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Old 04-05-2022, 09:23 PM
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Default Is my alternator shot? (‘70 TA)

I took my ‘70 Trans Am for its initial spring drive today and noticed that the voltage gauge in the dash was hovering around 13 volts. I put a voltmeter on it and it was a tick over 13 at idle. The needle never moved even when I tried running it around 1500 RPM. Is it time to have the alternator rebuilt?




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Old 04-06-2022, 01:18 AM
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Fully charge the battery. Make sure the belt is tight and retest.

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Old 04-06-2022, 06:44 AM
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Voltage regulator?

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Old 04-06-2022, 10:04 AM
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Bill Hanlon Bill Hanlon is offline
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What happens at that same 1500 RPM with the headlights on and the heater fan running full blast?

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Old 04-06-2022, 10:29 AM
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I have the same Craftsman engine analyzer I bought in 1987.

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Old 04-06-2022, 10:42 AM
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Assuming an accurate meter, a voltage above 12.6 at the battery indicates the alt is charging, 14V is ideal, If the wiring is correct. If you have a mechanical regulator, they can be adjusted slightly to provide 14 volts. If it's an internal regulator, replacing it should improve the charging voltage.

As Bill mentions, if the voltage is constant under load, the system is regulating, altho lower. IF voltage drops with load, could be a slipping belt.

George

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Old 04-06-2022, 07:45 PM
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I think 70 TA still had a external regulator, but if you do have an alternator with a built in regulator, a 1-wire alternator (or 3 wire with the sense winding connected directly to the output stud) instead of a properly wired 3 wire alternator can cause low voltage back under the dash.

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Old 04-06-2022, 08:08 PM
'ol Pinion head 'ol Pinion head is offline
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70 V8 Firebirds used one of 3 internally regulated alternators. 5 spoke open face front case 10SI. Earliest factory installation of an internal regulated alt on a Pontiac is the 1100800 on '68 Grand Prixs. Somewhat of a unicorn, have one in the trays of original alternators.

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Old 04-06-2022, 09:05 PM
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Thanks for feedback. Battery was on a tender all winter but I guess it’s possible that didn’t ‘fully charge’. When I took that photo, I had taken the car out for about a 30 minute drive. I didn’t see the needle move at all when I gave it more gas, but I don’t know if I revved it up to 1500 RPM. I’ll give that a try. Like OPH stated, this is alternator has an internal regulator.

If internal regulator is bad, can it be rebuilt. I’m pretty sure this is the original alternator case, or at least a date-correct one. I didn’t restore this car, but Gary (previous owner) was pretty fanatical about keeping things original.


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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone.
The frogs take up where they left off.
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Old 04-06-2022, 09:09 PM
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Default Is my alternator shot? (‘70 TA)

As George mentioned, I’ll also check the belt. I kind of remember the dash voltmeter gauge reading around 14 last year, before putting car away, which is what triggered me to break out my ‘classic’ Sears Engine Analyzer. It does appear that the dash gauge and the Sears unit are both showing same results.


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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone.
The frogs take up where they left off.

Last edited by racerboy; 04-06-2022 at 10:02 PM.
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Old 04-21-2022, 09:49 PM
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Subscribing. I have a very similar issue on my 69 bird.

PowerMaster 100 amp internally regulated alternator
I have run it as a 1 wire and currently being run as a proper 3 wire with inline diode on the excite wire. No change in voltage with the different wiring setups.
When I first start the car (battery known charged and less than 2 years old) I will see about 13.3 volts. This slowly degrades down to between 12.9-13.1 volts after about 10 minutes or so. It'll stay in that range from there.

The voltage stays stable regardless of the load. Lights on and off, electric fan running or not running, stereo on or off. I too am thinking bad alternator.

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