3G alternator help - low voltage

[gnathv]

Cycle glow plugs without starting and then check voltage, see if the are less than 12v

 

[Booyah45828]

13.45 will still charge, but it's preferable to see them at 14 or a little over. I think all 3g voltage regulators have a set point over 14.

Your glow plugs wouldn't be on while you're testing would they? Check to see if they are.

 

[renjaminfrankln]

Cycle glow plugs without starting and then check voltage, see if the are less than 12v

Bounced back quickly to about 12.25v and then slowly rose up.

Glow plugs not cycling when the truck is running, I have a manual relay installed with a dedicated switch.

Before I go any farther I am going to clean the contacts on both positive battery terminals. They look fine but I don't think i've ever had them off.

 

[gnathv]

That’s a good place to start. It seems that your batteries may be weak. Won’t know until fully charged though. Your alternator seems low as well. Have you charged them to 100% individually and then checked voltage and tested load?

 

[The_Josh_Bear]

I would also clean the grounds both at the terminals and at the block. Bad grounds cause more issues than just about anything.

As @Booyah45828 said, any regulator will have it's maximum charge point over 14v. And your batteries should have a "standing voltage" around 12.8v. This is fully charged, grounds disconnected and left overnight. That's the best way I've been able to test. This gives them each time to equalize away from the other battery, revealing it's true state.

Load testing is good too but since it runs the GPs and then starts the engine I'd say they load test fine.

Look up "how to full field 3g alt" and try that, it will give the maximum charging capability of the alternator(according to the set point on the reg), disregarding the voltage input to the regulator.
This will tell you a lot, like if it's capable of more and is just stuck low for some reason. If that's the case then it's not sensing the correct voltage(I think the yellow wire?). Whichever wire is the sense needs to go to the battery + terminal or the starter relay + on the fender. Both are fine.

 

[gnathv]

Maybe this will help. Just went out and measured my voltages.
Truck hasn’t been started since Friday. 30s at night upper 40s day.
My batteries are 5 years old Duracell (Sams club).

12.7 after sitting before start
11.2 during glow time not started
12 while afterglow truck running
14.5 after glow times out

what does yours do during these events.

I read that at 9.6 v under load it’s time to replace.

 

[renjaminfrankln]

Before I go any farther I am going to clean the contacts on both positive battery terminals. They look fine but I don't think i've ever had them off.

All contacts shiny clean, no change.

Re: Batteries I have not load tested them individually. But they seem fine. They sit at 12.5 after resting 24 hours and hold there for several days. Disconnected and separated for an hour or so today, both were reading 12.5. Truck cranks over fast enough to start instantly when its hot. With a good 10 sec pulse of the glow plugs it will fire up pretty quickly even sitting overnight with a morning temp around freezing.

13.45v is not bad and is keeping the batteries charged. But, it definitely should be slightly over 14 and not knowing why it is low does bother me. Also I think it has gone down slightly over time. One of the previous owners had installed an aftermarket cigarette lighter that was wired through the firewall straight to the battery. I had a USB charger that displayed battery voltage hooked up to it, so a pretty good direct source of batt voltage. Around a year ago, to the best of my recollection it was reading 13.7 or 13.8v when driving.

About a year ago I tore the truck down to do the heads. A project that took me some time. At that time I disconnected the cigarette lighter (because I realized it was not fused, yikes). I just recently re-connected the cig lighter with some proper wiring and put my USB charger back in there, now it is reading 13.5v when driving around, so that led to me beginning the troubleshooting process and this thread. (but just to be clear I am using a multimeter now to try and hunt down the issue)

 

[renjaminfrankln]

12.7 after sitting before start
11.2 during glow time not started
12 while afterglow truck running
14.5 after glow times out

what does yours do during these events.

I read that at 9.6 v under load it’s time to replace.

12.5 after sitting a few days (12.6 right now)
11.2 when using glow plugs
no afterglow
13.45 when running

 

[renjaminfrankln]

I would also clean the grounds both at the terminals and at the block. Bad grounds cause more issues than just about anything.

Look up "how to full field 3g alt" and try that, it will give the maximum charging capability of the alternator(according to the set point on the reg), disregarding the voltage input to the regulator.
This will tell you a lot, like if it's capable of more and is just stuck low for some reason. If that's the case then it's not sensing the correct voltage(I think the yellow wire?). Whichever wire is the sense needs to go to the battery + terminal or the starter relay + on the fender. Both are fine.

Re: Grounds they look fine to the naked eye and I get almost perfect continuity test between the battery posts and the alternator case. Actually went from 3 ohms to 1 ohm after cleaning.

Also earlier I threw a jumper cable directly from the battery negative post to the alternator body just to be extra sure.

The sense (yellow) wire goes to the battery + terminal via a ring connector. New ring connector crimped on by yours truly. The Green goes to the cab charging light... white goes to the stator terminal on the alternator (with a fresh disconnect crimped on).

 

[The_Josh_Bear]

I guess the only two things left are to full field the alt and if still 13.5v, the reg is bad out of the box. You didn't mention if you got a Motorcraft unit but all the knock offs are a crap shoot. Been there before.

There are 6 diodes in these 3g alts so if one went bad I would think you'd lose 2v. But one of the reasons 3gs are so great is that they are so robust, so I doubt that's it.

 

[gnathv]

Remember amps are related to voltage. If your amps are down your voltage will also be down. At 13.4 you’re charging, just at a lesser amperage if it’s the alternator. If it’s the voltage regulator your amps may be there. I think your alternator is getting tired or was rebuilt with tired parts.

 

[Booyah45828]

Full fielding it gives the alternator's max voltage it can create, but it doesn't check the rectifier or diodes for failures. Which is likely where your problem lies.

With the engine running, set your meter up to measure ac voltage, and connect the positive end to the alternator's B+ terminal and the negative end to the alternator case.

Anything more then .5 volts ac shows diode damage and the rectifier requires replacement.

 

[renjaminfrankln]

Full fielding it gives the alternator's max voltage it can create, but it doesn't check the rectifier or diodes for failures. Which is likely where your problem lies.

With the engine running, set your meter up to measure ac voltage, and connect the positive end to the alternator's B+ terminal and the negative end to the alternator case.

Anything more then .5 volts ac shows diode damage and the rectifier requires replacement.

Alrighty.

I did this test and I got 30 volts AC. So It sounds like i'm going alternator shopping!

 

[renjaminfrankln]

https://www.rockauto.com/en/moreinfo.php?pk=5778357&cc=1114759&jsn=935

Bosch AL7523X from rockauto... $75 for a 130 amp alternator sounds pretty good

 

[gnathv]

I would think you would hear the ac ripple through the radio if the rectifier was bad.