as user “pete23” noted, you don’t need to remove the glow plug to test it.
1. Locate and Prep: Find them on your engine. They look like small, thin barrels with an electrical connector on top. Carefully pull the wire or cap off the glow plug you want to test.
2. Set the Multimeter: Turn your multimeter dial to the resistance setting (Ohms, marked with the Ω symbol). Choose the lowest range, such as 200 Ω.
3. Perform the Test: This is the most common point of confusion. Just like “IH Forever” asked, “Where do I put the leads?” It’s simple:
- Touch one probe (it doesn’t matter if it’s red or black) to the metal tip of the glow plug where the wire was connected.
- Touch the other probe to a clean metal part of the engine, like the engine block or the hex nut on its body. This provides the ground.
4. Read the Results:
- Good Glow Plug: The reading will be very low, usually between 0.1 and 2.0 ohms. This means the internal heating element is intact.
- Bad Glow Plug: The meter will read “OL” (Open Loop), “1”, or show infinite resistance. This means the filament inside is broken, and the plug is dead.
Method 2: The 12V Test Light
As suggested by forum member “Sparky,” this is a fast way to check for a complete circuit if you don’t have a multimeter.
1. Connect the Test Light: Clip the clamp of your test light to the POSITIVE (+) terminal of your battery. You might need a longer wire, as “IH Forever” realized.
2. Test the Glow Plug: With the wire disconnected, touch the probe of the test light to the top terminal of the it.
3. Check the Result:
- Good Glow Plug: The test light will light up.
- Bad Glow Plug: The light will not turn on.
Method 3: The Old-School Dash Meter Trick
Forum member “pete23” shared a clever trick for older tractors that have a glow plug meter on the dash. It requires no tools.
- Isolate One Plug: Disconnect the wires from all the glow plugs except for one. Make sure the loose wires aren’t touching anything.
- Activate the Glow Plugs: Sit in the driver’s seat and press the glow plug button.
- Watch the Meter: If it is good, the meter on your dashboard should register a draw (move partway).
- Repeat: Repeat this process for each glow plug, one at a time. If the meter doesn’t move for a specific plug, you’ve found your bad one.
What if a Glow Plug Tests “Good” but is Still Weak?
As expert user “TomH” pointed out, a resistance test only confirms continuity—it doesn’t guarantee the plug is getting hot enough. He says, “Using a test light or measuring resistance tells us that we have continuity, but it is not an indicator that the glow plug is heating to its full ‘strength’.”
A weak glow plug might test with correct resistance, but won’t draw enough electricity (amps) to get properly hot. The definitive test is to measure current draw with a DC clamp meter. A healthy one should draw 6-8 amps. If a plug is drawing significantly less, it’s weak and needs replacing.
What if All My Glow Plugs Are Good?
If your tests show all glow plugs are working, the problem lies elsewhere in the system. Check these next:
- Glow Plug Relay or Solenoid: This is the high-power switch that sends voltage to the plugs. It can fail.
- Wiring: Look for corroded or broken wires leading to the plugs.
- Dash Meter: As “kevinj” suspected, the indicator meter on the dash itself could be bad, giving you a false reading.
- Other Issues: The problem could be unrelated to the glow plug system, such as low engine compression or a fuel delivery problem.
Find a Reliable Glog Plug at FridayParts
Follow these easy steps, and you can diagnose your glow plug correctly and ensure your working efficiency during winter. And you can consider the multimeter resistance test at first due to its safety and reliability.
If you find something wrong, turn to FridayParts. We supply cost-effective and high-quality glow plug replacements for your tractor, truck, or heavy equipment. With the right part, you can face the winter confidently and never worry about failing to start.
