99 Yukon wont crank

I need some in depth technical help please - here's the long sad story in 3 part harmony: Recently this Yukon (5.7L Auto Trans) would start fine in the morning, drive fine to work, start up fine after work, but shutting it off 1/2 hour later and trying to restart again would get no action from the starter at all. Let it sit and hour and then it started up fine. Happened again the next day only this time it took 2-3 hours before it would restart. I thought the starter was the culprit, so spent big bucks on a new starter and put it in.
It started up just fine with the new starter but the next time it was driven for a while and shut
off for about 30 minutes, it would not restart again... not even after waiting more than 12 hours. Here are the results of some testing that I have done:

No check engine lite was ever present, and plugging in my OBD2 reader showed no error codes. Security light seems to function normally - stays on for about 5 seconds after turning on the key and then goes off.
The fuel pump does run when the key is turned on.
The fuses are good.
The starter relay is good. Jumped across the output terminals of the starter relay socket and the starter cranks fine, motor starts but will run for just a couple of seconds.
The ignition switch on this vehicle was just replaced about a year ago.
According to the factory manual that I have, there is supposed to be a ground present on one side of the starter relay (coil side) this is pin 85 on the relay, and then when the ignition key is turned to start, there should be +12 volts on the other side of that relay coil which is pin 86. I dont see either when I put my meter on there. The diagnostics dont appear to take into account the theft deterrent system, which I suspect may be the reason. Should I be able to see theft deterrent trouble codes with the OBD2 reader as well? I tried adding a ground to pin 85 of the starter relay but it still wont start. What else can I do to figure out what is keeping the starter relay from working? Is there a way to bypass the theft deterrent system? HELP ! I'm stuck.
Thanks in advance.

How's yopur alternator? I think the wiring routes through the alternator to the starter.

correct me if im wrong but I beleive the antitheft will disable the fuel pump but the starter will still crank.
Try looking at every large guage wire terminal for corrosion (battery, starter, alternator, and all grounds. It just may not be getting enough current even tho the voltages look correct.

I also have a 1999 yukon that won't crank. key on I have a light in the gear position indicator. Key in run no light. If I put the key in the run position & then test the #8 fuse with a continuity light the vehicle will start and run, I can do the same at the yellow wire/black stripe coming out of the ignition switch. all wires seem to have power that should going to the ignotion switch. I also have similar results at the starter relay at the under hood power dist. center. I think that the problem is either in the ignition switch or the neutral safety switch on the transmition, but I am not sure and at $172 fro an ignition switch I want to be sure before I replace it. If you find any answers I would sure appreciate an E-Mail, GRIZZYTAZ2@ATT.NET

Welcome to AF. In the future start a new thread. See if you have current out of the ign switch to the neutral switch, if so go to the neutral switch and see what is happening there, if not the ign switch is bad. I assume the security lite is not staying on?

You can check if it is the Neutral Safety Switch by shifting into nuetral and see if it cranks. (just a quick and easy check- not a tell all meathod)

No check engine lite was ever present, and plugging in my OBD2 reader showed no error codes.


take to a good shop that has diagnostics if u can. a $5000 code reader will bring up more than a $500 reader will, engine lite on or not.

please post what u find out. i had kind of the same issue w my 93 1/2 ton. never did figure it out. th poor old girl is nothin but a wood hauler now lol