VM reads 8V - Alter. tests good + new Battery: VM reads 10V? - TCCoA Forums
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post #1 of 10 (permalink) Old 06-12-2018, 09:08 PM Thread Starter
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VM reads 8V - Alter. tests good + new Battery: VM reads 10V?

Perhaps the forum can help me. My 96TB lights suddenly went dim at night and the Voltmeter read 8V. I immediately assumed it was the Alternator. I had it tested at AAP and ORs and both said it was good. Battery was 5 years old and I thought it was time to replace. Replaced battery and Voltmeter read straight-up vertical position as normal. Drive car for 15 minutes and the Voltmeter went to 10 volts and read 12.4 volts to battery while running. Is there any temperature related issue that would cause this to happened with an alternator? Could alternator actually be bad, but only when it warms up?

Any advice is appreciated.

Glenn

1996 TB LX, 4.6, 120,000 miles, black with tan interior.
Dark tint and OEM "Sport" spoiler.
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post #2 of 10 (permalink) Old 06-13-2018, 05:54 AM
No, Mr. Lemmywinks, No!!!

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There could be a short that only shorts when it's hot; look st the wires going to the starter, and see if they're damaged.

The alternator could fail like that; I had one die suddenly while driving.

Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
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post #3 of 10 (permalink) Old 06-13-2018, 07:20 AM
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Where are you measuring the voltage? Car starts good?

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post #4 of 10 (permalink) Old 06-13-2018, 04:44 PM
No, Mr. Lemmywinks, No!!!

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Also, make sure the battery is held down; if it's not, look for a welded mark on your hood.

Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.


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Truth Isn't Truth. - Rudy Giuliani, 2018 Award winner, “Most Outrageous Diversion” by MSNBC for this quote
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post #5 of 10 (permalink) Old 06-13-2018, 05:32 PM
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Im not familiar with the later model t- bird electrical connections, but my VM read 8v when my ignition switch was separating from the wire harness. Hope you get it fixed.
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post #6 of 10 (permalink) Old 06-13-2018, 06:48 PM
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An alternator can be bad only when warm. Start by measuring voltage across both battery terminals while the engine is running. You want to see in the range of 14V when running. If you have 14V, and the gauge inside says different, then you have a faulty gauge, but I have never seen one of them fail, so that is not likely. You will likely find around 12V or less at the battery, which means the alternator isn't charging. Next step is keep the ground for the voltmeter on the negative of the battery and put the positive at the main output post of the alternator. If you have 14V coming out of the alternator, but not making it to the battery, you have a problem with the connection from the alternator to the battery. If you have only 12V or less at the output of the alternator, next check shut the car off, unplug the small connector, and check for power on the yellow and light green wires on the small 3-pin connector. The yellow wire comes from the underhood fuse box, and should have power at all times. The light green wire comes from the gauge/battery light, and should have power whenever the key is on. If both of those are good, last thing to check is make sure you have continuity across the white wire that goes from the small 3-pin connector to the single pin connector. If all that checks out good, and it still isn't charging, then you have a bad alternator.

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post #7 of 10 (permalink) Old 06-14-2018, 02:25 PM Thread Starter
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Thanks for everyone's replies. I replaced the alternator and test drove the TB and all seems good. VM needle is straight up.

I previously tested the alternator when the car was hot and the voltage regulator tested bad. The alternator ran fine until the engine warmed up.

Glenn

1996 TB LX, 4.6, 120,000 miles, black with tan interior.
Dark tint and OEM "Sport" spoiler.

Last edited by Glenn; 06-14-2018 at 02:58 PM.
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post #8 of 10 (permalink) Old 06-14-2018, 02:26 PM
No, Mr. Lemmywinks, No!!!

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Get a rebuild kit for $30, and you'll have a new one on the shelf for next time.

Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.


"War Is Peace. Freedom Is Slavery. Ignorance Is Strength." - George Orwell.
Truth Isn't Truth. - Rudy Giuliani, 2018 Award winner, “Most Outrageous Diversion” by MSNBC for this quote
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post #9 of 10 (permalink) Old 06-15-2018, 02:43 AM
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Whats a rebuild kit? I remember last time my alternator went out it was making grinding sounds and sparking.
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post #10 of 10 (permalink) Old 06-15-2018, 09:29 AM
No, Mr. Lemmywinks, No!!!

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That is the sound of bearing failure, if you're lucky, lol.

https://www.amazon.com/Victory-Lap-F.../dp/B000CFKSBO


This fixes everything except a worn slipring.

Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.


"War Is Peace. Freedom Is Slavery. Ignorance Is Strength." - George Orwell.
Truth Isn't Truth. - Rudy Giuliani, 2018 Award winner, “Most Outrageous Diversion” by MSNBC for this quote
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