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CAT 287B Alternator is good, but isn't charging

Joined
Nov 17, 2019
Messages
5
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Homer ak
CAT 287b, unknown year (I think around 2010?) Serial number ZSA02652
Battery won't charge.. New battery, alternator tests at 14v output on the bench, but once it's in the machine it doesn't charge the battery. If I disconnect the alternator from everything while the machine is running, I only get 1.5v output.
On the back of the alternator there is a three wire plug, two yellow and one green. The only one that doesn't have power when I put it to a multimeter is the green one, which I believe is just for the hour meter?

What do I do? All the connections on the battery and alternator are good, the battery is new and the alternator charges fine outside the machine. Belt is tight. HELP!
 

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hosspuller

Senior Member
Joined
Aug 27, 2014
Messages
1,873
Location
North Carolina
As a general rule … Alternators need a bit of voltage to its field coils to start charging. Then it self excites and the field wire is disabled. Until a CAT guru posts, Check your wiring again. Seems that one of the wires should have battery voltage when you crank the engine.
 
Joined
Nov 17, 2019
Messages
5
Location
Homer ak
Alternators need a bit of voltage to its field coils to start charging. Then it self excites and the field wire is disabled. Check your wiring again.

I've checked as much as I can see and don't see anything wrong. Is it logical that it's somewhere in that wiring harness?
 

hosspuller

Senior Member
Joined
Aug 27, 2014
Messages
1,873
Location
North Carolina
I've checked as much as I can see and don't see anything wrong. Is it logical that it's somewhere in that wiring harness?

Perhaps a relay or even a light bulb. Some equipment use the charging light bulb in series to back feed the voltage to the coil. When the alternator starts charging, the light goes out since the alternator voltage is equal to the battery voltage. If the bulb is burned out, it never passes the initial voltage to the coil. Does the charging light come on when you turn the key on ? (before cranking the engine)
 
Joined
Nov 17, 2019
Messages
5
Location
Homer ak
Perhaps a relay or even a light bulb. Some equipment use the charging light bulb in series to back feed the voltage to the coil. When the alternator starts charging, the light goes out since the alternator voltage is equal to the battery voltage. If the bulb is burned out, it never passes the initial voltage to the coil. Does the charging light come on when you turn the key on ? (before cranking the engine)
Great idea, I remember before the battery issues started the light would flicker while running the machine- I assumed this was because of dying voltage? But if it was the bulb getting screwy, that might make sense. I'll check that asap
 

kshansen

Senior Member
Joined
Mar 11, 2012
Messages
11,269
Location
Central New York, USA
Occupation
Retired Mechanic in Stone Quarry
Your 308 yellow should have battery voltage to it at key-on. Don't trust your DMM for this, use something like a headlight bulb as a test light to see if the circuit can supply enough current.
Very good point, a DMM will read voltage through a connection less than the thickness of a hair and have you thinking the circuit is good and when asked to carry even say 1/4 amp load it can fail!
 
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