Diesels are compression ignition engines: they rely on the heat generated from the compression as the cylinder moves to ignite the fuel. The heat that fuel sees is proportion to the speed of the compression. So while a gas engine might start with slow cranking (since the spark plug still fires), a diesel engine is more directly affected. Possible causes of slow cranking:
- Weak battery/batteries
- poor electrical connections in the battery to starter circuit
- Wrong viscosity engine oil (and/or hydraulic oil, on some tractors)
- Starter going bad
- Some diesel engine designs just don't handle cold starts as well. You may have to resign yourself to preheating in cold weather.
Of these, I would start by examining the batteries and electrical connections. It's easy and inexpensive to check. Also, your statement "Put battery charger on it and it shows batteries almost fully charged" could indicate a problem: a battery that has been on a charger for an appropriate length of time should show
full, not "almost full".
Some steps to check the battery:
If your battery is the flooded lead-acid type with removable caps on the caps, open the caps and fill to the appropriate level with DISTILLED water. (Many flooded lead-acid batteries have a sort of "split ring" inside or other indicator: you fill up to that point.)
Charge the battery, let it rest for a few hours and measure voltage. For a 12V flooded lead-acid battery it should measure right around 12.7 volts. If you let it sit overnight, it should still measure close to that voltage the next day (a significant drop in voltage indicates either a parasitic draw on the battery, or internal battery problems). Use a known accurate voltmeter for these tests (The Harbor Freight $6-7 voltmeters are good for rough indications, or indicating that this battery is showing the same it was yesterday, but I've found that they vary from one unit to another, sometimes by as much as 0.5 volts.) If you are concerne dthat your tractor may have some phantom loads, which are discharging the battery when not in use, you may want to try this test with the battery disconnected.
Measure voltage while cranking the engine. If it drops way down (like 10.x volts on a 12 volt battery) that's an indication of a problem: it could be in the battery or the connections.
A battery that drops to +/- 11 volts or so just while resting for a few days likely has a shorted cell. That battery is probably junk.
Letting a battery sit in a discharged or partially discharged state for an extended time causes sulfation in the battery, lowering battery output and shortening it's life. Heavy sulfation can sometimes mimic the symptoms of a bad cell. A You can try putting a charger/maintainer on the battery that has a desulfation feature. I've had luck recovering some moderately sulfated batteries with this. Note that if the battery has significant sulfation, it can take a week or more of desulfating it to really make headway in recovering it, if it can be recovered at all.
If you have a two battery system, you should always replace the batteries at the same time. This is true whether your batteries are connected in parallel (still a 12V system) or in series (creating a 24V electrical system).
Electrical Connections:
Poor electrical connections can cause your batteries not to charge fully from the tractor's charging system. Chronic undercharging causes sulfation (as mentioned above) and shortens the life of the battery.
Clean the top of the battery: sometimes the accumulation of acids, oils, dirt, etc forms a conductive path between the posts, causing a slow drain on the battery even when not in use. Even just wiping it clean with a rag or paper towel can help. I sometimes wipe it down with a mix of baking soda and water, which neutralizes any acids which may have collected on the top of the battery (sometimes the battery will outgas acid fumes when charging). If you see things fizzing while doing this, that's an indication you had residual acid on the top of the battery. NOTE: be VERY careful not to get any of the baking soda and water into the battery cells.
As others have already mentioned: Clean the connections at the batteries and at the grounds. Check where the negative cable on a negative ground tractor attaches to the frame, and look for other areas where electrical connections are attached to the frame. Also look at the positive connections. Pay special attention to connections at the batteries, starter, and alternator or generator. After going through the connections, charge the battery and try the tests mentioned above to see if things have changed.
Hydraulic and engine oil viscosity:
One of the first things I do when buying used equipment is change all the fluids and filters. You never know what the previous owner used or whether they actually replaced them at appropriate intervals. (In RARE cases, I'll make an exception to replacing fluids if I know the previous owner personally, and know they are the type to take meticulous care of their equipment.) Find out what viscosity is recommended for your make/model tractor in the temperatures you expect to see in your area. If your tractor is old enough that it recommends single viscosity oils, consider substituting a modern multi-viscosity oil to cover the temperature range you expect to see.
Starter issues:
I went through all the steps I could think of when I had trouble starting my 20+ year old tractor a couple of years ago. It had first developed issues starting in colder weather. My block heater had died, but the hard starting was happening even in temperatures where I had not needed to preheat in the past. For a time, I could overcome this by cycling the glow plugs a couple of times before starting. Eventually, the temperature at which it would start crept up to the poitn where I was having to crank the engine for significantly longer, and sometimes multiple glow plug scycles still resulted in difficult starting. I was stumped, so I brought it in for service. They found that the starter was going bad and cranking too slowly. The engine wasn't generating enough heat at the slower cranking speed to ignite the fuel (not an issue in warmer weather, and that's why the multiple glow plug cycles sometimes helped). They replaced the starter, and it's been starting easily ever since. The starter had deteriorated gradually enough that I had not really noticed the difference in cranking speed. I could certainly tell the differenc ewhen they replaced it.
