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Hey Engine Man: I Keep Burning Up Alternators on My Combine!

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 a letter Bohacz received from Rob Williams:

I have a 1660 Case combine. The alternator will not hold up on this machine. We been through four (alternators) in the last year! They start out fine, and then their output gradually decreases to nothing. The last alternator we installed lasted just 19 hours. A local mechanic installed a resistor in the line to the alternator. That didn't help. We put in a new battery in 19 hours ago, as well. I have looked for excessive loads, bare wires, etc. I am lost. Can you help me?

Response from the Engine Man:

If I were working on your combine, I would take the failed alternator apart to see what caused the component to go bad. Doing so will greatly help determine where you need to look. I do not know what brand alternator you have or if it has an external or internal voltage regulator. Another key would be to determine how many volts a new alternator put out while charging and before it ramps down and fails. For example, if the combine has a bad ground or the wire feeding the field circuit has excessive resistance, it will overcharge falsely thinking the battery voltage is low. If the condition is excessive, it will enter a state called "full field," which in simple terms means that the voltage to the field circuit will not be regulated (it will act as if the voltage regulator is shorted) and full output from the alternator will be seen. Since the engine rpm will be high while harvesting, a full fielded alternator will burn itself out and can even get hot enough for it to start to melt the solder holding it together inside. When this occurs, the voltage output would ramp down slowly over time (lower and lower output) as opposed to burning out like a light bulb does.

Also keep in mind that on many vehicles the dashboard circuit . . . if equipped with a CHARGE or BATT idiot light  . . . may be wired up to the field circuit. On many Delco applications if the bulb is burned out, the alternator will not charge (this is not your problem). But if there is a short in the printed circuit, it can full field the charging circuit.

Thus, I offer the following condensed advice. Take the now-failed alternator to an auto electric shop and let them bench test it and determine what failed. Then we will know where to look for the problem. If possible, locate a shop manual for the charging circuit for your combine, and then follow the troubleshooting directions for “No Charge,” which is the ultimate result.

This is the best I can do right now with the information I have. We need to act like a forensic doctor and do an autopsy on the alternator to give us a clue.

 

Do you have a maintenance question? Email Ray Bohacz at SFEngineman@Agriculture.com.

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