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Discussion Starter · #1 ·
Hi Everyone,

Any specific advice on putting a new alternator in my U900?

-any pointers, etc, and things I may overlook.

Thanks,

-ACUF
 

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ACUF, what is the failure mode of the original? if you have an external regulator, you may only need to replace the brushpack with an internally regulated brushpack. This would eliminate the original regulator and the harness between it and the alternator. I replaced my alternator (12V system) with one from an 80's model VW Golf which has the same external dimensions (clocking) as the original. The one I installed is a 90A rather than the original 55A so I upgraded the wire size on the B+ wire to the battery and transferred the original pulley from the old alternator. I have even seen a few of these that had the W terminal for the Tach output.
 

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I have even seen a few of these that had the W terminal for the Tach output.
But, probably not the same number of poles as the original. The tach would probably not read accurately.
 

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Speaking of the tach, do not turn the stud that the tach wire hooks to on the alternator even a tiny bit or you will break off the wire inside the alternator and your tach wont work until you open up the alternator and solder the wire back on. These alternators are as basic as they can be. A few diodes and a couple of bearings and some brushes, that's it. Good idea to grease the bearings when you split one open.
 

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Discussion Starter · #6 ·
Speaking of the tach, do not turn the stud that the tach wire hooks to on the alternator even a tiny bit or you will break off the wire inside the alternator and your tach wont work until you open up the alternator and solder the wire back on.

These alternators are as basic as they can be. A few diodes and a couple of bearings and some brushes, that's it. Good idea to grease the bearings when you split one open.
-OK,

The tach works currently, but the alternator doesn't seem to be charging my battery, and if I run the lights, etc, the battery will be dead when restarting. The battery is new. The alternator looks pretty beat, so I assumed it was a bad alternator. How do you get the tach wire off without turning the post?
Thank you,

-ACUF
 

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OK, first does your red alternator light come on when the key is on and the truck is not running. If not, that is your problem. It must work or you will not be charging period. The trickle of current through the bulb excites the windings. To get the tach wire off, clean the threads on the end of the stud real good and spray lube it. You may need to hold the end of the stud with pliers very carefully and when it comes loose clean up the threads you dicked up with a file, die or another nut the same size before taking the nut completely off. It may be easier to cut the wire flush at the alternator and do it on the bench, real easy if the alternator is split open. But check that bulb first, no bulb= no charge
 

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I guess my first post was implying a possible bad regulator. The brushpack should be checked for worn brushes. A common and easily repaired issue with the Bosch alternator. Also, as graphic66 stated, check the warning light operation.
 

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Discussion Starter · #9 ·
Yes, the charging light is on when I turn the key on.. So what does this indicate if the battery is new and doesn't keep a good charge when running lights, auxillary electrical components?
 

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Its as simple as a voltage check across the battery when the engine is off,and then when it's running to confirm whether the alternator is actually charging,not charging or over-charging.
 

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Discussion Starter · #11 ·
Its as simple as a voltage check across the battery when the engine is off,and then when it's running to confirm whether the alternator is actually charging,not charging or over-charging.
-So it should be sending 14V to the battery when running, 12V when not running?
 

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Yes, you should get 14-14.5V running and 11.5-12.5V not running. The difference should be noticable. You can also check with just the headlights, they should get brighter when you start it up. And if it's not charging your Alt. light should come on when running and when not running. Make sure that light works before you go any further. You could also just have the problem that has probably sold more batteries and alternators than just about anything, bad battery connections. They can look good but they can be corroded or loose. If you can move the battery post connectors by hand you most definitely have a problem there.
 

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Discussion Starter · #13 ·
OK, I can check all of those things. My volt meter may need replacing, but when I locate it I will try a test with that too. Thank you.

-ACUF
 

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Discussion Starter · #14 ·
So, The red light stays on when running on the 123 setting, but its dim, but when in the 0 position its bright red when running. the tach is working. the terminals are on tight, but will take them off and clean them.

