1991 firebird. driving home I noticed my headlights and gauge lights went dim. volts dropped to 8v.i made it home but when I turned the car off and tried to start it again right away it only clicked. I put the battery on a charger over night and the car fired fine but the gauge showed 12v. I assumed because of the way it happened that it was the alternator. I removed the alternator and took it to orileys to have it tested and they said it was good. after reviewing the symptoms I decided to try a new alternator anyway. didn't help. I then put in a new battery thinking it could have a bad cell. nope. no change. I put a meter on the battery and got 12.7v which is fine. I put a meter on the wire on the rear of the alternator and got 12.7v which is good.i put the meter on the wire on the rear of the alternator with the car running and the power should have gone from 12.7v to 14+v but it dropped down to 11v. I pulled the plug on the alternator and the wire that should have constant power had 12.6v which is good. I put the meter on the wire that should have power with the "key on" but without the key on and it was dead. I put the meter on this wire again with the key in the "on position" and still had nothing. so that's an issue. I searched the forum and found a few threads that were helpful. one mentioned the "batt" light on the dash. the thread said that if the bulb on the dash burns out it disrupts the circuit and the alternator doesn't charge. I turned my key to the "on" position and I have no "batt" light. im going to change the bulb tomorrow and hope that's it. its an annoying issue.

 

No volts bulb in the cluster. Check the 20A FAN fuse, that is the one that feeds the F terminal at the alternator.

RBob.

 

wish I would have seen your reply before I pulled my gauge cluster and found there was no bulb in the 91-92. putting that back together. the fan and alternator fuses are good. I checked then last night. once I get my cluster back in im going to stat at the battery and follow the wires up. isn't there a fusable link in-between?

 

rookie mistake made here. I removed the gauge face to see if there was a bulb behind the batt pic under the volts gauge. that gauge face also has the fuel on it. im putting it back on I realized that I can point the gauge hands anywhere I want. I don't remember exactly where my fuel level was. I started the car but the fuel arm didn't move to correct its position right away. will it find its correct position or will it only read from where I put it? as a just in case im going to take it to the gas station a fill the tank all the way and set the hand to full. at this point I don't know if that is needed but I don't want to run out of gas because im an idiot and these cars don't have gas idiot light.

edit- I did take a pic before I took it apart and was able to put the fuel pointer right where it was. however I didn't take a pic of the volt gauge with the key off and im not sure where it sat. so anyone with 91-92 will you take a pic of the volts gauge with the key off so I can see where it sits with no power.

 

with nothing fixing the issue I took back the reman alternator and exchanged it for another to make sure I didn't get a faulty alternator. nothing changed. I noted the voltage on the gauge and installed the original alternator. no change in voltage. I removed the new battery and installed the old one. no change. . I feel like I have ruled these two items out but sine the issue isn't resolved I cant say anything with certainty. I checked the voltage at all three locations and it was good with the key off but the rear of the alternator is still low with the motor running. I checked the continuity of the red wire from the back of the alternator to the battery and its not broken. im a little stumped as to where to go from here. I feel like the battery is good, alternator is more than likely good, wires are good. the fuses under the dash are good. anyone who wants to chime in is welcome here..

 

Well if your alternator is OK and the battery is OK, probably time to start looking at the condition of the battery cables, fusible links, look for a voltage drain. Break out the service manual if you've got one, and start following the charts.

 

I will look at the battery cable and fusable links next. now on the fusable links if they were bad wouldn't that stop the car from starting?

 

this is the schematic from the manual. it shows just 2 fusable links. they are on the one of the two blk wires that go from the starter. where do they terminate on the other end. also my car has a fusable link on a red wire very near the battery that I don't see here.

 

Looks like a generic diagram. Try this... 91 bird with a L03 right?

 

That's a lot better. Alright guess I've got work to do after work today.

 

2 off the junction box near the battery tested good for continuity and the 3 off of the starter were also good. as far as I can tell from the schematic there are only 5. is that correct?

