Gen 1 light on

Multimeter is reading 12.5-12.7 on both batteries
Gen 2 post is reading 25.5
Gen 1 post is also reading 25.5 (thought it was supposed to be 12?)

Gen 1 light stays on very bright, Voltmeter in truck stays in yellow.

any ideas on what is wrong here?

Check your voltage at your battery for each alternator. As in + and - of the front battery and then + and - of the rear battery. Move both leads to the battery you are testing and let us know what you get.

If you get below 14.4 at the front battery with the engine running, then you need to move onto the drivers side alternator and test it with the - lead on the front battery - post.

I was getting the same 12.5 at the front battery that I got when the engine was not running. Just to confirm, the next step I should take is to put the positive lead from my multimeter on the drivers side alt with the negative lead on the front battery negative post? What voltage should I be looking for when I do this?

That is the best way to do it.

Unplug the plug from the side of the alternator. Use your volt meter to check voltage at the red wire on the back of the alaternator, then check voltage on the red wire in the plug you un did. Both should be battery voltage with the key off. the brown wire on the plug should not have anything. Now, turn the key on and test all 3 again. They should all be battery voltage.

If they are, then you have a problem inside your alternator. If the listed readings don't happen, then you have a problem in the truck wiring. See what you get.

WOW barrman I cant forsee anybody getting that wrong PERFECT!!!!!! simplest answer you could give

Ok so I checked the voltage at the plug (while unplugged) of drivers side alt and got 12.5 volts(when engine is off and on).
Checked voltage with postive lead on large red wire on back of drivers side alt and negative lead on front battery (-) post I'm getting 25 Volts (with engine off and on).

You're saying I should be getting 12.5 or so at the drivers side alt but I am getting 25.5
does this mean there is a wiring issue? nothing has changed since I've got the truck wiring wise and this problem just recently started.... are there any fusible links that are involved directly with the drivers side alt?

What did the exciter wire do with the key on and off?

There shouldn't be anything in the wiring that would cause you to get 25 volts at the drivers side alternator. Since it was working fine before and all your 12 volt truck components aren't leaking magic smoke out, lets just ignore the high reading for now because it is probably just me not writting clear enough.

Back to the exciter wire. If it is 0 volts key off and 12.6 volts or so key on, your truck is fine and the problem is with the alternator.

Just like to point out that if your alternator is working you can take something magnetic while running and it will stick to it on the backside. Just a quick way to test your alternator. If it sticks its good if it don't you need to replace it. The alternator creates a magnetic field when operating properly and converts that into electrical energy, and if its kaput then no magnetism... just a quick way to test when you don't have a meter!

Your exciter wire has to work for the screwdriver to the back trick to work. If you are comfortable working with electrical stuff. Just jump from the hot wire bolted to the back of the alternator to the exciter wire with it plugged in. Do this with the engine running and you should hear the idle go down, the alternator start to make noise and get volted if the alternator is good and the truck wiring to the exciter plug bad. If there is no path for the voltage to go back to the battery, it won't work either.

Need some troubleshooting clues.....
my gen 1 light is barely starting to glow while driving without any accessories on. I have to cup hand around to see faint glow....
When I turn on heater blower to high the light goes brighter but not full brightness.
Is this the gen 1 going bad?
I haven't notice the slight glow until now.....
I looked at the TM and it mentions gen 2 in section 2-4 but I didnt locate troubleshooting for gen 1
here are stats for Front and rear batteries as indicated
not running....running no load...run w/lights....run w/hi blower... Running all
FRT 12.9 14.5 14.1 14.0 13.9
Rear 13.0 14.7 14.7 14.7 14.7
It shows that the frt battery is not getting full volts from Alt 1
Am I right?
Is this maybe a diode tree going bad?

Time to plan for a rebuild of GEN1. From the test readings, it may be the brushes. Diodes are either good or bad.

Take the alternator apart and test the components to find the bad ones.

The TM 9-2320-289-34 has a great section on rebuilding the alternators. Also Barrman has list some great DelcoRemy repair manuals that also help.

Gen 1 is drivers side the standard ground alt? or the isolated ground?
without revisiting the TM I get confused......
might just purchase new (rebuilt) to keep me going and pickup a rebuild kit so if(when) #2 goes I have an alternate plan.

The factory GEN1 and GEN2 are isolated ground alternators.

You CAN use a regular gronded alternator for GEN1. BUT you will not be able swap them around if needed. Also label the non-isolated ground so you or any future owners don't get confused.

Hawkman has some cores for sale cheap. You might want to buy one, rebuild it, replace your bad ne and then have a spare.

Rebuilt or new isolated ground alternators are running in the $200 price range.

Thanks
sounds like a plan!
didn't realize the isolated ground were that much more.
The only difference in the isolated is the fiber wafer applied to the post correct?
could I purchase a "standard" alternator, insert wafer and save a few bucks?

