77 f150
6 cyl
2wd
automatic
hi all, I went out and bought a new alternator, my Ford's got the external voltage regulator,
anyway, swapped out alternators, started it up, and unplugged the pos cable on the battery to see if it would run off the alternator,
nope died instantly
can I assume thats a accurate method to diagnose and if so, if its not the alternator what else could it be?
assuming its not a proper method, how do I check to make sure the battery is recieving a charge?
thanks ahead of time

 

You should never disconnect your battery with the engine running, at the very least it can burn out the diodes in the alternator, at worst it can interfere with the voltage regulator causing the alternator to have a voltage spike and potentially fry everything.

 

The best way to test your alternator is to buy a battery tester. You hook the 'gator clamps to your battery with the engine off (some have a load simulator) and it will tell you if your battery's good or not. Hook it up with the engine running and it will tell you if your charging system is functioning to spec. They're about $30-$40 - good investment if you work on a lot of vehicles.

 

Where did you get that idea to diagnose the alternator that way?

What is your original issue?

First, you did everything wrong.

The positive post is the heart of the electrical system...by removing it, you just killed everything, including the ignition. This is why the engine died instantly.

You are supposed to take the negative cable off the battery. But no matter how, it's wrong. This was an antique way to test the electrical system...with a GENERATOR! When was the last time a car came with one? Almost FIFTY years ago! Why don't do this, you ask? Montana_highboy said it all.

NEVER do this again, ever. Advise your friends, also.

To test the electrical system, all you need is a multimeter. Check the voltage on the battery terminals with a multimeter while the engine is running. It should read 13.75-14.4 volts. Anything lower or over, you have an issue that needs to be addressed.

A battery loader as per crazyeddie's suggestion is also good if you like gizmos.

 

While we find your parts, please enter your ZIP Code at CSK Auto
:]
Alvin in AZ

 

Taking a cable off while it is running will make the regulator think the alternator needs to run wide open to charge the battery and it could actually burn the alternator diodes out.

 

What can I say - you can't put a load on a battery or the alternator with a multimeter

 

That's because it's actually pretty rare for an alternator to put out volts, but no amps. It's easier, cheaper and faster for the average DIY to have a Harbor Freight DMM and test for volts engine off- engine on than it is to to use a load tester.

Josh

 

WOW -Good to know this is not an OK method to see if your alternator is working or not. In years past, I had been told to remove the Neg. Cable and if the vehicle still runs, than you alternator is OK and if it dies, than your alternator is shot. Looks like I'll never try that method again. Good Thread.

 

if you touch a screwdriver to the back bearing cap on the alternator while the truck is running , it will be magnetic if the alternator is good.

however this isnt telling you how good it is but it always let me know what i was checking first, the volt. reg. or the alt.

 

I'm not sure what that tells you. :/
BeenThereSeenThat one day and the alternator was bad anyway! :/
At this point I don't know what the alternator being magnetic means
or doesn't mean. :/

Here's a cool video a guy from Canada put up just for me! :)
YouTube - Alvin in AZ You Are Wrong!
He's an instructor in a automotive class and the regulator ain't
a mechanical type. The "discussion" was too much of a feeding
frenzy to be worth mentioning that fact at the time. LOL :)

I was saying the battery was part of the system, the system
relied on the battery as a voltage "clamp".

Someone care to dulipate the experiment on a vehicle with a
mechanical voltage regulator? :)

Alvin in AZ
ps- I think it's so cool to have my very own video on YouTube! LOL :)
pps- Please rate it 5 stars, if you rate it at all. Thank you. :)

 

For sure, not enough of a load anyway! LOL :)

But. ;)

I've seen guys use those battery testers several times and come back
with the "bad" battery in their hands saying the battery was "good",
when it wasn't! :/ No kidding.

