Heater Plug (Glow Plug) on Backhoe Loader (JCB) and Excavator Engines: How Preheat Works, Intake Heater vs Glow Plug and Failure Signs

A backhoe loader that starts fine at noon but cranks for a long time on a winter morning, then coughs white smoke before it settles, usually has a heater plug problem. Heater plug is the name most Indian mechanics use for the glow plug: a small electric heater that warms a cold diesel engine for a few seconds so the first drops of fuel catch fire.

The part is cheap next to the damage that long cranking does to a battery and starter, and a failing plug rarely stops a machine outright. It just makes every cold start harder, and in a warm month it can hide completely.

This guide starts with the signs, then explains how preheating works, the two kinds of heater found on construction machine engines, why the plug’s voltage has to match the machine, how a mechanic tests a plug and its relay, and what wears plugs out early. It carries no prices; the right plug depends on the exact engine, and a dealer quotes against that.

What are the signs of a bad heater plug?

Heater plug faults show up when the engine is cold and disappear once it is warm. If a machine starts badly after standing overnight but restarts easily after lunch, look at preheating before you look at fuel or compression.

A plug can fail without any of these signs in hot weather. The BorgWarner technical sheet on glow plugs points out that a diesel engine will start in warm, dry weather even with one plug dead, and the fault only shows when the first cold, damp mornings arrive. So a plug that died in May often goes unnoticed until the first cold morning of a north Indian winter, or until the machine moves to a hill or high-altitude site.

If the engine will not start at all, warm or cold, the heater plug is unlikely to be the only cause. Work through the full JCB starting problem checklist for a backhoe loader, or the excavator not starting guide, which sort faults by what the engine does when you turn the key.

What does a heater plug do on a cold diesel engine?

A diesel engine has no spark. It squeezes air until the air gets hot enough to light the fuel. BorgWarner’s figures put the squeeze at 30 to 55 bar, which heats the air to 700 to 900 °C, well above what diesel needs to ignite.

On a cold morning that heat leaks away. The cylinder walls and head are cold, the oil is thick, and the starter turns the engine slowly, so the air loses heat before the fuel arrives. The fuel then lights late or only partly, which is the white smoke and the knock.

The heater plug puts back the missing heat. Its tip sits in the combustion chamber, and a strong current from the battery brings it to a glow in a few seconds. BorgWarner states that the tip of a modern plug passes 1,000 °C within a few seconds. Fuel sprayed near that hot tip evaporates and catches, and once the first cylinders fire, the engine heats itself.

How much an engine depends on its plugs depends on its design. The Wikipedia article on glow plugs gives rough air temperatures below which an engine struggles to start unaided: about 40 °C for pre-chamber engines, 20 °C for swirl-chamber engines and 0 °C for direct-injection engines. The common-rail engines in current backhoe loaders such as the JCB 3DX and CASE 770 NX are direct injection, so they need help less often. Help still matters on a cold night, at altitude, or when a weak battery cranks slowly.

Glow plug in each cylinder or one intake heater: which does your machine have?

Construction machine engines use one of two ways to preheat. Both do the same job and usually show the same coil symbol on the dashboard, but they are tested and replaced differently.

The first is a glow plug in each cylinder. One pencil-shaped plug screws into the cylinder head beside each injector, so a four-cylinder engine has four. A thin metal strip or wire, called the bus bar, joins their tops to one feed from the relay.

The second is one heater in the intake. A single heating element sits in the intake manifold, the pipe that feeds air to all the cylinders, and warms the air on its way in. It has one heavy cable. Wikipedia notes that a glow plug system can use either one plug in the inlet manifold or one per cylinder. It also notes that engines larger than about one litre per cylinder usually use a flame-start system if they need a starting aid at all: a heater in the intake that lights a small amount of diesel to warm the incoming air.

