[Baba O'Riley]

My ‘71 Formula 400 needs a new alternator. Starts are slow and groaning despite a new battery and starter, lights are weak, and last night it wouldn’t restart after being driven 20 minutes. All power cut out, even the dome light, though that came back on after a couple minutes. Car wouldn’t jump from another battery but finally started, strong and fast, off an inverter. While running the voltage gauge shows just under 14V with headlights on, just over 14V with them off.

The original alternator was long gone when I got the car in ‘92, and what’s on there now is a Prostart 7127 piece of junk. I don’t show the car so I’m not into paying crazy $ for anything “correct.” Just something that works well and has a reasonably stock look (no chrome). Car is AT, it had air originally but AC is disconnected now. What would be a good replacement brand and, especially, good amperage for this car? What about the Powermaster units, are those okay? They sell units that turn 85, 100, and 150 amps. Thanks as always for any help, the expertise on here has saved me many times over the decades.

[unruhjonny]

have you attempted rebuilding it?

You might be REALLY surprised at just how ridiculously easy it is to rebuild one.
Just grab a paper clip - that's the secret tool to keep the copper connectors in place when re-assembing.

Also, snap a couple pictures so that you close it up clocked correctly.

[Baba O'Riley]

Thanks UJ, but it's just not a quality unit, it's never really performed well... a mechanic I tried on some other electrical stuff put it on as sort of a throw-in and he cheaped out, I want something better made.

[Stuart]

Originally Posted by Baba O'Riley

While running the voltage gauge shows just under 14V with headlights on, just over 14V with them off.

I'd like to see maybe 14.5 volts, but what you're measuring is only a few tenths lower and indicates the alternator is in fact charging. Are you sure the battery is OK?

[stellar]

Originally Posted by Stuart

I'd like to see maybe 14.5 volts, but what you're measuring is only a few tenths lower and indicates the alternator is in fact charging. Are you sure the battery is OK?

I would have the battery tested before changing the alt in this situation.

[Baba O'Riley]

Battery is new and tested fine last night.

[Sirrotica]

Bad, dirty grounds, or connections.

When someone bolts on everything under the hood pertaining to the charging/starting system, and you're still having problems, it's usually wiring, switches, or connections.

No one wants to take time to check them by disassembling the connections. They look at the outside of a connection, and deem it good from appearance. I can't tell you how many cars I've repaired that are just what you're describing as a mechanic in the past 50 years. Especially suspect wiring/switches that are 50 plus years old. VOM (volt ohm meters) will help you find the malfunction, rather than throwing parts at it.

[1965gp]

I have put this alternator on my 66 GTO, 70 GTO and 72 Firebird. Bolts right up, uses factory connectors and has extra power to accommodate auxiliary fans, gauges, stereo, vintage air, etc. so far I have been very happy with them.

It’s usually available next day at O’Reily.

[78w72]

as mentioned above, the sysmptoms you describe doesnt sound alternator related, with a new battery & starter the alt doesnt really have anything to do with starting. being it shows 14 volts tha alt is likely working fine. need to check all wires, connections, including grounds.

the engine should easily run with a bad alt for 20 minutes with a new fully charged battery. i had a an alt go bad on my daily while towing a car back from 3-4 hours away in the middle of the winter at 10°, drove for almost an hour on the 5 year old battery with all lights & the heater turned off (brrrrr) probably could have made it the other 45mins to an hour but decided to stop at an auto zone & buy a new battery off the shelf, installed it & drove the rest of the way home with no issues & restarted the engine a couple more times the next day, could've driven another hour easy.

i dont think its your alternator causing those issues with a new battery, if its good & charged you could have driven more than 20 mins with a bad alt & not had total loss of power or not even started when jumped.

[Formulabruce]

Bad connection at starter, and or negative cable to head ground. Also rear pass side of head to firewall should be checked.
Thick stranded wire can get "Glove syndrome " with corrosion on each strand reducing the amount the wire can conduct. I always solder them before replacing the ends. Its not easy, but worth it.
Your voltage seems very close.

[Baba O'Riley]

Okay, I think I'll pull the alternator and have it tested, then if it checks out okay I'll begin testing connects. Thanks very much to you all for your experience and expertise, have a great weekend with your Formula, Esprit, Base, or T/A.

[Sirrotica]

FWIW, just to add to my experiences, and as Bruce has already mentioned. The head bolt with the stud on he LF corner of the engine is especially susceptible to a corroded/loose connection on the ground circuit. Even though it may look fine, I have found many intermittent problems, and slow cranking problems from taking apart this particular connection, cleaning it and making sure the serrated washer is still there.

Because it is subject to a lot of heating, and cooling cycles the connection seems to loosen up, leaving room for moisture to rust the steel parts, causing high resistance. It may be fine for low demand on the system, but when loaded with the starter drain amperage, it becomes a bottleneck, and high resistance at the connection causes no, or slow cranking.

One other problem area of the early 70s cars is an ignition switch with bad contacts, causing intermittent starting problems. these were showing problems in the 70s when the cars were still fairly new. There are neutral safety switches, and if the car is a standard shift a clutch safety switch, all points that can cause problems in the circuits with starting.

[Ramairnacho]

Dude start car and disconnect battery if it runs still alternator good or use a voltage meter on battery to see if it charges. And 1 have a few open face 928s if you want to be correct

[Ramairnacho]

If you just want an alternator lifeti.e warranty Uto zone alternator is what I would do for u.der$100.ha!

[Sirrotica]

Disconnecting the alternator while the car is running it a very good way to ruin a functioning alternator, seen it done back in 1970 on a Jeep. It ruined the alternator. It doesn't always blow the diodes out, but it certainly can. This was a Prestolite alternator, and it was functioning before the hack mechanic pulled the cable off. The dealership had to eat the repair, because the Jeep came in for a tune up, the charging light wasn't on, until the guy pulled the battery cable off while the engine was idling, trying to show off how he tested alternators without any instruments.

A much safer field test is to use a thin piece of steel like a hacksaw blade, and place it at the bearing end of the stator shaft. If there is a magnetic pull, the alternator is charging, if no pull then it's not. No chance of ruining a perfectly good alternator by blowing the diodes.

[Ramairnacho]

I believe volt meter at battery is best way

[Sirrotica]

Not every car has a voltmeter, and I worked on all kinds of construction equipment that didn't have voltmeters on them either. The easiest way to test them was to just see if they were making the magnetic field at the end of the alternator with a screwdriver, etc.

If you want to know at what rate it's charging, then you need to get out your VOM, and hook it up.

[Ramairnacho]

Originally Posted by Sirrotica

Not every car has a voltmeter, and I worked on all kinds of construction equipment that didn't have voltmeters on them either. The easiest way to test them was to just see if they were making the magnetic field at the end of the alternator with a screwdriver, etc.

If you want to know at what rate it's charging, then you need to get out your VOM, and hook it up.

Please don't be upset with me just trying to.help. and that's interesting testing them magnetic field. I never heard of that before.

[stellar]

Testing with a hacksaw blade or a screwdriver at the alternator needle bearing tells very little. If there is no magnetism then it will not be charging but it doesn't mean the alt is bad. If there is magnetism it doesn't mean the alt is good or charging correctly. The only it tells is if the rotor in the alt is active.

[Sirrotica]

As I said, when you get the magnetic field, then you get the VOM out and check the rate, and components. If there is no field you at least know something in the charging system is awry.

It's a field test to see if the charging system is capable of putting out current, not a thorough determination of all components.

Pulling a battery terminal off while it's running isn't definitive either, just a field test that can render a good set diodes junk, needing replacement. The point was that the rotor field test won't harm the alternator...........