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"Generator" light stays on with engine running......

SUNDAY:
Tried jumpering terminals 1+2 while the engine was running. The battery stayed at 12.08 V

And as far as i can tell the gen light stayed on.
 
Well,then something is wrong with the Alternator most likely..

If the wiring was faulty,jumping those 2 wires together would have started it charging and you'd have seen the voltage increase..

The only other possibility I can think of is the thick red wire coming off that stud on the rear of the alternator is not letting current go to the battery to recharge it..
If you had your volt meter connected to the battery terminals while you jumped the wires,it would not have seen any increase in voltage,if that thick red wire had a break (or a fried fusible link) in it..

Make sure all the wires that are connected down at the starter solenoid are intact and the fusible links are not burnt up..sometimes they look perfect externally but aren't conducting electricity inside because the copper got too hot and melted..

You can try reading the voltage right at the stud on the rear of the alternator with your volt meter (the positive probe to the stud,the negative probe to ground ,if you still see no increase in voltage there when you jump the wires with it running --then I
guess its time to pull the alternator off, and go have it tested somewhere..

It will suck if a recently bought alternator turns out to be defective,but if you dont do these tests first,the next one you put on wont charge either,if it is a wiring problem..
 
Having it abruptly fail when switching alternators is a smoking gun. I'd be looking at the wires you disturbed during the installation, and if you find no apparent failures there, I'd be taking that new alternator back to the store.

The odds of some unrelated, undisturbed wire failing exactly when you swapped alternators are miniscule. As for a cheap parts-store rebuilt alternator, seeing one of those fail out of the box isn't particularly rare.
 
This problem should have been resolved in less than three answers.

That the Battery or Generator light stays on indicates a bad diode trio in the alternator. These "Sample" the output of the six-pack of Power Diodes that do all the work of charging a battery and also play a role in the output amperage of the alternator.

Notice I didn't say "charging a dead battery" or a filling in for a bad battery. Alternators DO NOT LIKE making electricity from a low voltage position.

To FULL FIELD your alternator look for the D shaped hole in the back of the alternator plate close to where the battery cable comes off the alternator going to the battery.

That D shaped hole is designed for a device called a Thexton Full Fielder, IN what it does is that it shorts the DIODE TRIO out of the circuit and makes the alternator put out maximum charge. This is only a test port, one which obviously the rebuilder never used on the test bench before shipping it out.

IRC, and i may not.....there were three designs of alternator Molex-style receptacles in those days, and you might have the wrong alternator for your application too!

ALWAYS CLOSELY COMPARE YOUR PHYSICAL WIRE CONNECTION AND ALSO THE PHYSICAL ATTACHMENT POINT BEFORE PUSHING YOUR CORE AWAY INTO THE GIANT "GEEZ... WE DON'T KNOW WHAT EVER HAPPENED TO YOUR OLD PART".
 
I could have told him about the "D" hole,problem is on many applications you cant get at that test port to ground out the diode trio & regulator..since the OP isn't well versed in GM wiring ,I tried to explain things simply so as not to confuse him ...Its easy to stick a screwdriver in the wrong "hole" and have it get ground up in the rotor!.

When I worked in parts stores,I had a habit of testing every new or rebuilt electrical item like a starter or alternator before handing it over to a customer--we had the equipment,and it took only a few moments,it avoided a lot of potential problems..at least I could say "well.it worked when it left here "!..

One store I worked at also stockpiled cores ,they didn't send them back to the rebuilder or warehouse for at least a few weeks--so if the customer did get a "wrong part" or his old one had a bracket or other needed part that was left on his "core",it could be retrieved..

We also got the third degree if we failed to make sure the customer was in fact getting the right part--we liked crossing the part number off the old one to the replacement,that was the best way to ensure they were compatible..
 
So, what is involved with testing the alternator?

2) Discuss the reliability of auto parts store testing.

3) Can i do any or all of the testing at home?
 
vandelay industries said:
So, what is involved with testing the alternator?

2) Discuss the reliability of auto parts store testing.

3) Can i do any or all of the testing at home?
Click to expand...