Which one a particular machine has is decided by its engine, not its badge, and the engine maker’s operator manual says so. Machine specifications rarely do. Of the 51 backhoe loaders listed on DesiMachines, none publishes a heater plug rating, and only the CASE 770 NX Magnum mentions cold starting at all, with a field cold-start kit and starts down to -20 °C. A few excavators name their system in their features: the Bobcat E27 and its E30, E37 (Ultra HD and Xtra HD) and E50z siblings list automatic glow plugs, the SDLG E680Hi and E6135Fi list an engine pre-heater, and the Hyundai R30Z-9AK lists an integrated preheating system.

Engine size gives a rough guide. The JCB 3DX, the backhoe loader most of India calls a JCB, runs a 3.0-litre, four-cylinder JCB 430 engine according to its specifications, which works out to 0.75 litres per cylinder (our arithmetic). The Kubota D1305 in the Bobcat E27 and Hyundai R30Z-9AK is 1.26 litres over three cylinders. Treat that only as a hint, and confirm on the engine itself or in its manual.

How do the preheat lamp, relay and timer work?

A heater plug draws far more current than the ignition switch can carry, so the switch never feeds the plugs directly. It tells a relay to do it. The relay is a heavy-duty electric switch, and on most engines a timer or the engine computer (the ECU) decides how long it stays closed.

Figure 1: A typical preheat circuit with a glow plug in each cylinder

1. Battery: supplies the current. The plugs must be rated for the machine’s system voltage.
2. Key switch: the preheat or heat position, or simply the ON position on engines with automatic preheat, signals the relay.
3. Relay with timer or ECU: closes to send heavy current to the plugs, then opens after a set time or once the engine is warm.
4. Preheat lamp: the coil symbol on the dashboard; it stays on while the plugs heat and goes out when the engine is ready to crank.
5. Bus bar: the strip or wire joining the plug terminals, so all plugs are fed in parallel.
6. Glow plugs: one per cylinder. On an intake-heater engine a single element replaces items 5 and 6.

The plugs are wired side by side, not one after another, so one dead plug does not switch off the rest. That is why a single failed plug makes a cold start rough instead of impossible.

BorgWarner gives the timings for a modern system: preheating lasts about 2 to 5 seconds at normal outside temperatures, longer in the cold. Many engines then keep the plugs on after starting, which BorgWarner calls post-heating, for up to about 3 minutes or until the coolant reaches 70 °C, to cut smoke and noise while the engine warms. If you switch on and never crank, the controller switches the plugs off after a set time.

The habit that matters for an operator is simple: wait for the lamp to go out, then crank. What each dashboard symbol means is covered in our guide to excavator warning lights. And never spray starting fluid into the intake of an engine with preheat; the not-starting guide linked above explains why it can break parts inside the engine.

12 V or 24 V: why the heater plug voltage must match the machine

A heater plug is made for one voltage. Fit a 12-volt plug to a 24-volt machine and it burns out in seconds. Fit a 24-volt plug to a 12-volt machine and it barely warms.

Machines in India use both systems, and the split follows the machine type more than the brand. These counts come from the battery and alternator figures published for machines listed on DesiMachines. Two 12-volt batteries are counted as a 24-volt system, and 17 large excavators whose two published figures disagree are left out.

A backhoe loader such as the JCB 3DX or CASE 770 NX runs 12 volts, as do most mini excavators. A 20-tonne excavator such as the Tata Hitachi EX 210 Infra, carries two 12-volt batteries, the usual sign of a 24-volt system, and so do most wheel loaders. The 8 to 15 tonne excavator class is split, so check that machine’s battery figure before buying plugs.

Voltage is also behind one of the commonest failures. BorgWarner lists operating the plug at too high a voltage, a jump start being the example it gives, as a cause of a broken heating coil. On a site with mixed machines, jump-starting a 12-volt backhoe loader from a 24-volt excavator or tipper can kill every heater plug in it. Jump-start only from a source of the same voltage. Battery and starter checks are covered in the guide to the self motor and alternator.