 

THIS IS WHERE I`M AT NOW.
1) alternator is good
2) battery is good
3) fusable links by battery and starter are good
4) fan works and fuse is good
5) continuity on red wire from battery to back of alternator- constant power
6) one wire in alternator plug has constant power
7) WIRE IN PLUG SHOULD HAVE KEYED POWER BUT DOESNT
NEXT STEPS
1) the junction block near the battery is pretty nasty. im not sure its enough to affect anything but I'm swinging wild here.
2) I need to find out why the " key on " wire isn't exciting the alternator with the key on. will start tracing it.
3) replace the positive battery cable.

any other suggestions?

 

There are all the charging system diagrams, including power distribution to the FAN fuse from the diagram I posted the other day. If they don't load large enough to read, right click and view image to see them full size.

Good luck!

 

If the alternator and battery are good then you have 2 things to check/clean or just replace if they are old.

Negative battery cable:Connection to battery and connection to engine block. if cable or connections are bad you lose ground and the alternator will not charge the battery. (open ground)
Positive battery cable: Connection at battery, starter, and especially the smaller lead which provides the charge to the battery from the alternator. This small wire should go to from the positive terminal at the battery connection to the bulkhead fitting at the radiator core support. install new battery cables, clean the bulk head fittings and terminals at the core support and use new hardware at the negative battery cable connection to the engine block and the charging wire connection at thecore support not a 30+ year old rusted ones. ( I use stainless steel )

for cheap insurance add a ground strap from the engine block to the sub-frame.
with the engine running and charging system properly wired and grounded you will see 14.74 volts at the battery. you can turn on the headlights and radio and voltage should not drop. ( means the alternator is charging the battery)

 

I changed the pos and neg battery cables tonight. the pos was in good shape. the wire that goes from pos to junction box was worn. the neg cable had about 1/4 inch of bare wire showing. I was putting everything back together to test the systems but the plug on the alternator wasn't seating. I looked closely at it and it was cracked. I poked the crack with my finger and the plug fell apart. im not sure this was the issue, it could be but the plug is 28 years old and maybe this weeks multiple in and outs might have been to much for it. either way I have to order a pigtail and solder it in.

my plug has two wires. the pigtails I'm finding that say they are direct replacements have 3 wires. I know I can cut the third wire but im curious what the third wire is for? why do they say it fits my car? are the tpi and tbi plugs different?

 

GM 12124898 is the connector part number. The seal can be removed and individual terminals removed/serviced as needed, plugs are available for unused terminal positions, etc. The Aptiv website sucks, but Mouser generally has the parts to service GM connectors, if you can find the part numbers.

 

installed the new plug and fired the car. problem NOT solved. im still not charging. had 11v at the alternator. how does alternator get "excited" to charge?
battery good
alternator good
new pos batt cable
new neg batt cable
all 5 fusable links good
fan works
fuses good

 

8A-30-14 under Circuit Operation Charging. Paragraph 2, 3rd sentence. The first test in Table 1 isolates that possibility.

Start with the Troubleshooting Hints on 8A-30-13, then move on to the diagnostics on 8A-30-14

If you get through all of that, and the problem persists, you don't have a problem or you missed something.

What are the actual symptoms you get driving the car? Does the battery eventually go dead if you don't put it on a charger? Or are you just reading what you consider less than optimal voltage with a meter?

 

Post #2, the F terminal on the connector you replaced. Note that the letters (S, F, L, and P) are molded into the connector shell..

RBob.

 

driving home my gauge lights went dim. I noticed my headlights had also gotten dim. I was able to drive home. when I shut the car off and tried to restart it the battery was dead. the battery took a charge. its running on straight battery power. you can watch the volts drop as you drive.

 

You say you aren't getting 12 volts on the brown wire at the generator with the key on, but your fan works. This means you must have an open circuit in the brown wire(circuit 250) between the fan relay and the generator. Refer to the diagram. If you have 12V at the fan relay on the brown wire, which you must if the fan works, and no power on the brown wire at the generator, then the wire must be broken. You asked what excites the generator, or more accurately the regulator, to begin charging. The regulator is activated by the 12V ignition signal on the brown wire(circuit 250) when the key is turned on. No power on circuit 250, no charging.