MAX Voltage output test
Use a digital VOM and attach it to the B+ post on the back of the alternator.
Ground the other end anywhere that is bright metal
- I like the brake lines for a ground as they attach to the master cylinder.​
Start engine (make sure the battery is decently charged)
Insert a short piece of paper clip* into the small "D" shaped hole in the back of the frame and watch the voltage.
* No longer than 1 inch enters alternator.
- you can't reach very far with a 1" probe.​
Just make sure that the probe also touches the side of the D-shaped hole, as that's the ground for the alternator regulator test you're doing here and it carries very little amperage.

DO NOT BE AFRAID OF ANY SPARKS AS THEY WILL NOT HURT YOU AND THEY SURELY CAN'T BE FROM ANYTHING SOPHISTICATED that you might have poked inside the housing.

You WILL hear the alternator LOAD-UP and perhaps even the idle speed may drop a bit - this is good.
​
Down and dirty DIODE test: (very accurate, really!)
Start engine, let idle
Check the DC output from the B+ lug on the back of the alternator using the AC Volt scale.(yup - that's right, the AC side of the meter)
AC volts should never exceed 0.013 VAC output on the B+ lug. Never! .
If your result is higher than 0.013 VAC, the diodes are in trouble.
Replace alternator (as many times as it takes to get a good one!)

If you have a Cold Weather/Low RPM, Digital Exciting System (DES*) in your vehicle, the AC Voltage diode test is a little trickier.
This DES System didn't show up until about 2001 and UP models of GM HD alternators IIRC.
*This first showed up in Cadillacs I think, circa: 1999 or there about.​

You can test the DES system if you are sure that the battery is very close to charged and the engine/battery is not below 40ºF while it is running.
The DES is a supplemental system to artificially excite the alternator to put out a lot more amperage at idle and at low engine RPM in cold weather and right after start-up, by hitting the stator with high frequency signals to increase the 'pull-break' cycle inside the magnetic fields inside the alternator. .

DES MAY come in and out at low stoplight-crawling speeds and some alternators will really make a loud squeal if it has a slightly loose iron field core lamination, etc.

The noise itself is harmless - more a bother really - but it can attract the attention of nearby drivers.

Normally one can hear a standard alternator 'cutting fields' when it is idling - kinda a low volume whistle/squeal. It too is perfectly normal.

________________________________________________________________________​

Now about the store testing: Ahem!

If you think that your parts 'dude' or 'dudette' behind the counter was just yesterday asking if you want to SuperSize your Fries ..... well.......

In-store testing is prolly decent enough as most testers are computers with onscreen instructions that tell the operator what to do, and then if the green bulb lights up - it is OK.​




HTH.
 
1) Ok i got it tested at Pepboys. i watched her hook it all up, so really i don't think there's anyway to screw it up.

i don't really understand the results other than that the alternator is toast:

Lamp/diode trio FAILED
Regulator test FAILED
Rectifier test PASSED

................... Low Actual High
Battery volts 14.20 11.47 15.60
Ripple volts 0.00 0.00 2.00
Lamp on volts 0.00 5.64 8.00
Lamp off volts 11.00 0.00 0.00


2) The thing worked just fine for two weeks. i would think that AC Delco was high quality vs. the offbrand autoparts store remans. Live and learn, i guess..............

3) What determines the amp rating of an alternator? Because i'm thinking of using the rear half of an alternator that is laying in my garage----which i know is good, and combining it with the front half of this failed alternator----because i can't change the pulleys right now (don't have an air impact).
 
Like most anything, an alternator is subject to quality controls before shipping.... and almost all are 100% tested. There can be exceptions.

FWIW ---- I know that the Bosch units reman'd west of The Mississippi are done in a facility that I helped set up in the early 1970s, and to this day, I know the QC is 100% Pass/Fail. There's that, not that Delco Rebuilt are any better or worse. What's in a name any more any wsy?

Wait till you can change the sheave (pulley) and install the unit correctly and keep the whole assembly under warranty.

...... and everybody should have a Harbor Fright electric impact for these times.

Can't your parts store whip it off for you too?
 
Yes. It was the alternator. Problem solved.

i just think it's strange how it worked great for 2 weeks----then plop. AND it was an AC delco reman........
 
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