Easy Alternator test? Options

[ChrisReale]

Is there an easy multi-meter test you can do to see if the alternator has crapped out? I guess the fact that my battery doesn't charge is proof enough, but I just wanna make sure.

[So.Cal.914]

When your car is running put the probes on the pos(+) and neg(-) side of

the battery it should read about 13.5 volts.

[Rand]

Also check your voltage regulator before replacing the alternator.

[TheCabinetmaker]

I assume you checked the belt first?

[Scott-thundercat]

check the battery voltage before it's started and after- usually goes up by at least 1 volt (otherwise the battery wont charge).... or take it to kragen or autozone and they will test it on the car really easy.

[McMark]

As mentioned, the voltage regulator can be bad as well. Also, make sure the warning light bulb in your dash is operational (should light when the key is turned on). Without the bulb, the alternator won't energize.

Glad to see you're making progress on this. PM me if you need anything.

[smooth_eddy]

I assume you checked the belt first?

(IMG:style_emoticons/default/agree.gif)


This was my problem. I think it should flex about a half inch in the middle of the belt. Eddy

[Brew]

Also, make sure the warning light bulb in your dash is operational (should light when the key is turned on). Without the bulb, the alternator won't energize.

Mark, I'm not doubting you in any way, but...Huh? Really? If the bulb burns out, the alternator wont work?? Is this a common occurence? Should I be carrying a spare bulb? Sorry, this design just seems waaaay stupid, and I woulda never thought to check the bulb if my battery wasn't charging.

[type47]

Also, make sure the warning light bulb in your dash is operational (should light when the key is turned on). Without the bulb, the alternator won't energize.

Really? If the bulb burns out, the alternator wont work??

i think he means:
alternator will work, just the "juice" (which goes to the battery thru the circuit in which the alt light is located) won't get to the battery if the circuit is open due to a burned out filament in the bulb.

[McMark]

Nope, if the bulb is burned out the alternator will not excite and will not charge.

Jim, that's wrong. If the bulb is burned out the alternator will do NOTHING. The electricity created by the alternator does not run through the bulb. Once the alternator is excited, you can take the bulb out and it will keep on charging. But the alternator needs that little kick start to start making power. There's a ton of articles about alternators on the web if you want a clearer understanding.

[SGB]

Absolutely true. As McMark says, But it RARElY burns out (I've never heard of it happening). I think it is more valuable knowledge if you messing with the instruments.

[Spoke]

To check the alternator assuming that the belt is ok and everything is mechanically sound, do a test which simulates the voltage regulator applying voltage to the armature to increase alternator voltage.

Sorry, I don't have access to my schematic. (It's at home and I'm in South Korea) So my instruction will lack the actual wire designation. I did this test to separate voltage regulator issues from alternator issues. My low voltage issue was the voltage regulator.

Look at the schematic for the voltage regulator (VR). The top most connection from the alternator provides power to the VR and senses alternator voltage. The center lead in the schematic is the armature positive lead and inside the VR, this lead is connected to what looks like the contacts of a relay. The VR connects this lead to the alternator voltage to increase the voltage and opens the lead if the voltage is sufficient.

If you connect the armature positive lead to the top most connection from the alternator, you will simulate the VR trying to increase alternator voltage. Connect the leads together first then start the engine. If the alternator is ok, the battery voltage should rise to about 15-16 volts. If it doesn't, you either have done this test incorrectly or your alternator is bad.

If you are unsure about the VR connections, do not do this test.

Spoke

[cary]

Revive an old thread ..............
Just trailered the car home. Electrics went dead.

Saw the volt meter dropping. Decided to pull over and have enough charge to drive it on the trailer. I was concerned earlier when I only had 12.25 volts at the battery with the car running. Wrote it off to the battery being charged with the tender. Guess not ............

I didn't notice the alternator light didn't come on when the key was on. Not Good. Went out and swapped the green to red bulbs. Bulb fired up in the green spot. So bulb is good. Pulled the #9 fuse, it was good.

I did do a fuse box update to a blade type fuse panel. When its light tomorrow I'll drop the fuse panel and look at the red/white wire going to #9.

Any other thoughts?

[Michael N]

Also check your voltage regulator before replacing the alternator.

(IMG:style_emoticons/default/agree.gif) checked my voltage regulator and found that it just wasn't seated all the way. Sure saved me a bunch of time and effort. Hope you have similar luck.

[cary]

Did that cause the alternator light not to fire up?

[Dave_Darling]

Yes, when the VR is not plugged in, the alternator light won't light.

--DD

[type47]

I'd like to listen to a discussion on how the VR works. Here is a link to the PP elec diagram containing VR and Battery:

http://www.pelicanparts.com/914/parts/Elec...lectric_74E.jpg

[cary]

Yes, when the VR is not plugged in, the alternator light won't light.

--DD

That's were I'll start tonight when I
get home. I took it off to put on the relay cover.


On a positive note the car ran like a
dream until battery went below
10 volts.
Took it to the PDX Pelican group monthly
Burgers & Beer last night.

[Dave_Darling]

I'd like to listen to a discussion on how the VR works.

It compares the battery voltage to the alternator voltage. When the alt voltage is higher than the battery voltage, it disconnects the field windings of the alternator, making it no longer charge. When the alt voltage is lower than the battery voltage, it connects the field windings of the alt, making it charge. It effectively turns the alternator on and off, over and over again, pretty quickly.

That is a simplified version, at least. There are more details, and I think those start taking into account how quickly the electric fields in the windings build up and decay, and how the compared voltages are actually biased so you charge at ~14V rather than ~12V, but the description is a half-decent overview.

I believe the alt light runs off of the alternator voltage wire that goes to the VR. I'm not sure exactly when the power for the light has to go through the VR, but it probably has something to do with the way the light is used to "bootstrap" the alternator to get it to start charging.

--DD

[cary]

Back in business.

The new Beru VR positions further forward on the relay board. So it wasn't even plugged in the right sockets. Maybe only one leg was plugged in.

One must remember that the alternator light has to come on when the key is switched on. This shows the VR is in the circuit.

Fired her up and we had a 13.7 volt reading on the battery. That's more like it.