The air conditioner will begin to blow cool air after about 4-5 miles at normal speeds but when I increase speed to get on the highway, the air begins to blow hot air. After about 10-15 miles the air will start to blow cold air again. I purchased a can or R134 w/ a gauge and when I connected the gauge the reading said POSSIBLE OVERCHARGED.
Any advise would be appreciated due to the 100+ degree weather.

 

It doesn't cost that much to get a professional to look at it, that's my advice. I've seen coupons in the paper for places like Firestone for a $19.95 A/C performance check.

 

those cans don't do much. To properly charge the A/C, the freon needs to be evacuated, a vacuum pulled on the system so it will suck in the new freon to fill the system

 

did you turn the truck on and put the ac at max ac and to the last speed cuz if u dident then thats why its saying over charge

 

You can't tell if it's properly charged with the little gauge that comes with a can. You need a set of high and low side gauges and some thermometers. If you don't know what you are doing, it's really best to pay a pro.