Glow Plug Troubleshooting

A diesel starts by compression, but in cold weather that heat is not always enough. Glow plugs add warmth to the combustion chamber, and a weak set shows up as hard starting and white smoke on cold mornings.

Key points

Not every diesel has glow plugs, so check your manual
Test the battery, fuses and relay before the plugs
Disconnect the battery before working on the circuit
Do not use starting fluid unless the manufacturer allows it
Broken plugs in the head are a mechanic's job

What do glow plugs do?

A glow plug is a small electric heater that screws into each cylinder or into a pre-chamber. When you turn the key to the heat position, current flows through the plugs and they warm the chamber so fuel ignites more easily. Many tractors have a dash light that tells you when to crank. Others use a timed relay.

Not every diesel tractor has glow plugs. Some use an intake heater or a different cold-start aid, so check your manual.

A glow plug can fail alone or in groups. Often one bad plug goes unnoticed until a second goes, and then cold starts become poor. Replacing a full set at once is common practice when the plugs are old.

What are the symptoms of a failing glow plug?

The tell-tale signs show mostly in cold weather:

Hard starting when cold, but normal starting when warm
Long cranking before the engine fires
White or gray smoke at start-up that clears when warm
A rough, misfiring engine for the first minute
Dash light that behaves oddly or never comes on

How do you test the glow plug circuit?

Start with the battery. Weak batteries and corroded cables cause many cold-start problems, so check those first. Then check fuses, the relay and the control switch. A relay that clicks when you turn the key to the heat position is a good sign, and one that does not click may be bad.

A mechanic or experienced owner can check each plug with a multimeter by disconnecting the wire and measuring continuity or resistance, following the figures in the service manual. A plug that reads open is dead. Do not guess at the numbers. Your service manual lists the expected values for your engine.

Take notes on how long the dash indicator stays on and how many seconds the tractor needs to start. A change from last winter is useful information.

Is it safe to test glow plugs?

Be careful with the electrical side. Disconnect the battery before removing wires, and keep tools away from battery terminals. Never use a jumper to power a glow plug in a way the manual does not describe, since plugs can get extremely hot and ignite fuel vapor or burn skin.

Wear eye protection and gloves, and let the engine cool before touching parts.

If a glow plug test shows normal readings but the tractor still starts poorly, look at the fuel system and compression, since a worn engine can be hard to start even with good plugs.

How do you replace a glow plug?

Replacement involves removing the electrical connector or bus bar, unscrewing the plug with the correct socket and installing the new one by hand first to avoid cross-threading. Tighten to the manual's torque, and use the correct part for your engine.

A plug that is seized in the head can snap during removal, leaving the tip inside the cylinder. That is a job for a mechanic. Heating the engine slightly, using penetrating oil as the manual allows, and working gently can help, but there are no guarantees.

What about starting fluid?

Many cold-start problems tempt owners to spray starting fluid into the intake. On a diesel with glow plugs or an intake heater, that can cause a violent ignition and engine damage, and it can ignite fire at a heater. Do not use it unless the manufacturer says it is safe for your engine.

Better cold-weather help is a block heater, a warm battery, correct oil and fresh fuel as the manual describes.

Check the manual for the starting procedure on your model, including how long to hold the heat position. Over-heating plugs by holding the heat too long can shorten their life.

When should you call a mechanic?

If replacing a plug does not fix the problem, or if you have a broken plug tip in the head, stop. The cause may be a bad relay, a weak starter, a clogged fuel filter or a failing injector. Do not keep cranking, since the starter can overheat.

If you run a small tractor on a few acres, a cold-start problem is an annoyance. On a ranch in winter with livestock to feed, a reliable start is a welfare issue, so have the system tested before the cold hits.

A ranch tractor used for early-morning feeding would do well to have a spare set of plugs and the right socket stored in the shop before winter arrives.

When to get help: Call a qualified mechanic or the dealer service department for seized or broken glow plugs, unresolved cold-start problems and any electrical fault you cannot trace with the manual. Use only the parts and torque the manufacturer specifies.

Sources and further reading

Organization homepages are linked so you can search them for current guidance on this topic.

More in Tractor Maintenance & Repair

← Injector Problems and Symptoms Cold-Starting a Diesel Tractor →