My 2004 Yukon started itself when I connected the battery cables, no key in ignition.

[strutaeng]

I can see the starter relay welded shut cause the engine to CRANK with no key in the ignition but you wouldnt get fuel pump relay enable and ignition system enable from just that which would keep you from getting an actual engine START.

Ah, yes. I was thinking that after I posted that.

In the case I was thinking when the battery was connected it would just start cranking by itself (but wouldn't fire because no fuel...)

Puzzler for sure

 

[Hugh937]

This is a cool one. Alot of things have to happen in order for the starter to crank and engine actually start (ignition and fuel). And all of those come together at the ignition switch (not to be confused with the ignition lock cylinder). The ignition switch likely has burn/welded contacts. Its fairly easy to change out and doesnt cost much either. Just have to do a relearn for the factory security system.

Thanks. I would never have guessed that. I think I just might do this. I am just puzzled because of all the things that have to happen to do this. But the key switch is a common link amongst what it would take to do this.

 

[Hugh937]

I agree with these other guys it is most likely the ignition switch
it's probably the cheapest easiest thing to try at any rate.
I would get a oem one from the dealer if they are still available, you can get it at the parts department and ask them to re-key it to your key.
some aftermart ones are not able to be rekeyed
plenty of youtube video's on how to, you just remove the shround around the ignition, turn the key forward inset a pin a hole and pull it out, insert the new one.
then you can do either a 10 minute releaern or 3X10 min relearn, then it should start, once you have it relearned, try disconnecting the battery and reconnect see if the problem is gone.

Thank you. And thanks for the specific information on changing the ignition switch. I have never had to do that in any of the (dozen?) vehicles I have had before, and the newer the vehicle, errggghhh. Anyway, I think its worth a trip to get a GM one if available, (for some reason the local GM dealer seems to have or be able to get older parts easily). One thing I did not think of doing is looking at the scan codes to see if any showed up (other than one for a dead battery.) The ignition switch seems a common link to the problem. You can see why I would not trust working under the hood with the battery connected now. I don't know much about the relearning process, is it in the original owners manual or should I just go out and spring for a Haynes or whatever, like I should have. I have some shadetree knowledge that is really no good anymore. Thanks for the response. Going to look for the owners manual and see if it has relearn in it.

 

[Hugh937]

Those models had funky ignition switch tumblers too if you or the previous owner had a lot if weight on your key ring set

Thanks for the info. I know a woman last drove it, but don't know what she had on the key chain. And it was not fobs, since I don't have a set of them.

 

[Hugh937]

Try swapping the starter relay. I had a relay get stuck ON once and it was weird as heck. I can't remember which one it was, headlights maybe? I simple swap revealed it was the relay.

I thought about this for just a second when I found the dead battery, since it was REALLY dead, i.e. completely non-rechargable. I need to take a look at that because I bet the relay never has been changed. Thanks

 

[Doubeleive]

Thank you. And thanks for the specific information on changing the ignition switch. I have never had to do that in any of the (dozen?) vehicles I have had before, and the newer the vehicle, errggghhh. Anyway, I think its worth a trip to get a GM one if available, (for some reason the local GM dealer seems to have or be able to get older parts easily). One thing I did not think of doing is looking at the scan codes to see if any showed up (other than one for a dead battery.) The ignition switch seems a common link to the problem. You can see why I would not trust working under the hood with the battery connected now. I don't know much about the relearning process, is it in the original owners manual or should I just go out and spring for a Haynes or whatever, like I should have. I have some shadetree knowledge that is really no good anymore. Thanks for the response. Going to look for the owners manual and see if it has relearn in it.

The relearn process is just placing the key in the run.position for 10 min or 3x ten minutes depends on the year, nothing crazy has to be done

 

[Fless]

The relearn process is just placing the key in the run.position for 10 min or 3x ten minutes depends on the year, nothing crazy has to be done

Just plan to have a battery charger on it when you do that.

 

[jwth]

This happened to me on my 2003 yukon.
I ended up changing the ecu after a lot of checking and chasing.
gremlins!

 

[Cranker57]

I had the same thing happen. the engine started to run when I hooked up the battery. I had been underneath the truck working on tightening the big bolt on the starter. I went underneath again and realizedthat the wires had moved while I was tightening the bolt and shorted out. I streaightened the wires and the problem was solved.

 

[Doubeleive]

I had the same thing happen. the engine started to run when I hooked up the battery. I had been underneath the truck working on tightening the big bolt on the starter. I went underneath again and realizedthat the wires had moved while I was tightening the bolt and shorted out. I streaightened the wires and the problem was solved.

probably different, starter going and engine actually running are two different things, the starter just needs 12v and off it goes