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Hey Engine Man – My Tractor Is Very Hard-Starting!

Successful Farming Engine Man Ray Bohacz has engine grease and field dirt under his fingernails from a life spent repairing vehicles and running a farm. When he's not busy in the shop, he's working on maintenance articles and videos for Successful Farming magazine and answering questions from readers.
The following is an email Bohacz received from Jordan Anderson:
I have a 1978 John Deere 4240. It is very hard-starting and always has been. I have tried a new starter, new batteries, and I've installed a hydraulic pressure release.
None of these have aided very much. Do you have knowledge that may help me?
Response from the Engine Man:
When you say hard-starting (and based on the parts that you changed), I am assuming that the engine is cranking over slowly, or poorly. If that is so, then it is a completely different problem than an engine that cranks over at a good speed but does not want to fire.
The most common cause for slow or hard cranking is a poor ground circuit or bad electrical connection. With your John Deere, I would tend to think that it is weak ground either at the cable or where tractor ground/starter mounts.
A tractor, in the same manner as a car, uses a DC electrical system. That means it has a voltage supply and a ground. The ground circuit is just as important as the voltage supply since it is the rest of the pathway. The proper way to check a ground circuit is with a voltmeter (not an ohm meter or just by looking at it). The test is called voltage drop.
A down-and-dirty method not using a voltmeter would be to use a single jumper cable. Move it from the starter housing to a good, clean ground and then to the battery negative, etc. If the engine cranks better, you know it's a ground issue.
Please keep in mind that with a diesel, the ground circuit is challenged the most when starting the engine. After that, there is very little electrical load, unlike a gas engine in a car or truck. Thus, it is easy to get fooled by a weak ground since no other problem is apparent.
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