serpentine belt tension 5.7L

Hi gents - 1998 K2500 5.7L 255,000miles. Lots of parts replaced, and overall very pleased with mileage I've gotten out of it as a reliable daily driver this year, since changing head and intake gaskets last March. I look over the engine every time I check the oil, which is about every other time I fill up. About three weeks ago I noticed that front of the AC compressor pulley stopped, then started turning again, not the pulley itself, but the very frontmost part of it, if that makes sense. Combined with the frequent mild smell of burning rubber, plus the idle is starting to feel a touch rougher, it was almost imperceptible at first, but now when I'm holding the brake at a stoplight or even just idling in park, the idle rises and falls just a bit, compared to rock steady before. Would you think this is related to improper belt tension, and if so, ideas of where to start? Thanks in advance.

Did you have the AC on? Did you have the defrost selected with fan on? The ribbed pulley should turn with belt. When clutch energized the center compressor will drive turn with belt or engine rpm. For belt info this page. If you have the OE service manual there is more info.

TommyJ1980 said:

Hi gents - 1998 K2500 5.7L 255,000miles. Lots of parts replaced, and overall very pleased with mileage I've gotten out of it as a reliable daily driver this year, since changing head and intake gaskets last March. I look over the engine every time I check the oil, which is about every other time I fill up. About three weeks ago I noticed that front of the AC compressor pulley stopped, then started turning again, not the pulley itself, but the very frontmost part of it, if that makes sense. Combined with the frequent mild smell of burning rubber, plus the idle is starting to feel a touch rougher, it was almost imperceptible at first, but now when I'm holding the brake at a stoplight or even just idling in park, the idle rises and falls just a bit, compared to rock steady before. Would you think this is related to improper belt tension, and if so, ideas of where to start? Thanks in advance.

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The air conditioner compressor clutch will lock and unlock, any time the AC (or defrost) is on. When the AC or defrost is not on, the compressor clutch should be unlocked and freewheeling, as much as the normal belt tension will let it.
The belt tension on the 400s is regulated by the belt tensioner assembly. Several things could be going on:
The tensioner or its pulley could be failing,
The compressor clutch could be going bad,
The idler pulley could be beginning to lock up and that could be making the belt slip and affect the idle.
It's normal for the idle speed to fluctuate a little bit when the compressor clutch locks up, because it is putting a little bit extra load on the engine.
I recently had the idler pulley lock up and kill the belt on my crew cab. Fortunately it happened at idle speed, waiting for the light to change so we could turn into a parking lot. On a hydro boost equipped truck like my one ton, no belt means no power steering or power brakes, so it took a bit of muscling to get it into the lot and somewhere I could work on it. Luckily it wasn't too hot, because the temp gauge was climbing and the parts store was a few blocks away. But it easily could have been way worse, could have happened at night or on the road.

There is a low pressure switch that attaches at the accumulator and you can unplug that for now, till you decide what to do with the rest of the recommendations. That should turn off the a/c cycling system. If you go deeper, you need to plug back in.