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2001 Toyota Tacoma knocks when AC belt is tightened

Previous owner had disconnected the AC My mechanic hooked it up but when the belt is tightened it make a knocking sound louder when under load. He could not find the cause. When the belt is loosened it goes away.
The truck has 123,00- miles on it and had been sitting for 6 months before I got it. Finally just had him put the belt back on and the knock hasn’t got any louder.

I assume you mean when the A/C is on? I am surprised your “mechanic” could not find the cause. A simple stethoscope could probably tell whether or not the knocking comes from the compressor. Cold be one of the compressor’s piston is going bad or the A/Cs pulley is on its way out.

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Check the harmonic balancer also. One thing you do not want is a loose or worn balancer knocking on the crankshaft nose. That can lead to crank damage which means major motor work.,

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Find the source of the sound step 1. One could hope it is a bad tensioner.

Sorry I forgot to mention he replacedvtge compressor but the knock was still there

That’s a good bit of info. New compressor. No belt. No noise. Related?

In that case Barkydog may have the right idea… belt tensioner … or any other accessory with a pulley.

However, it is possible to over tighten a belt and causing a pulley out of alignment and causing the knocking. If all accessories run OK, battery charges, power steering works, A/C work with the belt loosened a bit I would keep it that way. A stethoscope is still the best way to identify the source of an unwanted noise. Heck, a simple 2ft piece of garden hose will do it.

I don’t like to say this mechanic is in over his head but it just does not seem like it would be that difficult to locate the source of the knocking.

I will just reiterate what I said earlier about checking the harmonic balancer. If it’s not too late already.