Alternator Intermittent Charging

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Jun 14, 2024 | 07:08 AM

NM33013

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Alternator Intermittent Charging

Hello, I have a 1990 F150 with the 300 6 cylinder and 5 speed manual. The alternator is intermittently charging. I started the truck, drove almost 20 miles and all was fine. Went to the store, on the way home, the amp gauge dropped into the lower range for a while with the battery light on. It then jumped back up and started charging again...only to drop again. What is the issue I'm looking for? Is it the alternator itself...voltage regulator maybe? The truck has been in the family since new and just turned over 450,000 miles. Not sure if the alternator has been changed in a long time.

Jun 14, 2024 | 08:47 AM

My4Fordtrucks

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Joined: Jun 2015

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Quote:

Originally Posted by NM33013

Is it the alternator itself...voltage regulator maybe?

Yes? Can you duplicate the problem with the truck parked? Does the voltage drop when checked with a DVOM? You could take the alternator off and have it tested.

Jun 14, 2024 | 03:11 PM

Prototypemech

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Joined: Nov 2013

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From: Mojave, Ca

I suspect your alternator brushes are worn. They're spring loaded to touch the slip ring. As they wear, the spring can't provide enough tension to make a good contact and the system stops charging. At a certain point in the brushes life, it makes contact when the alternative is cold, but as it warms up and expands 0.001", the brushes move away and it stops charging.
The good news is that the brushes are cheap and easy to replace, but at 450k miles, I bet the bearing in that alternator are toast too. Maybe just get a new high quality (not AutoZone) alternator and replace it. If it's not a 3g alternator, you should upgrade to the 3g and get the better performance.