-ACUF
 

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'88 U-1300L, '70 406, '78 406, '78 416 project, '82 406, '57 404, '65 404, '70 404, '68 Haflinger.
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Oh good, you are testing the special Unimog "fry the alternator" option by running the engine in the "0" position of the switch! We've always wondered it that function works. Guess it does LOL

The "0" position is off. You should never turn the switch to "0" before stopping the engine, and Yes, the light should come on bright in the "0" position when the engine is running. I tried the 'fry your alternator" position once too. It didn't work on my truck.

ACUF, you really must have a working voltmeter to do any testing on an electrical system. Without that, questions and answers are pretty much useless.

Bob



So, The red light stays on when running on the 123 setting, but its dim, but when in the 0 position its bright red when running. the tach is working. the terminals are on tight, but will take them off and clean them.

-ACUF
 

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Discussion Starter · #16 ·
Yeah, I will start looking around for the volt meter. It will be helpful for this test, but it sounds like even if the alternator wasn't broken, now is. Luckily I have a new Bosch to replace it with, and Man, I will never turn the key to the 0 position when running!!

So what actually happens to the voltage, when the engine is on, but the ignition is off? -you say fry.. but what really happens to the electrics?

Thank you all for the feedback. Very helpful.

-ACUF
 

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That's a really good question and I have not heard a definitive answer, but...

Alternators are coil-based devices, and coils, inductors, are energy storehouses using the fields surrounding them. When the key is off, the load is disconnected from the alternator, allowing voltage to build, and not providing a dampner for those spikes. And alternators are AC devices so require diodes to convert to DC. Diodes are susceptible to damage from high voltage spikes. So, the diodes get zapped. In reality, diodes have improved greatly over the years so a modern diode is probably well rated for their task. In the early seventies a good diode might handle 100v junction v. Today, it's easy to get a cheap 1000v diode. On the other hand, our trucks were from the early years of diodes so a 30 year old alternator could easily be damaged. IMHO.

Kinda like the 'knowledge' that setting a battery on concrete will ruin it. That was true decades ago when battery cases were made of hard rubber which became porous, absorbed acid, and conducted, thus shorting itself into concrete. That's no longer a problem. Maybe the same with diodes?

Bob

....chop...
So what actually happens to the voltage, when the engine is on, but the ignition is off? -you say fry.. but what really happens to the electrics?

Thank you all for the feedback. Very helpful.

-ACUF
 

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That's a really good question and I have not heard a definitive answer, but...
Very simply (and Bob did say it), when the ignition is OFF (0 position), the alternator has no load - it's driving an open circuit. The voltage can (might, could, ect) build to a point where it destroys the regulator (internal or external) or arcs the winding.

Any AC generator could possibly destroy itself in a no-load situation - your truck, your home back-up generator, your power companies big units, Hoover Dam, 3-Mile Island, any of them...

Kinda like the 'knowledge' that setting a battery on concrete will ruin it. That was true decades ago when battery cases were made of hard rubber which became porous, absorbed acid, and conducted, thus shorting itself into concrete. That's no longer a problem.
I always wondered why my Dad told me this - being a 1970's educated electrical engineer, it never made sense to me. I always guessed it had something to do with the cold concrete floor somehow screwing up the temperature stability of the battery under charge. Thanks Bob - now I can remove the various pieces of plywood I always sit the batteries on when they're out of the vehicles !!!
 

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...Kinda like the 'knowledge' that setting a battery on concrete will ruin it. That was true decades ago when battery cases were made of hard rubber which became porous, absorbed acid, and conducted, thus shorting itself into concrete. That's no longer a problem. ....
I had this discussion with a co-worker a while back that insisted that this would kill a modern battery.
I found info on several battery manufacturers websites calling it an old wives tale with old basis of fact (older battery construction). It does not apply to modern batteries.
 

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Discussion Starter · #20 ·
Well, thank you all for the feedback on the alternator and my misunderstandings of overloading it. So, would running it in the off postion also fry lights, and other electrical things? or would the fuse system stop that?
 
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