Alternator changed.
It has been a while since I report back. I am still working on the coach.
As you remember, I was trying to solve the problem of low voltage on the front voltage meter. After few tries, it is clear that the alternator is broken. So I took it out.
@stump mentioned how useful an impact wrench is. So I bought one of the Milwaukee medium torque impact wrench. 600 ft/lb of nut busting torque is enough to handle most scenarios. The big brother has over 1400 ft/lb of torque, but it is too expensive and too big and heavy. I don't think I will ever need that much torque.
With the help of the little impact wrench, taking the alternator out is piece of cake.
After alternator is out, I called to many shops to get a quota to rebuilt. The one @wolfe10 recommended is in Pearland. It is about a hour away from my house. I called them, they are super nice and seems very knowledgeable about power line alternator. They charge minimum $390 to rebuilt the alternator. I mentioned few people on the forum rebuilt their power line alternator for under $300. The owner said that is few years ago. Everything is more expensive now.
I went to another alternator shop that is about 30 minutes away from my house. They are pretty reputable too on alternator rebuild. They bench tested the alternator to confirm that the alternator is broken. The bearing is also need to be replaced. But it seems like they don't want to rebuild the alternator, so they start recommending new units. a KEI heavy duty alternator for $670. A Delco Romey 28SI alternator for $380.
Wait a minutes, If it cost at least $400 to rebuild the alternator, why not buy a new one? If buy a new one, why buy from a local retailer. There are definitely better place to buy online.
So I took my alternator home and start searching options for new alternator or if I can rebuild by myself.
The manufacture of powerline alternator is now HD Power Solutions. They are still in Fort-worth area. I called them few times. They are super nice people. Very helpful. But they said to rebuild the alternator by them, it will be at least $500. They can sell me a newer updated model 25-16 HD alternator for a discounted price of $900 plus tax. That is beyond my budget for a alternator. True it is nice for the alternator to last another 30 years. But I seriously doubt if I am still going to drive the same coach after 30 years. So it is not an economic way for me to invest another $1000 for a 30 years service life.
Their tech support guy is very friendly and helpful. The guy I talked to seems to know everything out of his mind. He has been there for over 25 years. I think he might be the one who build this particular alternator 
. So he knows inside out the alternator. He even give me his cell phone number so that I can call him any time I want. Absolutely best tech support. With his help, I quickly determined the alternator is broken and it could be as simple as broken brush or a broken rectifier. With it out of the engine bay, I took the voltage regulator off to check the brush. It is pretty obvious that one of the brush has broken metal ribbon. See the picture. So the first step is to find a replacement brush. They seems still have parts available for this model. Bearing $45 each needs two of them. Brush $15. I might take time to gradually rebuild the alternator by myself later.
So rebuilding alternator might now be an option for the near future. Buying a new one seems to be the better option. My requirement is clear a 200AMP alternator with J-180 mount. Oh boy, there are so many options for the alternator. Price range from $100 all to way to $1200. From 100 AMP to 500 Amp. It is confusing.
Since the alternator shop mentioned Delco Remy 28SI as a replacement, I start looking for this particular model. It seems like this model is very popular for 18 wheeler truck. Nothing too fancy, just a reliable alternator. But finding a geniune Delco Remy online seems to be challenging. There are so many website offering that model, and the price varies greatly. I think those major auto parts store should have decent price. Autozone, AdvanceAutoParts, Oreilly and NAPA. After looking around on these store website, one particular model on AdvanceAutoParts stands out, it is carquest hd alternator 28SI. Judging form the model no, it is very likely a re-branded Delco Remy 28SI. And most importantly there is a 25% coupon for fathers day. So I places order on Advanced auto parts. It come out at $250 shipped to home. Not bad for a 200AMP alternator from a trusted store. I know I can return to AAP local shop if there is anything wrong.
While waiting for the parts to arrive, I needs to take the pulley out from the old alternator. Again the guy from manufacturer over the phone helped me hand by hand to get the pulley out of the old alternator. Great Tech support. He is surprised that I have an impact wrench. That little boy make things so much easier. As @stump said, it will totally worth the cost if you use it just once. I totally agree with that. With the pulley out, I took the body apart, as you can see from the picture, there are a lot of rust and greasy inside the body. Not sure if it is rebuild-able by DIY.
After few days, the alternator arrived. To my surprise, it is not a Delco Remy 28SI. Instead it is manufactured by WAI. It seems like WAI is a big auto parts group, they might did the manufacture for or even own the brand of Delco Remy. So it is not a DR 28SI, but it is from the same manufacturer. It should have same quality.
Installed pulley follows Delco Remy 28SI manual. Then installed on the engine. The original cable is too short. I have to cut the zip tie to loose the case to reach the terminal. It pretty tight for the positive and sense wire. I have to pull the wire really hard to get enough to reach to the post. Hope this will not cause any issue in the long run.
Since the tachometer is not reporting accurately. I bought a cheap laser tachometer.. Put tiny reflective tape on the wheel that drives the alternator so that I can use the device to measure the RPM.
After everything is put back, I am super nervous to start the engine. I am afraid I might break something else. But I am glad to see that everything seems to work just fine. The voltage meter show 14V reading now. The tachometer shows 1173 RPM at idle. The Tachometer on the dashboard shows about 860RPM. I think the alternator drive wheel is different from the engine RPM. But both are pretty close. The tachometer might be working fine.
I know it is too long of a post just to change an alternator. Somebody might be able to change it within few minutes. But for people like me who are not very familiar with heavy duty truck, it took some time to get over the initial fear.
Thanks for your reading. Hope it can be helpful to somebody else.
Now I can proudly remove 1 items from my 933 to-do list 
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Topic: 1991 U300-- bought it!