 

I started tracing the brown wire. the loom wont come off. its crumbling when I try to remove it. once I get this fixed I will have to re-loom it. I got from the alternator to the firewall. I have about 10 more inches to pull out. so far nothing looks wrong with it. I broke a vacuum hose/tube off of something attached to the firewall near the a/c box. so that's once more thing. every part of this car is brittle.

 

I had a similar issue years ago. A buddy asked me one day, "have you done a voltage drop test?" And i argued, "the battery is 12.6, 14.7 at the alternator, every connection is spotlessly clean, etc." But when i finally listened to him, i found that the wire from alternator to battery positive was loosing voltage. I simply added a second, 10-gauge wire from the back of the alternator to the battery. Never had a problem again.

Theres tons of info on the interweb about doing a voltage drop test. heres a sample; https://axleaddict.com/auto-repair/A...e-Drop-Testing

 

Working on a 28 year old car can be annoying. All of my lines are brittle. I snapped off the vacuum line on this while fishing a wire from the harness.. Not even sure what it is yet. Mounts on the firewall by the a/c box. Once I get this charging thing done I need to go through and replace all the vacuum lines.

 

my brother was in town and we had a brief discussion about my issue. he suggested I change the fan relay even if I thought it was good because its difficult to test a relay at home with certainty. I replaced the relay and it did change something that wasn't working which makes me wonder why im still not charging.
ive got 12.6v at the battery
ive got constant power at the rear of the alternator
ive got constant power in the first wire in the alternator plug
now that I replaced the relay I have keyed power in the second wire on the plug.

it seems like everything should be working now. I put the meter on the battery and get 12.6v then start the car my volts do not go up. now that ive replaced the bad relay do I start over again with replacing the alternator or battery? could the failure of one component cause the failure of the other?

 

Where are you putting the negative lead of your meter when taking these voltage measurements?

 

when I test the battery its on the neg terminal. when I test the alternator I ground it to the alternator. I also grounded the alternator to different bolts on the motor to see if it made a difference.

 

OK, good, because it was sounding like your alternator was not grounded, as hard as that is to believe.
Do you actually have a heavy cable going from the (-) battery terminal to the engine block? You should also have one going from the block to the body.

 

If you have battery voltage at both wires to the regulator, the alt case is grounded, and you have battery voltage at the B+ terminal, the alternator must produce current unless it is faulty. Recheck voltages to be sure you don't actually have 14+ at the B+ terminal with the engine running. You may still have voltage drop on the Alt output wire to the battery.

 

I replaced the fan relay and it fixed the missing keyed voltage. I had 12.7 at the batt, 12.6 on the rear of the alternator, 12.6 in the first wire in the plug and 12.6 keyed in the brown wire. so all the power going into the alternator was correct but it still wasn't charging. I replaced the alternator AGAIN and had 13.8 at the rear of the alternator. I called success. I drove the car and after about 15 minutes the volts started going to down again. I thought something might be burning the relay out. I put the meter back on it a few minutes ago and I have all the correct voltage going to the alternator. I was wrong about the alternator and the relay so maybe the battery is crap too. this has been ridiculous. I didn't test the battery or alternator. I just noted that there was no voltage difference when new components were added. so with a new relay and new alternator and new battery cables im going to once again add a new battery unless someone has a better idea. its weird. there are only like 4 major components to this charging system and it seems like they have all gone bad at once. im annoyed but im like 16 days into something that should have taken a day. I would dance around a bonfire of burning dollar bills naked right now to get this crap over with.

 

What usually happens is the alternator does not charge the battery (for some reason), and then the battery gets low or dead. A "dead" battery can actually measure 12.6 volts, but it no longer has the current capacity to crank an engine. If the battery was on it's last leg, then when you get the charging system working again, it now tries to charge a dead battery, and you now kill the alternator. Some repeat this process a few times until every piece of the charging system (and battery) are replaced. Everything works now because it's all new, but the initial cause never got figured out.

