Ask the Mechanic
Tractor Dash Lights: Friend or Foe?
READER: I really rely on the red warning lights on my tractor that let me know if my oil pressure is low or my alternator is not charging the battery. However, I would like to know how these lights are told when to come on or go off. I always enjoy watching them when everything is OK, but I like to see them when the key is on with the engine not running. It gives me a secure feeling (see photo, left).
STEVE: Yes, dash lights are interesting to watch. I find it unusual to stare at something on a tractor that is not working when in use (like dash lights), but I also like to glance at the dash lights when they are working when the tractor is not running with the key on. This is very different from the newer tractors that have a string of lights that can light up to alert the operator that there is something wrong with the tractor. Helicopter lights?
The oil light on your tractor is signaled to come on when the oil pressure is low. Somewhere on the engine, there is an electrical switch (see photo, right) with at least one wire on it. When the oil pressure is OK, the light has no ground because the oil pressure keeps the contact points open, so the light stays off. But, if the oil pressure drops for any reason, the switch creates a path to ground, and the points inside the switch touch, creating a path to ground, so the oil light, which has constant voltage, burns as long as the key is on.
The charge light works in a similar way. The charge light has voltage with the key on, but it needs a path to ground to light the bulb. When the alternator is charging, a little wire from the alternator to the charge light cannot supply a path to ground to the charge bulb because it has voltage. But, when the alternator stops charging, the wire to the bulb has no voltage, which now becomes a ground, so the light comes on. If your charge light is dim, then your alternator is probably going out, which allows the wire to become a ground -- but not a good ground.
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-- Email Steve Thompson care of dan.miller@dtn.com, and be sure to include your contact information and phone number.
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