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Old 05-11-2019, 10:45 PM   #1
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Alternator only outputs 1 hour - Lehman 120

Original Motorola unit, freshly rebuilt from a shop, replaced regulator and diodes.

After approximately 1 hour of runtime the voltage drops from 14v to 12.5. Appears the alternator is outputting current and then does not. Is it overheating? Unit is not hot. Any suggestions?
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Old 05-12-2019, 01:08 AM   #2
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It sounds like a defective regulator/rectifier.

That is not uncommon. It is actually MORE common nowadays than it used to be; offshore manufacturing has provided lots of cheap but lower quality aftermarket parts.
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Old 05-12-2019, 09:10 AM   #3
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Some FL alts have the exciter powered through an oil pressure switch. Lose that power and alt will go silent.
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Old 05-12-2019, 10:51 AM   #4
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Never fix things that aren't broken.
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Old 05-12-2019, 01:16 PM   #5
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Quote:
Originally Posted by Ski in NC View Post
Some FL alts have the exciter powered through an oil pressure switch. Lose that power and alt will go silent.
This was the first item I tested. Pressure switch is good and has power. Low OP buzzer is wired to same switch. Is there some type of temperature sensor in the ALT that would cut power, possible this is faulty?
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Old 05-12-2019, 01:18 PM   #6
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Never fix things that aren't broken.
Yes, scope creep is my life. This one was a fail of my own creation, switched the battery selector while the engine was running, fried the diodes and or regulator, requiring the rebuild.
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Old 05-12-2019, 01:19 PM   #7
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Originally Posted by Civilitas View Post
It sounds like a defective regulator/rectifier.

That is not uncommon. It is actually MORE common nowadays than it used to be; offshore manufacturing has provided lots of cheap but lower quality aftermarket parts.
I guess I'll pull the alt and return to the shop for an extended load test.
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Old 05-12-2019, 01:21 PM   #8
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Low OP alarm switch is likely not the same switch the powers alt exciter. Alarm switch should be NC contacts, alt NO contacts. NC means normally closed, NO means normally closed, referring to state of contacts with no pressure. Shelf state.
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Old 05-12-2019, 02:05 PM   #9
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Here is the FL 120 schematic.
The low OP switch powers the alternator.
Attached Files
File Type: pdf FordLehmanEngineElectricSchematic.pdf (49.9 KB, 20 views)
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Old 05-12-2019, 05:47 PM   #10
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Quote:
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I guess I'll pull the alt and return to the shop for an extended load test.
There are different ways these can fail. If it is in the rectifier portion, a quick test is to look for AC voltage leaking through. Put your meter in ac voltage mode and ground lead to ground and hot lead on the battery terminal on the alternator. Should be less than 0.5V of AC.
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