OL AG '89 said:

Thanks
sounds like a plan!
didn't realize the isolated ground were that much more.
The only difference in the isolated is the fiber wafer applied to the post correct?
could I purchase a "standard" alternator, insert wafer and save a few bucks?

Click to expand...

It is a little more complicated that that, but that is the jest of it. about five washers, a ground stud, and some bolt isolators. You cant buy the parts at autozone.

My local rebuild guy can take an old Caddilac alternator and turn it into an isolated ground unit. He charges $125 for the unit.

I haven't built one myself as I don't have ready access to the parts.

O.k. found a Delco 1105500 at rare electric for $189 plus shipping, no core charge. New unit, not reman. That looks like a good with all the prices I've seen in other searches. I'm trying to find out the manufacturer of the unit.
But I will probably order at least one...

Exciter wire reading

Barrman said:

What did the exciter wire do with the key on and off?

There shouldn't be anything in the wiring that would cause you to get 25 volts at the drivers side alternator. Since it was working fine before and all your 12 volt truck components aren't leaking magic smoke out, lets just ignore the high reading for now because it is probably just me not writting clear enough.

Back to the exciter wire. If it is 0 volts key off and 12.6 volts or so key on, your truck is fine and the problem is with the alternator.

Click to expand...

hi barrman, thanks for all the valuable info. I have a question that differs slightly from these other guys. I am not getting voltage to the exciter wire on alt 1 with key on. I checked and my heater blower fuse is good, I cleaned the spades, and the blower fans works nicely, at all speeds. at start up, i am no longer getting my gen 1 light to light up. it was working as of yesterday, but i swapped out my gen 1 with another used one (which was pulled off a running truck some time ago). I pulled the bulb and swapped it out for a known good one. still nothing.

what is my next move?
also, my volt meter in the truck is playing funny games. sometimes it reads in the yellow, about half way, and then the next time i start the truck up, it wont move at all and just sits there. dead.
batteries are full charge, load tested very well.

thanks in advance for any info. my head is spinning trying to absorb all these posts.

MORE INFO. ok, still no gen 1 light. when i started the truck this morning the volt meter was in yellow, low yellow. i went up under the hood, and wiggled the 2 wire plug on the alt 1 and then looked in the cab, the voltage was in the green...i started to think i fixed it somehow.
turned the truck off, got breakfast, came out, turned key on, still no gen 1 light, started up, and then voltage meter was back in the yellow again. i tried the wiggle wire trick, and no luck...still in the yellow.

question: where does the brown tickler wire go from the alt 1? to the fuse panel under my dash? i have been looking at so many diagrams, and i think that is where it goes....does anyone have any ideas? thanks

more info: pulled out the bulb and socket from the gen 1 light, and voila...gen 1 light works now!!! and my guage in dash reads in the yellow and then green. one question is that my guage needle is herky jerky, and not smooth. if i tap the glass it moves. not sure if there is a connection. any ideas? does everyone else's needle jump or does it move smoothly?

Barrman said:

That is the best way to do it.

Unplug the plug from the side of the alternator. Use your volt meter to check voltage at the red wire on the back of the alaternator, then check voltage on the red wire in the plug you un did. Both should be battery voltage with the key off. the brown wire on the plug should not have anything. Now, turn the key on and test all 3 again. They should all be battery voltage.

If they are, then you have a problem inside your alternator. If the listed readings don't happen, then you have a problem in the truck wiring. See what you get.

Click to expand...

Excellent break-down! I followed this procedure and discovered that, when disconnected, the + wire from the alternator had 0V potential between it and ground. I traced the circuit and found a melted fusible link -- actually three melted fusible links. Replaced them and now I'm good to go.

I have a gen 1 light that won’t go off, here is my current situation.

1. When I turn the key both my gen lights turn on to excite the circuit.
2. Once started, the gen 1 light stays on. Gen 2 turns off.
3. Yes, I've blipped the throttle.
4. I have also confirmed the gen 1 alt is not charging while the engine is running.

I’ve followed Barrmans procedure and here is what I get:

key off:
-wired plug: red side 12v
-wired plug: brown side 0v
-plug on alt: red side 0v
-plug on alt: brown side 0v

Key on :
-wired plug: red 12v
-wired plug: brown 12v
-plug on alt: red side 5v
-plug on alt: brown side 5v

please help me decipher these readings as barrman did not specify results like mine.

I am not sure what you mean by wired plug and plug on alt.

There is the plug plug on the side that has 2 wires. One is always battery voltage and one (brown stock, white with replacement plug) that is 0 key off and battery voltage key on.

There is also battery voltage on the red wire that bolts to the back of the alternator.

5 volts doesn’t compute for any of the wires. Something on the truck side isn’t right. I would suggest following each wire all the way to a battery or the firewall through plug. Then use a 10mm socket to unplug that plug. GM put some really nasty black goo on that plug to prevent corrosion. Latex or nitrile gloves are your friend here unless you want to have that gunk on you for a while. Use a finger nail file to clean every single contact in there. Put it back together and see if you get battery voltage now.

If not, keep on tracing the circuit.