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

vandelay industries

vandelay industries

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My "generator" (battery) light now stays on while the engine is running. (the red warning light that has the icon of a battery, but the owner's manual calls this a "generator light")

1) this light glows at, i think, 1/2 the intensity of the other warning lights---brake, choke, oil etc. So it actually may have started yesterday during the day, but i did not detect it until last night when it got dark outside. As far as i know this light situation started yesterday.

2) the only differences i notice are that the left turn signal is a little slower than normal and the dash lights are dimmer than normal. Everything else, including headlights seem normal. And the truck starts and runs no problems.

3) New AC delco 321-357 alternator installed about 2 weeks ago. Belt is tight and new.

4) Battery should be good as i tested it before and during the alternator changeover. But i have tools and can test again.
 
First thing is recheck the new alternator and battery. I always revisit the last thing I did if a new problem arises. Check connections and do a running voltage check. After that I would be looking at grounds.
 
The "generator" light coming on dimly is usually an indication of one or more of the three diodes in the alternator have failed--it'll still put out "some" charge,but not 100%,and it can let the battery discharge to ground,through the faulty diodes..

This is assuming all the wiring is good,hooked up correctly,etc..sometimes a poor ground can do this also--I've had a few GM vehicles that I had to add a ground wire right from the alternator case to the battery negative terminal to overcome poor grounds before..
 
Ok battery was 12.5, then after starting it up........no increase in voltage.

The battery negative cable looks good and is tight to the alt bracket (And the engine would not have started if it was bad?) And the little "branch" wire running off of it ( main battery neg cable) that runs to the radiator support looks good and is clean and tight. i don't know of any other ground wires that might cause a problem? The ground running from firewall to head is ok.

Next?
 
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Pull the alternator off and have it tested at the nearest parts store.
 
First I would test for voltage at both of the wires on the alternator plug (assuming this is an older truck with the 10SI alternator that has a two prong plug on its case)--with the engine running you should have 12V present at both wires on the plug (leave it plugged in !)..

If there is only voltage at the thicker red wire on the plug,and not the other tan or white wire,the alternator will not get the field coil "excited",which starts it charging..and that will also make the "gen" light come on...its possible that wire has a break in it somewhere..
 
A quickie "no tools" test you can do,is simply jump those two wires on the plug to each other with a wire or cotter pin--if the alternator wasn't charging due to no field excitation,you'll hear it start charging ,and the engine will slow down a bit..

I had a '77 GMC that I had to do this to many times on the side of the road,till I found out where the field wire had become all corroded inside and no strands were left to send 12V to the alternator plug..
 
Ok, i just tested alternator output with the engine running with the voltmeter. Red to the B+ terminal and then black to the body of the alternator. So with the engine off, 12.4ish and with the engine running no change.

But this is a brand new Ac delco alternator (starts pouting and crying :cry:)

diesel4me said:
First I would test for voltage at both of the wires on the alternator plug (assuming this is an older truck with the 10SI alternator that has a two prong plug on its case)--with the engine running you should have 12V present at both wires on the plug (leave it plugged in !)..

If there is only voltage at the thicker red wire on the plug,and not the other tan or white wire,the alternator will not get the field coil "excited",which starts it charging..and that will also make the "gen" light come on...its possible that wire has a break in it somewhere..
Click to expand...

How do you do this?

diesel4me said:
A quickie "no tools" test you can do,is simply jump those two wires on the plug to each other with a wire or cotter pin--if the alternator wasn't charging due to no field excitation,you'll hear it start charging ,and the engine will slow down a bit..

I had a '77 GMC that I had to do this to many times on the side of the road,till I found out where the field wire had become all corroded inside and no strands were left to send 12V to the alternator plug..
Click to expand...

And this?
 
Well,I already told you how...

Assuming you DO have a '73-87 truck with the alternator pictured below,these 2 wires are the ones I'm referring too--anything newer than 1987 ,this does not apply,the alternators are a lot different for later fuel injected and computerized trucks..

alternator plug pigtail.jpg GM 10SI alternator.png
 
You'll have to forgive my ignorance; i'm not real clear with electricity, but i think i'm different in that i like electricity/electrical systems, just never got around to it. Although it looks like now I AM going to have to get into it.......

diesel4me said:
First I would test for voltage at both of the wires on the alternator plug (assuming this is an older truck with the 10SI alternator that has a two prong plug on its case)--with the engine running you should have 12V present at both wires on the plug (leave it plugged in !)..

