1999 Chevy Tahoe Lt Engine size = 350
ABS and Check engine light codes = P0502

Question = I was driving and the
ABS light came on. When I applied the brakes the started grinding. Afterwards the transmission started slipping and wouldn't shift higher than third at 3000 rpm's. Does the speed sensor control the shift rate of the tranny and if so how do I replace it.

Answer:
Hello and welcome. Sounds like you have multiple problems here. Code P0502 is a vehicle speed sensor circuit low. It is on the tail end of the trans.(or transfer case if 4wd) That does play a part in the shifting patterns and abs system. First thing you need to do is get the ABS computer scanned for codes as well, that is the key to start diagnosis.

Grinding in the brakes may be just normal brake problem, but if abs activation is what you feel, then a problem in one of the front speed sensors is most likely cause. Need to get to a repair shop and have all these different systems checked out.
1999 Chevy Monte Carlo 3.1L V6.
Question = Whenever the a/c heater is set on defrost or the passenger compartment and you accelerate or go up a hill at 2000 rpms or more it starts blowing on the floor.  Then when you are cruising it goes back to normal.  I noticed under the dash someone bypassed a vacuum line.  Is this a blend door, control head or vacuum problem.

Answer:
Yeah, you're obviously loosing vacuum, or the vacuum signal is getting routed the wrong way. With the changes in rpm or load on the engine, the vacuum pressure changes as well and if not routed correctly, or not connected properly to the back of the control head, you will lose it and the mode door/actuator will change to it's default position with is probably the floor setting.

If you say someone bypassed a vacuum line, I would get that corrected first and see if that fixes your problem. If not, you could have bad control head, or bad actuator. I would bet, though, that it is just a vacuum problem.
1999 Chevy Tahoe Code P0502- Vehicle Speed Sensor

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