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I just bought a 1998 Chevy 1500 Z71 with the 350, 4x4, Auto Trans.. and driving home from work this morning, it was doin some weird stuff. I was goin down the road and set the cruise at like 50-55, and I noticed every 5 seconds or so the motor would rev itself up like somewhere around 2-400 rpms. It was cruisin at around 1500 rpm. I figured maybe it was because I had cruise on and I was going up a hill or something. So I took my cruise off, and it still did it.. the whole way home. It kinda seemed like the faster I was going, the more it did it. Also, another problem its got it, I noticed sitting in my drive way, with the truck running in park, when I hit the brake.. it clicked really loud. I know it would be like the button/sensor or whatever it is for the brakes to come on.. but why would it be so loud? Any imput is greatly appreciated!
 

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I was having the same problem with my grand prix. Copied and pasted from the grand prix forum

Under the dash around the brake pedal should be a sensor that tells the PCM when your on your brakes... its not working or not aligned properly.
there should be 2 sensors\switches down there, play with them a bit, if the sensor is thinking the brakes are on all the time your TCC Will be messed.
Also if your rear brakes lights are on then you know the sensor is touching something when its not suppose to. (1 sensor activates the brake lights, the other tells the PCM when to engage the TCC).

If that doesn't help then replace the TPS sensor underhood.

That helped me, might help you to.
 

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i bet you had the heater on
my 1999 chevy 2500 is doing that now also
but only when the heat is on the 3 lower fan speeds
if it is on high it dont do it

as far as the click
theres a button behind the pedal shaft to controll your brake lights
thats prolly were the noise is comeing from
 

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i had that problem with my 2000 GMC sierra. I took it on a 300 mile road trip and ever since then it stopped with the revving. It was alot worse when it was cold out. Once it got warmed up u could just slightly tell.
 

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hemi is right
when your heat is on defrost, the AC compressor is cycling on and off.
this is because the compressors would seize up after not being used in the winter months.
you can verify this by running the truck with the windshield defrost on and hood up. you will see the AC clutch engage every few minutes.

as for the clicking it does sound like the switch above the pedal, it just looks like a button.
 
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