With 4wd, you need to be aware of the "pump rub" issue in the magnesium tail section of the transfer case. It can cause a pinhole in the tail section, very slowly leak out the ATF (in the transfer case), and ruin it. There are various pump rub kits, or if a tail section is ruined there are aluminum replacements. I don't live in a cold climate, so I'm certainly no expert about winter prep. Bio-diesel is very good for the motor but gels much quicker in cold weather. Avoid it in the winter if you can. You'll have to study up on anti-gel additives (like Diesel Kleen 911, I think), and using Diesel #1. The glow plugs and controller need to work well in cold weather. Turn key on, wait for glow plug light to go off (should be about 5-10 seconds in cold weather, I think). I always suggest a lift pump and more/bigger fuel filtering and water separation. I use a Kennedy Diesel lift pump and Donaldson filter on a Nicktane auxiliary filter head. Beware that some setups flow too much and cause problems with the return into the tank, so be sure to read reviews on the so-called "bigger, better pumps". I assume you know that the LB7's had problems with the injectors. Again, there's lots to read on that, too. Again, I'm lacking experience with the LB7 since I have an LLY with it's own list of potential problems!!
While it costs more than rebuilding, many have had better results with just replacing with a new unit. I replaced mine with one made for 2011 (ACDelco 178-0853). It may require a little bit of filing on the rod eyelet so the brake light switch works properly, but there was also a significant improvement in my brakes. Read reviews for that by others, not just me (Amazon and elsewhere).
All of the above . . . plus try new bulbs. Sometimes when they "burn out" the filament jumps over to another filament and strange things happen. Sometimes you can't tell just by looking at the bulb, either.
Blend door actuators? Notorious for going bad. I just replaced one, but it was the easy one which just put heat to the driver side all the time. Rat nest? Other debris like leaves in the vents? But sorry, I don't know where to start looking...
Basically, the "something special" you need is ... a Duramax pickup truck. Then move whatever it is that you really like from your old truck to the Duramax truck (if that something is compatible). Maybe you can get davester to help you He already knows how to convert from 2wd to 4wd!!
Almost always it's a loss of prime. Try priming it (hopefully you know how). If the filter head hasn't been rebuilt or replaced in the last few years, it's probably leaking air. Also, the soft fuel lines are getting old and leaky. Sometimes even the hard lines are rusting, and sometimes from the inside out. If you don't have a lift pump, the pump on the engine sucks fuel all the way from the tank and through the filter. A "leak" anywhere will cause a loss of prime. I say "leak" in quotes because air leaks in and fuel does not (usually) leak out at all.
Certainly sounds like a stepper motor. While a more common problem in the older trucks (I replaced all of mine years ago), it seems like any stepper motor can go bad. If you have something that plugs into the OBDII port and displays stuff, you could watch the speedometer speeds shown there. If they look good there, then it suggests the stepper motor for sure.
As long as whatever you are doing doesn't cause jerks or clunks, then it's fine. By that, I mean (for a Duramax, but I'm sure similar for you), for me, between about 28-48 mph the torque converter is locked up in T/H, but unlocked if not in T/H (generally). If I am under slight acceleration/load and switch out of T/H in that range, the drivetrain gets a jerk/clunk. So I don't do that. I don't think it's as touchy engaging T/H in that speed range, but still I make sure there's no load so the transition is as gentle as possible.
Of course!! [On edit] Ok, maybe I should qualify that a little. You probably shouldn't shift to a lower gear that would cause the engine RPMs to go above redline. But I also think the computers are going to protect the engine from doing that anyway; still, why test it? I guess you shouldn't be shifting it every 2 seconds, either, but I don't think that was your question.
2004 was a transition year from 1st gen (LB7) to 2nd gen (LLY). Look at the RPO codes in the glove box to see. Each has their own distinct list of issues. Maybe the biggest for each: LB7 - injectors (1st set were probably replaced under warranty). LLY - overheating (towing up long grades, especially in the west); injector connectors (#2 and #7 notorious; but the injectors hold up very well!); headgaskets (especially if overheated or had tuner). I usually recommend the LLY (I have one) only for people who are likely to work on it themselves (maintenance, looking at known potential problem areas). Regardless, if it's had a tuner I would walk away. I hate that it sounds like I am trying to scare anyone away from these trucks. I've had mine since new and plan to keep it "forever" because it's a GREAT truck (but I've taken steps to prevent overheating, worked on the #2 and #7 connectors, protected the known wire rub-thru spots, etc.).
As others have said, the G80 is sensitive to having some friction modifiers, not too much and not too little. It does have clutches that engage the locker. Use either genuine GM grape juice, or I've been using Mobil 1 synthetic gear oil, 75-90 that says "LS" on it, for a long time without any issues. Unless the Lucas specifically says it has friction modifiers for limited slip, then the G80 is definitely going to chatter. Or they might be the wrong modifiers. Stick with what's known to work well!!
The charging line to the camper (camper wiring option or trailer connector at bumper) is always hot. Disconnection happens either because (1) you pull the plug, or (2) the camper or trailer has a battery separator. For example, my Lance camper has a separator that connects when charging voltage from truck is over about 13 volts (13.2 or 13.3 I think). After stopping for a minute, the voltage drops and the separator disconnects. If this just started, the battery separator in the camper is a good place to look. Otherwise, just pull the plug! Or, possibly, somebody added a battery separator on the truck side and it has failed. I had my battery separator fail a few years ago. It failed "open" so it wasn't charging the camper battery.
First, everything Davester said!!! For both the diffs, I like Mobil 1 Synthetic 75-90 LS. The rear diff needs some (but not too much) friction modifier, so the gear oil that already has it (like the Mobil 1) is the easiest way to get it right. Or you can pay something like 3x more and get genuine GM "grapejuice". As Davester said, the transfer case needs Dex III and not Dex VI. If it hasn't had an after-market pump rub modification, then it needs it. I think GM might have one, but it's not as good as the after-market upgrades. For the Allison, a TES-295 ATF is the best you can get (Castrol Transynd or Mobil Delvac; there are others). Also very good would be a synthetic Dex III ATF (like Mobil 1 Synthetic ATF "for older GM" because they might not be able to advertise Dex III since that spec was retired). Don't let a dealer talk you into Dex VI (could develop leaking seals later, unless your transmission was made after the "change", but you'd have to find that, and find your serial number). Either way, TES-295 is best. At 170*F, the proper fill level is about 1/4th up in the hot has area of the dipstick. Filling it higher might cause the transmission to run hot. Also, just replacing the external filter would cause only a few ounces to be lost, so for sure don't add a quart!!
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