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Poorly starting tractors - not just a battery issue
IF a tractor is a persistently poor starter, but you are fairly certain the battery is fine then it is possible the problem lies with alternator failing to charge correctly. Mick Roberts reports.
There is nothing worse than turning the key on a frosty morning to be greeted by just an ominous knock or click from the tractor’s starter. It is particularly irritating if you invested in a new battery which has failed to cure the starting woes.
“It’s worth noting that a ‘lazy’ battery is unlikely to start a diesel engine,” says John Dixon, Workshop manager and lecturer at Wiltshire College, Lackham, near Chippenham.
“Unlike a petrol engine, which can start on one turn, a compression ignition system requires about a 100rpm cranking speed to create the cylinder temperature of 500degC to ignite the fuel.”
Sometimes a battery will lose charge overnight due to a ‘parasitic load’ from a faulty electrical item draining power. This can also be caused by an internal fault in the alternator, which is connected permanently to the battery.
“If this is the case, the alternator will be hot to touch in the morning - check this first.
“If the alternator is cold and the battery is good, but not being charged, the next logical step in the fault-finding process has to be to check the alternator is charging and the power is getting back to the battery,” says Mr Dixon.
“With the help of a low-cost multimeter and, ideally, an ammeter, it will take about only 10 minutes to check the charging circuit. But, before doing anything else, check the fan belt is in place and tensioned correctly.”
Where to start
The test process starts with the battery - even if it is new or you think it is fine. Set the multimeter to the 20V range in DC - indicated by a letter V, followed by a long dash with three dots below - and place the probes across the terminals with the red to postitive (+) and black to negative (-). Check the voltage - it should be at least 12V.
Now, leaving the probes in place, start the tractor and the multimeter reading should rise to a charging voltage of about 14 to 15V. If this does not happen, it is not being charged, so we need to find out why - starting with the obvious things first.
“Check battery connections are clean, tight and the earth (negative) strap from the battery is attached securely. Check all the wiring and connections, particularly the ‘Euro Plug’ and terminal at the back of the alternator,” says Mr Dixon.
If all this is okay, attention then turns to the charge light warning circuit. This is crucial for charging because it is through here the alternator receives its initial ‘exciter’ current, without which it cannot function.
Checking this requires a small ‘jumper’ lead with a male Lucar terminal crimped to one end of a piece of wire. Plug this into the small D+ slot in the Euro Plug and touch the other end to earth. With the ignition turned on, the warning lamp should light up. If it does not, it indicates there is a fault in the charge warning light circuit.
“But, and it’s a big but, the bulb could be blown and in many older tractors this will stop electricity flowing through the circuit.
“Often changing the bulb requires a huge amount of work to get at it, so it’s best to eliminate all other possibilities before firing the dash apart,” says Mr Dixon.
“We can test the alternator is actually charging using the multimeter in combination with an ammeter. This involves the ‘jumper lead’ with the addition of a female Lucar connector at the other end. This is used to connect the D+ terminal on the back of the alternator to the corresponding D+ terminal on the Euro Plug (which is removed for this test).”
Probe
The multimeter is connected in a parallel circuit with the red (+) probe attached to the large B+ terminal on the alternator and the black (-) probe to earth. The ammeter needs to be connected now, in series, with the red (+) lead on to the B+ connection on the alternator and the black (-) lead into the B+ terminal on the Euro Plug, so it becomes part of the circuit.
Start the engine - increase the revs and the voltage shown on the multimeter to rise to between 14V to 15V and then remain steady about at 14.5V.
With the engine running, the ammeter should show a current of between 20 to 40amps, depending on the charge state of the battery. As the alternator continues to charge, the ammeter should start to drop back as the battery regains its charge.
Now (with the engine still running) turn on the lights and other electrical items to see how the alternator responds to the load. At the same time, check the charge warning lamp on the dash. If this begins to ‘glow’, there is a faulty diode in the alternator’s rectifier.
The ammeter should rise again. Conversely, the ammeter gauge should fall as the individual loads are turned off.
“If the ammeter shows the alternator isn’t responding to the load it could indicate a high resistance fault, which is likely to be caused by a bad earth, poor connection or broken or damaged wiring,” says Mr Dixon.
“This will cause the alternator’s rectifier to reduce charging output too soon, because it incorrectly equates the high resistance is being fed back from a fully charged battery. This means the battery will never be charged to its full capacity.”
If the battery still struggles to start the engine, it could be the wrong ‘Type’. Check the battery is a SLI (Starting, Lights and Ignition) type with the correct cold cranking capacity (CCA).
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