How does a mechanic test a heater plug and its relay?

Testing goes from the cheapest check to the most work. Do it with the key off and the engine cold, and disconnect the battery before removing anything.

1. Start with the lamp and fuse. Turn the key to preheat. If the lamp does not light, check the preheat fuse and the lamp before anything else.
2. Then the relay. A working relay usually clicks when the key reaches preheat. A test lamp or multimeter on the bus bar should show close to battery voltage for the preheat time and then drop. No voltage means the relay, timer or wiring is at fault, not the plugs. Voltage that stays on with the key off means the relay is stuck, which drains the battery and cooks the plugs.
3. Next, each plug on the engine. Remove the bus bar and measure resistance from each plug’s terminal to the engine body. A reading of open circuit (no continuity) means the coil inside has broken and the plug is dead. A good plug reads a very low resistance; compare it with the value in the engine’s workshop manual and with the other plugs, since one that reads very differently from its neighbours is suspect.
4. A current check is quicker on some engines. A clamp meter on each plug’s feed during preheat shows whether it pulls current like the others. This is the method BorgWarner’s own plug tester uses: it compares plugs by current draw and regulation without removing them.
5. Finally, look at any plug you remove. The tip tells you why it failed. A tip that is melted or broken off usually points to an injector fault, which the next section explains.

On an intake-heater engine the same idea applies to one part: check the relay, then the element’s resistance and current. Wiring faults between relay and plugs are common on older machines; the wiring kit guide covers harness damage.

What wears heater plugs out early?

A heater plug works only for seconds at a time, so a healthy one lasts a long while. When plugs keep failing, something else is usually causing it. BorgWarner’s list of failure causes matches what mechanics find on machines:

• A jump start from a higher-voltage source, or the wrong voltage of plug, breaks the heating coil.
• A relay stuck closed keeps feeding the plugs, and they overheat and burn open.
• A plug made only for preheating, fitted to an engine that also post-heats after starting, fails quickly.
• A dripping or worn injector, or fuel injected too early, overheats the plug’s tip until it melts or snaps. Replacing the plug without fixing the injector means the new one fails the same way. The fuel injector guide covers those faults.
• Too much torque on the plug or its terminal nut cracks or snaps it, sometimes leaving the tip inside the head.

Long, repeated cranking flattens the battery, and a weak battery gives the plugs less voltage, so the next start is worse still. Keep the battery healthy; the battery guide explains sizes and life.

Replacing a heater plug: what to tell the dealer and how it goes in

Plugs are specific to the engine, so give the dealer more than the machine model:

• The engine model and serial number from the engine plate.
• The machine’s system voltage, 12 or 24 volts.
• Whether the engine has a plug in each cylinder or one intake heater.
• The part number on the old plug, if you can read it.

BorgWarner’s advice is to replace the whole set when one fails, because plugs wear out at about the same rate, and once the bus bar is off, doing all of them costs less labour than coming back for the next one in a month.

Removal is where plugs break. If a plug will not turn at normal effort, stop. BorgWarner’s method is to soak the thread with synthetic oil, overnight if possible, then warm the engine and try again gently with a proper socket. Clean the carbon out of the hole before fitting the new plug, and tighten it with a torque wrench to the figure in the engine manual. A snapped tip left inside the head is a cylinder-head job.

The cost moves with the type of heater (a pencil plug or an intake element), the voltage, whether it is made for one engine family only, and whether the relay or timer also needs replacing. A dealer quotes against the engine details above.

When the machine itself is due for a change, compare backhoe loaders and excavators listed in India by engine and electrical specification, or see how two common backhoe loaders differ in the JCB 3DX vs CASE 770 NX comparison. Other engine parts, from sensors to the stop solenoid, are covered in the parts guide.

Sources: BorgWarner (BERU), Technical Information: All about Glow Plugs; Wikipedia, Glow plug. Engine and electrical details are taken from our listings at the time of writing. Specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.