Time for alternator?
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Elder User
Joined: Jan 2011
Posts: 951
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From: Center, Texas
Time for alternator?
My dad has a 95 Ford F150 5.0 with 270k on the clock. He has owned it for the last 170k so the alternator has at least that many miles on it. Over the last couple of days he said his dash battery meter would go low and then back up and today his battery light came on. We put the volt meter on the battery during cranking and running and the battery tests 12-12.2 key off, drops to 10.99 during cranking and then 11.99 running but revving around 2k it would go to 12.2. We arent familiar with alternator testing but we know it should be putting out around 14 volts. What we dont understand is that wouldnt the battery gradually pull down instead of holding in the 11.99-12.2 mark if it wasnt charging at all? Are we incorrect in checking on the battery itself or is there some way to put the volt meter on the alt itself. Thanks.
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Joined: May 2013
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Simple answer - a working alternator will always put out more voltage than the battery at rest.
If you are reading 12.2V @2000 rpm then your alternator is bad - for whatever reason. Have it bench tested before replacing it in case there's a wiring issue.
If you are reading 12.2V @2000 rpm then your alternator is bad - for whatever reason. Have it bench tested before replacing it in case there's a wiring issue.
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Elder User
Joined: Jan 2011
Posts: 951
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From: Center, Texas
UPDATE: Got the alternator tested and it tested good. Reinstalled and everything was good. Charging at 13.9-14.3 and battery checked out at 12.4-12.5. Drove the truck and everything was fine. Dad drove it to work last night (30 mile commute one way) and about halfway the started discharging and battery light came on and check engine light would flicker on about every 2-3 minutes. He said you could accelerate and it would go off but if you let off the gas the battery light would come back on and guage would show a discharge. Its cranking fine and battery is still checking 12.4 this morning after the drive home in which the same symptoms occurred. All wires around the battery and fender well where the cables goto alternator look good. Any suggestions? Thanks.
Senior User
Joined: Oct 2007
Posts: 133
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From: Jacksboro / Knoxville, TN
So you took the alt off to have it tested? Advance / Autozone have testers where you can just drive up and they'll test it. You'll have to keep it running at 2000RPM or so while they test.
Anyway, I'd have it tested again. Sounds like it is going bad to me.
Anyway, I'd have it tested again. Sounds like it is going bad to me.
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Elder User
Joined: Jan 2011
Posts: 951
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From: Center, Texas
Update: Took truck to have alternator tested with it installed and it failed, was only putting out around 8 volts or so. Put new alternator off, charging good. Check engine light is still coming on and now the truck is loosing power under acceleration. Gotta find out whats going on there. He has one of those old scan tools that flashes so maybe we can pinpoint something. Thanks.
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