If there is only voltage at the thicker red wire on the plug,and not the other tan or white wire,the alternator will not get the field coil "excited",which starts it charging..and that will also make the "gen" light come on...its possible that wire has a break in it somewhere..
Click to expand...

So for this one, and yes i have the plug with 2 wires, if the plug is still plugged in, then how exactly do i test for voltage at both wires?

diesel4me said:
A quickie "no tools" test you can do,is simply jump those two wires on the plug to each other with a wire or cotter pin--if the alternator wasn't charging due to no field excitation,you'll hear it start charging ,and the engine will slow down a bit..

I had a '77 GMC that I had to do this to many times on the side of the road,till I found out where the field wire had become all corroded inside and no strands were left to send 12V to the alternator plug..
Click to expand...

So in this case the plug is not plugged in? And then you use, say a paperclip, to "connect" both wires?

Maybe it's best to assume that i know NOTHING about electricity and tools at this point.
 
Just jump the 2 wires pictured on that plug with a jumper wire,or something like a cotter pin--(anything too thin like a paper clip may get hot and burn your fingers !)--do this with it plugged in as it normally would be, with the engine running..

If the alternator was not charging due to it not getting power to the field wire (white one shown above on the plug),you'll probably hear it kick in and start charging,which will make the engine slow down a little..usually once it starts charging it will continue too,until you shut the engine off,then it will go back to not charging if that field wire isn't getting 12V..

You can use a test lamp or volt meter to check the wires on that plug also--with the key "on" (with or without the engine running),there should be voltage present at both wires..(this test can be done with it unplugged or plugged in,it doesn't matter..

I assume you know how to use a test lamp--you can use a store bought one,or just use any 12V bulb and socket with 2 wires,just ground one wire to the negative battery terminal,and probe the wire you want to test with the bulb's other wire--if power is present the bulb lights up..
Volt meter works the same way--ground the meter's negative probe wire to the battery negative,and set the selector on the meter to DC 12V or whatever range is closest ,and probe the wire with the meter's positive probe...the meter will indicate if any voltage and how much there is at the wire..
 
You can back probe those old style connections easily as well, just leave it plugged in and insert a thin wire or pin into the backside of the terminal and use that as your test point. It carries the voltage without all the amperage that comes with using a jumper. Some multimeters come with thin probes that will backprobe a connection like that without additional probes.

If you're close to the parts store and have a ride just pull it and let them bench test it, those fancy test machines they have work pretty well.
 
Thanks everyone for the help so far. i am studying and thinking about all this. i probably won't be able to get to it until this weekend though. i do have another car i can use.
 
The basic charging wiring harness is pretty much the same on all 73-87 GM trucks (and cars too)...they may use shorter or longer wires to reach the alternator depending on its location (straight six needs longer wires,V-6 is shorter ,as an example)--some trucks had two fusible links and wires at the starter solenoid ,other later ones had only one...
 
UPDATE for saturday 2/25:

Ok, i test lighted both wires in the plug with the ignition on; Both wires have power, but the smaller brown wire is dimmer than the larger red wire.

So, i used the voltmeter; The larger red wire has 12.4-8 ish while the smaller brown wire, 6.3-5 ish.
 
That brown wire is the one that excites the field windings in the alternator and starts the charging process...

Far as I know both wires should have 12V present,but seeing the brown wire also has some resistance from the "generator" bulb being wired in series with that wire ,it may read lower than 12V--but 6V sounds kind of on the too low side....

Some resistance is needed--on vehicles with a volt gauge they have a resistor made into the gauge or wiring,instead of using the bulb..if there were no resistance, the brown wire would send a full 12V to the field windings,which will make the alternator put out maximum voltage and amps..

That brown wire "senses" how much voltage is in the battery and wiring,and "tells" the internal voltage regulator to increase or decrease its output to keep the battery fully charged and the system voltage within normal limits..(12V to about 14.5 V)...

Your alternator should be charging--did you try bridging those 2 wires on the plug together with a jumper while the plug was still plugged in ?...
I would,with the volt meter hooked to read the battery voltage--if you see the voltage at the battery increase from 12V to 14V or so,when you jump the wires,then your alternator is OK,its the brown wire not getting enough current to the regulator..
 
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