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!!
One problem is there is no such thing as a "power brake booster pump". "Hydro booster" is the power brake assembly that mounts between the brake master cylinder (where you would see/monitor/fill with brake fluid), and your brake pedal. The hydro booster is powered by power steering fluid pumped from the power steering pump, which of course also powers the power steering (down at the steering gear box). For my 2004 Duramax Silverado, I used an ACDelco 178-0853 hydro booster. It is listed for 2011 Silverado, but fits on 2001-2011. It works better and is cheaper! My pedal feel is much better than before. For a new power steering pump, I used an ACDelco 20756713. It is listed for 2008 Silverado, but fits back to 2001. It pumps much better at engine idle (e.g., if you would like to turn and brake at low speeds, like when parking). It has 12 vanes instead of 10 vanes in the older pumps, and maybe a better valve of some sort. These GM brakes have very poor results in the rust-belt. They probably have to be serviced (cleaned and re-lubed, especially the slider pins) every year. And replaced when they rust out (including all the lines). Sadly, I don't trust dealer service departments. If you have poor results from one shop and can't do it yourself, try another shop. Hopefully you finally find the guy that actually knows what he's doing.
Well, I'm not going to be much help but since nobody else has responded... I know nothing about the 2013 Duramax, but the older trucks did not have a built-in pyrometer. Did they add that at some point? I know for the older trucks you have to drill/tap into a manifold (carefully) to add the sensor. Even if there was a pyrometer added from the factory, you couldn't just splice in an aftermarket gauge. I assume a gauge like that would want a direct line to the sensor. IF there really is a factory built-in sensor, then you would have to have an OBD monitor that knows (or can be programmed) the PID codes. That monitor would plug into the OBD port. OBD is for communications only and doesn't have raw sensor signals.
I certainly don't know for sure, but one possibility is the ignition switch (e.g., ACDelco D1426D for "classic" or so-called "old body style", or OBS). This is just the electrical contact part of the ignition switch, not the key/tumbler part. They are known to go bad (I carry a spare when I travel even though I've never had a problem). When bad, they cause strange things to happen. Diesel, right? Fuel pump? Added lift pump? Stock is vacuum from the tank, through the filter, and to the CP3 (or CP4 on later models), then into the high pressure rails. Also, it's extremely dangerous to shoot any sort of starter fluid into the intake because it can run away and destroy the engine.
It doesn't sound like your fill-ups are too far off. With 45 gallons, it wouldn't be unusual for the gauge to show "empty" when there's still, say, 5 gallons in the tank. That leaves 40 from empty to full, and 20 at about the 1/2 mark. You said you put in about 18. Ok, still a little off, but not much. Fuel gauges are often pessimistic to help make sure you don't run out of gas. Try running it down to 1/4 and then fill to see how much it takes. Try running it down to 1/8 and fill. Then on down to where the warning light comes on (assuming the 1/8 showed quite a bit of gas left). You might keep track of miles on the tank at these gauge positions to make sure you don't run out of gas (in case there really is something wrong with the gauge or sender). You could even carry a couple gallons when running it low. Then you'll get to know how far you can comfortably let it go down to, and how many gallons are actually left in the tank when low or "empty".
I know everybody knows this, but somehow I don't think I was careful for the first 30 years I worked on my cars... but I have now for the last 20 years. When taking rims off/on (e.g., brakes or tire rotation), clean up the contact points on the hub and the rim. Remove any dirt and rust so the rim goes back against the hub without any undesired crud.
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