When turning key,power comes onbut car won't start

you mean it will not crank?????(normally)_
starter motor dead key turned fully CW to the right ST position?
tell all things good and bad, this tells the elect guy how bad DC power is, the extent, and saves time. and labor.!
i guess a typo here./? IDK.
My box truck won't start with turn off key in ignition.?

say crank if starter is dead,
if does crank make sure glow plugs are good.
if only transmission is dead say so and how far dead.
all gears fail forward and reverse>?

If by lights, you mean the dash lights come on, then ignition switch must be getting power. If dash is not working with key on, check the 30 amp fuse link which powers the ignition switch.
If switch is good and starter is good, the start wire from switch goes to, if automatic, to inhibitor relay (the park/neutral function) and then to starter. There is also an inhibitor switch on the shifter or on the transaxle that turns on the inhibitor relay when key is on and shifter in park or neutral. The inhibitor switch is powered by a 10 amp fuse D.
If you have a manual transmission, the start wire from ignition switch goes to the clutch interlock relay and then to the starter. This relay is turned on by the clutch interlock switch on the clutch pedal.
You can check for power on either relay, the inhibitor relay or the clutch interlock relay, with a test light. Pull the relay out, have the key held in start position. Now both relays would have power showing on two terminals where the relay plugs in. If inhibitor relay only has power on one terminal or no power at all, either ignition switch is not sending power or fuse D is out or the inhibitor switch is not working right. If clutch interlock switch does not have power on two terminals either the ignition switch is not sending power or the clutch interlock switch is bad.
If the relay checks are good, check for power on the small wire to starter with key held in start. If you have power there, the starter solenoid must be out.

This vehicle is equipped with a chipped ignition system.On your dash warning lights,when turn key into on position you'll notice a yellow key symbal, it should flash then go out. If it flashes then goes to solid on, it means the system does not recognize signal from key. This can be because of wrong key(original valet key will not start car) or key for whatever reason has been deprogrammed. Which can only be done by dealership unfortunately. If system is working(light does go out) then you need to look at possible brake switch sensor,park/neutral safety switch on shifter, and for signal at started when key turned to start position

the clicking that you hear is coming from the starter relay. sounds like your ignition starter switch or ignition lock cylinder is bad.

well if you have a dvom/voltmeter check the power at the starter terminals. the main terminal should have power all the time the key wire should have power only when the key is turned to crank engine. if you have power at both then i would start looking at grounds

TEST POWER LEADS TO STARTER FOR POWER
AT STARTER (BATTERY POSITIVE TO STARTER BATTERY NEGATIVE TO ENGINE BLOCK)
THEN TEST FOR SIGNAL POWER TO SELENOIDE ON STARTER
(KEY TURNED TO START POWER SHOULD GO TO SELENOIDE)
IF NO POWER TO SIGNAL WIRE CHECK FUSES AND RELAYS
ON STARTER CIRCUIT
IF THIS CHECKS OUT MAY BE AUTO TRANSMISSION NUTRAL PARK SWITCH
TRY TO START IN NUTRAL OR WHILE SHIFTING TRUE GEARS SLOWLY WITH SHIFTER
IF STANDARED CHECK NUTRAL SAFETY SWITCH

Ground a test light to negative ( ground ) then remove relay and check for power, then have some one turn key forward as to start vehicle and check again there should be another hot when key is turned to start let me know if it helps thanks

First, make sure you can turn the engine by hand. If you can't then the starter can't turn it either. Have someone else turn the key and look for the source of the click...may be a relay. Go over the starting circuit with a volt meter. Check for power at the starter main terminal and then with the key turned to "start" check for power at the trigger terminal on the starter. Check battery voltage and check relay operation.
You have a neutral safety switch on the transmission if its an automatic and a clutch switch if it's a stick. Check them as well. Make sure battery grounding on fender and block are also clean and secure.

If he had it started two times without the key , i would think it was the starter either . Do you an the guy who replace the ignition switch know how the starting system on your tahoe works ? Do you know what a neutral safety switch is ? Try moving the gear shifter to neutral an see if it starts then . That is if your tahoe has a automatic transmission . When you turn the key to the start position this sends a voltage signal to the PCM - engine computer which inturn will energize the starter relay , closing the contacts sending B+ voltage to the starter solenoid , spinning the engine over . Using a factory scan tool you would be able to see if that start signal from the ignition switch is making it to the PCM . Also would be able to energize the starter relay with the scan tool to see if the computer can energize the starter relay . You could also do testing with a DVOM - digital volt ohm meter . Find the PCM an connector 2 pin #59 , this is the circuit for start from ignition switch . Have someone turn the key to start , is there B+ voltage ? Find the starter relay in the under hood fuse box, pull it out an look on the bottom . You will see four sets of numbers 30 , 87 , 86 , 85 . If you take a piece of wire an jump 30 an 87 in the fuse box the starter will crank the engine , or spin the engine . If you do that and it won't start , cables , connections or the starter is the problem .

1) Look for external ford starter solenoid or,2) Try on neutral, the parking transmission switch, 3) Look for fuse or a Relay on engine compartment