94 GMC sierra wont start. just single click. battery checks good. Starter taken off and checked at autozone, good. all connections were tight and no corrosion on bat terminals. any ideas

Does the engine crank over and will not run or does the starter not crank? If no crank then check the battery and connections. Make sure the vehicle is in park. Possible bad starter or dirty or loose connections. If it cranks over but will not start the spray a little starter fluid into the throttle and try starting it. If it runs, then you have a fuel delivery problem. If it does not run, then an electrical problem may exist and you will need a scanner to help find the problem.

Sounds like your battery is very low on charge, or you have a bad connection between the battery and starter or ground. Check that the battery cables are clean and tight on the battery, and that they are securely connected at the starter (positive) and frame/engine (negative). If everything looks good there, charge your battery, then bring your car to AutoZone/Advance Auto/O'Reilly's, etc and have them check your battery and alternator.

Did you check your battery? Just because all the lights come on does not mean you have enough power to crank the starter. Remove battery and have it load tested or put a charger on the battery for about 30 minutes and preferably a battery charger that has a 50 amp boost cycle, when trying to start the car. If it starts you probably have a bad battery. If it doesn't the start checking the positive and negative cables for corrosion or breaks. If all checks out then it's probably the starter is bad.

Clunk or click? A click would be the starter solenoid trying to engage the starter. Recheck battery and starter connections for tightness. If they are good, it is possible you got a defective starter and should have it tested. If it is good, make sure you have power at the solenoid wire (small purple wire, I think) when key is held in the crank position.
Aside from that, are your battery cables in good condition?

I had an 84 silverado & a 91 sonoma with the same problems. It was a ground wire. Make sure all body, & frame wires are Attatched, tight & clean of rust & corrosion, no frawed wires. Also make sure battery terminals are clean & tight. Good luck.

Replace the starter, the solenoid cant engage the starter because it is locked up, hence the extreme voltage drop with the fan when you try to start it with the fan on.

Good chance that your start has got to the end of it's life and needs to be replaced. After so many starts the brushes in the starter wear out and the electrical connection from the brush to the armature no longer have the ability to carry the amp to turn the starter gear. Check the wire connections to the starter first and try giving the starter a tap with a hammer and see if that would help get it to work. A few light tap may get you a few more start before you have to replace it. Good luck and hope your get your truck running 100% again soon.

loose connections and starter relay in power distribution box under hood. Make sure you dont have a loose or poor connection on a ground or power wire from battery.

try tapping on starter while someone truns key, sounds like bad starter if wires are clean and tight. good luck.

does the starter make a "clicking noise" when it wont turn over? check your battery cables at the battery and at the starter solenoid....if it not a connection , then it sounds like a bad starter relay/solenoid