Too many different guys in too many different places to count it all as
"operator error" IMO. YMMV

I tested the battery using just an analog volt meter with the starter as
"the load". The circuit was good because I'd checked the circuit using
a volt meter and had already fixed any opens and/or resistances etc.

I've never experienced a shorted-out starter, so that's another point. ;)

The thing that many don't realize is a battery doesn't fade away near
the end of it's life, it bounces back an forth between "ok" and bad.

-----------------------

Get an analog meter and do this to a vehicle that's working "right"...

1) Put the meter's leads right on the battery's own posts. :)
(not the clamps danggit ;)
Should be about 12 volts.
2) While watching the needle have a helper start the sucker.
The voltage shouldn't go down all that low, say 10 volts.
3) While watching the needle have your helper rev the engine a little.
The voltage should go up above the original reading.
Where it goes to is a matter of the battery's condition and/or
(especially if it's mechanical) the voltage regulator's setting.

Ok. :)

Can you see what the meter would show if a battery clamp was removed?
~12 volts the whole time, right? :)

Can you see what the meter would show if the battery had one or more
"dead" cells in it? Might be ~12 volts until the starter's load was applied,
right? :)

---------

If the clamp is dirty and not able to carry the current the starter needs
to turn an engine over, which is a lot. LOL :)

Can you see how the voltage would be ~12 to start with and when the
starter was engaged the ~12 volts wouldn't go down much, if any, just
like the "clamp off" situation? :)

If you then put the meter lead on that clamp, the voltage would read
~12, then when the starter was engaged it'd drop to near zero (or so).
Can you see it? :)

This is the number one problem I've run across...
http://shop.oreillyauto.com/product_...oss/sk7001.jpg
...the "little cable clamp" on the clamp, not able to carry the current.
Don't take much to ruin that connection.

------------------------

BTW, motor oil works real good at keeping a good connection "good".
The green and red felt washers help especially if they have a few drops
of motor oil soaked into them.

Alvin in AZ
ps- I'm the weird guy that makes a point to show up when someone's got
their hood open and have used a meter many times to find the problem. :)

 

The biggest problem with the "Load" testers is the "magic 8 ball" answer... CHARGE AND RETEST.

Pffftt... who the hell has time for that? If the battery is more than 3 years old and has less than 12.2 static volts (in my opinion, with 12.6 full and 12.4 weak) junk it and buy a new one.

We have load testers at work, from a Harbor Freight special to an $800 Snap-on. Worthless in my opinion.

Josh

 

I don't wanna blow $100 or so dollars on a new battery - I'd rather take the time to charge and test it before just tossing a functional battery. But ok, so leave the "load test" out of the picture. It still has a voltmeter and does the same thing as a multitester. It's a little more user-friendly since it shows you on the gauge what qualifies as a "good charge", "bad charge", "charging system good", "charging system bad", etc.

 

I'm with you on not wasting things and money, prob'ly way stingier than
you even. LOL :) But I haven't lived "in town" since high school either.

The end of its -useful life- IMO is when it first starts its "death throes".
Not later after a couple more weeks (or more) of "turning up" dead.

But the first time it turns up dead danggit ;) when there was no other
reason for it to be dead like that. And yes, I can determine that using
nothing more than an analog volt meter and the battery's history.

A guy tests it on his fancy rig while the bad sucker is on an "upswing"
and declares it good and sends me or my helper packing with the sorry
sucker?

That only happened to me a few times (when it really counted!) before
I learned how to prevent that. ;)

Pour half the acid out and refill with water. :/

Besides it's not 100 bucks, it's pro-rated.
And it doesn't have to be dead to get your money.
BTDT with a "good" Exide that liked to spit acid.

------------------------------

What's the guy gonna do?
Pay you mileage for his mistake?
What about your lost time?

You know how to prevent that from happening to you now. :)
Next time a guy wastes your time and effort it's your own fault. :)

Alvin in AZ is a retired railroad signalape
"I've forgotten more about how to pour water than most will ever know ;)"