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Installing a new alternator in a U900

3582 Views 28 Replies 8 Participants Last post by  bandit86
Hi Everyone,

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

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


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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?
IF the switch system connected the alternator output to the DC accessories when in position -0-, running with the lights on would actually provide the load desired. But, seeing it's position -0-, it does not connect anything :)

Here's an example of a proper, but unexpected, alternator load - MY truck is set up with a master battery disconnect. Running the engine with that switch open (no battery) denies the alternator it's load and could possibly harm it. BUT, leave that switch disconnected (open), turn on all the lights and the alternator would be happy and harm free. That 20A (or so - I have all aftermarket high energy bulbs) load simulates the battery.

(edit) BTW, a simply fuse system will NOT reliably protect anything from high voltage spikes - only (extraordinary) overcurrent conditions.
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Also, how are these spikes created? and, if the key was in the off position, then now worries about currents running through the trucks electrics, right?
Also, how are these spikes created? and, if the key was in the off position, then now worries about currents running through the trucks electrics, right?
The generator windings create a electromotive force (EMF) as they rotate, through the principle of electromagnetic induction. This is the fundamental of all rotational voltage generation as discovered by Faraday/Lorentz/Maxwell in the early 1800's. When there is no place for this force to dissipate, it can grow larger then the designer of the generator intended. If it gets too large, damage to sensitive electronics can occur.

In a modern alternator equipped vehicle, this damage could occur to the regulator semiconductors. In older, generator equipped vehicles, the possible damage could be to any connected electronics, indicators, coils, mechanical regulators, ect. The potential peak voltages from a automotive generator probably wouldn't be too great since it's so small and limited in rotational speed - most probably less then 100V.

In a Mog, any damage would probably be limited to the alternator or generator itself unless you had some kind of ultra-modern, cheap (no protection circuits) gauge, GPS or other consumer device attached to the DC line. If the key was -OFF-, yes, no worries as nothing is connected. If there WAS something connected, then we probably wouldn't need to talk about damage as there would likely be enough of a load to prevent the root cause in the first place :)
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Very excellent feedback, as usual.

Thanks Great Collective Unimind.

I will keep the updates coming.

alright, my charge light does not light up. i shorted the connector to ground, the light works. Would this be because I did the wrong sequence shut off thing? or would the charge light still light up? I dont remember that light ever working...

The black wire has a good ground on it, the big reds are 12V, the charge light about .4V when connected, 12V on the wire when not connected.

I have a plug with 3 wires that is not connected to anything near the air filter, black, brown and I can't remember. Also a silver coloured metal housing to the right of the air filter on the fire wall, would that be the voltage regulator?
The alternator on that machine has an internal reg, or at least it did when I had it.

Not sure about the wire behind the air filter. If you can take a picture of it maybe that would jog my memory.

Jason, did you know this mog is not a 70? According to the guy who did the appraisal it's a 73 or 74. Would that mean different alternator?
Not sure how it could be a 73 or 74 when the German ownership said 1970. How did the guy determine the year? Unless he had the build card pulled I have a hard time believing it.

Build sheet. Built for a paper pulp company. He is sending me the info but the postal service is on strike, will be a while before I can verify
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