So I have a 97 4.2L and I installed a bigger alternator to power my lights and stereo. So I got the 250 amp alternator, but to achieve that power, it came with an overdrive pulley (smaller pulley, spins faster, more power at lower speeds). So now the problem is, when the truck is just sitting there, by itself. That extra load on the belt causes the engine to shake, and that shakes the whole truck. It's more of a vibration, but it's getting to the point where it's a loud annoying vibration. So I was wondering if it was possible to change the idle speed up a couple hundred revs or even just a hundred. Just so the shaking stops. Because when I do give it just a touch of gas, it stops the vibration. I tried looking around, but couldn't find any information on it.

 

Look under the factory top hat for your throttle cable n you should see a screw that will adjust the idle speed

 

This is a throttle stop-not an idle speed adjustment. Turning that screw can cause your tps to go out of range. Idle speed is determined by the PCM controlling the IAC valve. It's not adjustable but worst case scenario it can be reprogrammed.

 

^+1. Gone are the days where you could adjust a carb's idle and fuel mix with a screwdriver. Even if you change tire profile size, you need to make adjustments at the PCM, which can only be done at the dealer. It should then 'learn' about the extra load of that monster alternator.

 

That's what I thought. That sucks. I hate having to spend money at the dealership.
Thanks anyways.

 

Got quote from dealership: $79. Probably 15 min. job - BUT, if it saves you gas, it might pay in the long run.

 

If your old pulley fits the new alt. I'd put it back on. The engine likely has additional load on it just with the new alt. having more power output. I don't think spinning it faster is necessary and slowing it down may correct your problem.

 

The point of the overdrive pulley is so I can get the power out of it. 95% of the time I dont need it. But the other 5% I do. I need 100 amps of power with just the lights. Another 130 amps with my subs, and the vehicle still needs like minimum 60. So changing to the old would defeat the purpose. 80 bucks isn't that bad.