88 Blazer high idle

I've got a 1988 K5 Blazer 4x4 350 TBI with a 4 speed and when it warms up it idles at about 1500 rpm. I checked for a code , got 24 vehicle speed sensor, I don't think that would mess with the idle would it ? If it is the Throttle Position Sensor , or the Idle Air Control Valve , or something else the computer controls I would get a Trouble Code right ? Any ideas
Thanks , Steve

first thing to look for when you have a high idle is a vacuum leak. Unmetered air will cause a lean condition in the exhaust. The O2 sensor sees that it is running lean and will add more fuel. Unmetered air is extra air. Computer adding extra fuel... more air, more fuel will lead to higher idle.

So I got the speed sensor in the mail today , installed it , now the idle is about 400 RPM , it even died at a few lights . The computer controls the idle , doesn't it , how do I turn up the idle ?

the computer does control the idle. I would look at the Idle Air Control (IAC) on a scanner to see what the counts are. There is an idle adjuster screw under a tamper proof cover. Disconnect the battery for a while and then reconnect the battery and let the computer relearn the idle setting.