 

Forget about 12.6 / 12.7 volts. You should have at least 14.5 volts at the battery when engine is running, this means the alternator is charging the battery. Anything less than 14.5 volts means you have a charging / ground issue (something is wrong)

• You need to test the battery/alternator voltage when the engine is running & parked.
• Did you bench test the battery? maybe not an issue since it sound like you have re-charged it after it died but if the battery if over 5 years old it should be replaced.
• Did you make sure you have a new 10 gage wire from the positive battery post to the bulkhead fitting at the radiator? and from the bulkhead fitting to the "bat" post on the alternator. This is the wire that provides the charging current for the battery.
• With the engine running you should have 14.5 volts at: the 'bat' post on the alternator, the bulkhead fitting @ the radiator, and at the battery. if not you can now isolate the problem.

 

This is obvious but I never saw it mentioned; if you put a new good alternator on a heavily discharged battery it can burn out the new alt. Other than that, good luck.

 

from the look of this everything going into the alternator is fine. I have all the correct inputs but no out put. that says bad alternator. I bought it from orileys and had them test it before I took it. I feel like there is some issue with the test. im going to remove it and take the new and original back up and ask if I can watch the output on their test. might even see if I can put my meter on it while they test it. something here doesn't add up.

 

The store's bench test doesn't put any load on the alternator. In service is completely different. The alternator has to support heavy current load. Of course, you never install a new alternator without fully charging, and verifying health of, the battery first. The old bench testers we used to have were different. They had a carbon pile that could apply a load to the generator and actually test the stator and diodes for current and voltage. The test they do at most parts stores now just spins the unit and looks for voltage. Not the same at all. On vehicle testing with something like the Snap On AVR is much more useful.

If you have 12V at the wires of the small connector, you should have charge voltage of about 14v at the B+ with the engine running. If not, the alternator is faulty.

 

I have the correct voltage at the plug. I'm not getting anything but battery ( 12.7) at the rear output though.

 

It has to be a faulty alternator. Be sure the batt. is fully charged before you install the new alt.

 

I feel your pain, i went through this recently with my alternator, then the one that got burnt up when i replaced it. At some point during troubleshooting i had jumped 12volts to L port on the alternator plug. While this actually excited the alternator and made it charge (for about an hour), it fried the unit. And refused to re excite after this.

The wire running to the L plug has a higher resistance because its meant to run through an indicator bulb in the dash which provides enough resistance through that wire to help excite the alternator but not fry it. By hitting it with 12v straight during testing i killed my replacement alternator.

After much swearing and anger i replaced it with a CS130 that is a one wire setup (not a conversion, but 1 wire straight out of the box) and its excellent. I rigged up a wire that ran to the gauge on the dash and everything seems to be working fine.

 

alright update and a little progress. I took what I believed was a faulty remanufactured alternator back to orileys. I asked them if I could put my meter on it while they tested it to see what voltage it put out. he said the test machine has a safety lid that covers the alternator so I couldn't touch it while it was being tested. he also told me the test doesn't show the any voltage numbers. it just read good or bad. that's lame. I ordered a NEW alternator and installed it. I got 14.3v at the alternator. It was good to see. my car still isn't charging though. I have the wire from the rear of the alternator and the battery remaining. my guess is that all of this draining and charging has killed the battery. its old. im picking up a length of 10g wire tomorrow with a new battery and expect by the end of the day to have my bird back. im not sure what happened but it seems like every piece of my charging system went down. im almost there.

 

Some progress is better than no progress...

 

finally solved! I ended up rebuilding the entire charging system but its fixed now. the final change was installing a replacement wire between the alternator and the battery junction box. when the new wire went on I immediately had 14.8v. during this I ended up replacing 1) alternator 3 times
2) battery twice
3) both battery cables
4) fan relay
5) 10g wire between the alternator and the battery junction box

thanks to all the guys who stuck with me and offered their help. that's what these forums are all about.

 

Eureka !

 

What is the battery junction box you speak of here? The fusible link? or the fusebox?

 

That would be this little bugger, off to the right of the battery. VIN F engines do not have one.