Tractor batteries and winter starting

A tractor that fired up on the button in October can refuse flat in January without anything having been touched in between. It's almost never a component that gives up overnight: it's a margin that has disappeared. A tired battery, a corroded earth and thick oil don't show at 15 °C, because there's still reserve in hand. At -10 °C, there isn't any left.

Cold attacks from both sides at once, and that's what makes the diagnosis misleading: the battery loses a large part of its available power while the engine demands two to three times more torque to be turned over. Here's how to take the problem apart in the right order.

Two curves that cross

A lead-acid battery doesn't store less energy when cold: it gives it back more slowly, because the chemical reaction slows down and the internal resistance rises. In parallel, thickened oil doubles or triples the drag on the starter. And a diesel needs a minimum cranking speed, of the order of 150 to 200 rpm, for compression to be enough to ignite the fuel. Below that it turns, but it won't fire.

A battery at 70 % state of health gets through the summer without anyone noticing, and becomes useless at the first real frost.

Size it in CCA, not in amp-hours

The amp-hour describes endurance, not starting. What counts here is CCA, cold cranking amps: the current the battery delivers for a given time at -18 °C while staying above a floor voltage. That's the figure printed on the label, next to the test standard.

• Never go below the original value. CCA is sized for the displacement, the compression and the starter. Going above is risk-free as long as dimensions, terminals and technology are compatible.
• The exact value is on the original label or in the manual. A 25 hp compact and a 100 hp utility tractor aren't in the same ballpark, and big machines often run 24 V or two batteries in series.
• Two batteries in series form a set. You don't replace just one, and you don't mix ages or technologies: the weaker one imposes its behaviour and destroys itself within weeks.
• The clamp isn't decoration. An agricultural machine vibrates enormously; a poorly secured battery sheds its plates in a single season.

Glow plugs: understand before you keep cranking

On a direct-injection diesel, the glow plugs — one per cylinder — create a hot spot in the chamber. On older or larger engines, it's an intake heater in the manifold. Same logic, same symptoms when it fails.

• Bad starting cold, fine when it's mild: the most reliable sign of a failing preheat system.
• Thick white smoke and misfiring for the first 30 seconds, then everything settles: unburnt fuel, so chambers that aren't hot enough.
• A warning light that goes out instantly, or a cycle of variable duration: suspect the glow relay before the plugs. It's what fails most often, and by far the cheaper part.

Testing is done with a multimeter: a healthy plug shows a very low resistance, of the order of an ohm, and draws a current in the tens of amps; an open-circuit plug reads infinite resistance. On a four-cylinder, a single dead plug is enough to make starting a struggle in hard frost.

Aerosol starting fluid: why you don't

This is the get-you-going trick that does the rounds of every farmyard, and the one that breaks the most engines. Those aerosols contain an ether-based mixture that self-ignites at a far lower temperature than diesel — and that's precisely the problem.

• Ignition arrives far too early. On an engine with glow plugs or an intake heater grid, the hot spot is already there. The ether lights the moment it meets it, so well before top dead centre, while the piston is still rising: the combustion pushes the wrong way.
• Peak pressure goes through the roof. This is no longer progressive combustion but detonation. Classic outcome: a bent or broken con-rod, broken rings, a head gasket, a glow plug torn out. The tractor starts once, the damage shows up later.
• It masks the real fault — low compression, dead preheat, waxed fuel, tired battery — which carries on getting worse in the meantime, and the ether washes the oil film off the cylinder walls as it goes.

Some engines without preheating are fitted from the factory with a metered starting aid: a calibrated device that injects a measured quantity at a precise moment. That has nothing to do with an aerosol sprayed into the air filter. The rule: if your engine has glow plugs or an intake heater grid, ether is out of the question, no exceptions.

Diesel waxing up: what actually blocks

Diesel contains paraffin waxes. As the temperature drops they crystallise: the fuel goes cloudy, the crystals grow and clog the filter. It isn't the tank that solidifies, it's the filter that blocks. The characteristic symptom: the engine starts, runs for a minute or two, loses power and stalls.

• Fuel sold in the cold season is already treated to hold up lower than summer fuel — although the blend and the grades on sale vary from one country to another and from one season to the next. A tank filled in September and never used is still sitting on summer-grade fuel.
• Running with a full tank limits overnight condensation, and so the free water that freezes at the bottom and in the pre-filter.
• Drain the water separator before winter and then regularly: often a clear bowl with a quarter-turn screw, two minutes' work.
• Fit a new filter before the frosts, and check the heated pre-filter if the machine came with one before you blame the fuel.

On additives, the principle is simple: agents that change the shape and size of the wax crystals so they pass through the filter instead of matting up. Two conditions that get ignored: add them before the fuel goes cloudy, into diesel that is still clear and if possible warm, and stick to the dose — overdosing doesn't take it any lower. Poured into a tank that has already gelled, an additive does nothing. And above all, never cut your diesel with petrol or paraffin: the high-pressure pump is lubricated by the fuel itself, a less lubricating mix seizes it, and that's the most expensive repair on the whole system.

Cables and earths: where the current gets lost

A tractor starter draws several hundred amps. At that current, a parasitic resistance of a few hundredths of an ohm collapses the useful voltage. It's the most common fault, the cheapest to put right, and the one people look for last.

The test that settles it is a voltage-drop measurement, multimeter on volts, while somebody operates the starter: on the positive side, between the battery + terminal and the starter feed, aim for less than 0.5 V; on the earth side, between the - terminal and bare engine block, less than 0.2 to 0.3 V. Above that, you've found your problem: a sulphated terminal, a corroded earth strap, or an earth path that runs through a rubber-mounted engine bracket — a classic case, which calls for a dedicated strap. Brass brush, a firm tighten, a thin film of grease: that's what you do before buying a battery, as covered in the classic reasons a tractor won't start.

A battery on a machine that sits

A tractor used only for topping and snow clearing spends most of the year standing still, and that's where the battery dies. Self-discharge runs at around 3 to 5 % a month, on top of which come the standby loads — control units, clock, alarm — quite capable of emptying a pack in one to two months.

The point few people know: a discharged lead-acid battery freezes. The electrolyte in an empty pack is close to water and turns to ice at around -10 °C, which distorts the plates and cracks the case. Charged to 100 %, the same battery holds up a very long way below that: keeping it charged is also the only protection against freezing. A trickle charger costs £25 to £60 and easily doubles the service life; a battery isolator kills the standby drain for about £20.

The procedure at -10 °C

1. Terminals and open-circuit voltage first. Below 12.4 V you recharge, you don't crank.
2. Ignition on, full glow cycle to the end, light out, before you crank.
3. Depress the clutch so you're not dragging the congealed oil in the gearbox — on many machines it's also a mandatory safety interlock.
4. Crank for 10 seconds maximum, then wait 30 to 60 seconds: short spaced attempts beat one long one, the starter heats up and the battery needs to recover.
5. Three attempts without success, you stop and investigate. Persisting flattens the battery and cooks the starter.
6. If it fires, hold it slightly above idle for a minute or two with no load, then work it gently while the hydraulics and transmission come up to temperature too.

The rest of the winter prep is in the tractor storage checklist and in the seasonal maintenance checklist. For the values specific to your machine — original CCA, glow duration, plug testing, oil grade for the ambient temperature — L'Atelier's technical assistant gives them from the make, model and year, rather than leaving you to guess off a neighbouring engine.