Hi everyone I have a question on how to test the alternator to make sure it still good?

Does any one know how to do this? I was think of using a voltmeter, but I'm not sure.

I am trying to test my alternator on my truck it is a F-350 with a 460 engine.

ANy help would be good

 

The quick and easy way is to start the truck and put the voltmeter leads on the battery. It should read about 14 volts with all accessories on. If it's 12 or less it's either the belt or the alt.

 

What ever happened to removing the battery cables & it the vehicle was still running the alternator was good?

Did that go to the wayside with the invention of computers?

 

I have never heard that before...

Are you pulling my leg wouldn't that hurt the alternator?

 

That test I mention was for older vehicles. 40's,50's,60's, & 70's.
These cars were not ran by electronics or computers. The battery started the car & the alternator ran everything else.
If you thought you had a bad alternator, you would fire up the car & remove the positive batter cable from the battery. If the car remained running, then your alternator was fine. If it died, then you had a bad alternator.

 

It works, but it's easier and safer to test with a meter.

 

Thanks for the clarification

 

Further clarification. Alternators didn't become prevalent until the mid 60's and almost always had a mechanical EXTERNAL Regulator up until approx 1972. With the advent of imbedded electronics (early 70's) alternators got an electronic regulator internally which was highly sensitive to back feeds from disconnecting the battery while the alternator was running. The voltage had no where to go and you'd end up with fried Diode packs, rectifiers, and voltage regulators.

Generators with the old MECHANICAL regulators you could do this with, but almost always with an alternator and an electronic regulator you'd end up frying the reg or the diode packs.

DO NOT DISCONNECT BATTERY WHILE ENGINE IS RUNNING unless you like a lot of smoke and have plenty of $$$$$ to replace the alternator.

Use a voltmeter across the battery terminals and read the voltage with the engine OFF. Should be 12.5VDC no lower. Start the engine. If the voltage at 1000 rpm is above 13.5 to 13.8VDC
minimum with a maximum of 14.4 to 14.8VDC you are ok. higher than 14.8 and your regulator is bad.

NOW while the engine is running, crank in accessories one at a time and let the voltage stabilize for a few seconds between each. When you have everything on and running 1000 rpm you should have NO LESS THAN 12.5. Any lower at 1000 rpm means a diode or the rectifier might be flaky or the battery may have a weak cell.

Easiest thing for you to do if you're not handy with a volt meter is to take it down to a reputable shop and have them test the charging system, battery, and starter out. They may or may not charge you for this. I know Advance Auto and Auto Zone don't charge.

Larry
Larry

 

Nice post . Should be a sticky titled" How to troubleshoot your alternator"

 

I use to always unhook the altenator to see if it was producing. Will not cause any damage to it at all!!! However the arc involved with unhooking on the newer cars and trucks can be volitile to the computers.

 

i agree,, i have never seen an altenator smoke from pulling off a terminal.....what if you had a a dirty battery terminal and it wasent makin a connection,..your altenator would go up in smoke.???..i dont think so...

 

larry is right, there's a pretty good chance you will be getting yourself a new alternator if you take the cables off. Just test it with a voltmeter or run it down to a local auto shop.

 

One Way to check if you can get to it is to take a screw driver and place it to the back of the alternator with the motor running if its magnetic then you know its doing some sort of charging.

 

I agree NEVER REMOVE A BATTERY CABLE with the engine runing!!
There is a potential for a huge voltage spike that will take out
electronic components like PCM, etc.!!! The battery acts as a filter
for the voltage spikes from the alternator. The Di/Dt (change in current / change in time) can create a huge voltage spike from
inductance if the battery is pulling a high current when you disconnect the battery cable!!

 

The screwdriver trick is a good quick check. Idle the engine and hold tip *close* to rear bearing cap and you should feel a slight magnetic draw. Has worked every time for me...Be careful of the moving parts!