Took a look at some of the threads that look similar to my problem. Wondering if it's a Passlock issue. 2002 LS, cranks and sounds like it's about to start then dies. Thought it was a battery issue, able to jump start it so I got the battery replaced. Put the new battery in and it wouldn't even turn over (clicks, no power to the dash or other components).

After about 15-20 minutes I noticed the light under the hood came on, suddenly there's power to everything. Tried starting it, no luck. Tried jump starting it and the darn thing fired up. Shut it down for a few minutes, cranked it up and everything was fine. Shut it down again, left it alone for a short while, came back and it cranked right up.

Next day, my Dad moved it a few feet to shuffle the cars around in the driveway. He said it started right up, ran without any problems and he parked it. I came by to pick it up later that same day, cranked it up and again, no start. Turns over just fine, my guess is that if I jump start it, it will work.

It seems somewhat random, everyone's said to check the usual (alternator, plugs, fuel pump) but I don't think there are any issues with those components because when it does manage to start up, everything runs just fine. 136K on it, just need it to last another month or two. Plan on selling it later this year. (If you're anywhere near Cypress, TX and want a deal on a 2002 Blazer LS, message me!)

Long story short, is this a Passlock/ignition switch issue or something more serious (re:expensive)

 

Sounds as if it is a weak fuel pump to me. In my experience with these trucks, jump starting the vehicle with another, running vehicle, will bump the system voltage of the vehicle being started enough to boost the output from a weak pump. Once running, the higher voltage is consistent given a properly functioning charging system (alternator). A fuel pressure test will tell the tale.

Testing the ignition switch contacts using the information found in the link below my signature image will either confirm or rule out the ignition switch as a problem.

 

Thanks swartlkk! I'm hoping it's not the fuel pump (planning on searching the forums on how to replace). It's a curious problem that almost seems electrical in nature (I would think a bad fuel pump wouldn't even let the truck run at all) like I said, sometimes it runs without fault, and sometimes it just refuses to crank. I'll update when I can.

 

Swartlkk, in your link for the ignition switch contacts it shows what can happen with a faulty switch, but how can I actually test it without opening up the switch? I think I have a VOM (somewhere in the house) that I can use, can I check it that way?

I was wondering if maybe a different key would work better, so I tried my spare key and the Blazer started right up. So I took that key out and used my regular key and it will crank but not fire up. I switched back to the spare and it wouldn't start up again. It's hard to get a consistent result when troubleshooting it. I can feel/hear the relay in the engine fuse box clicking too, so I don't think that's an issue. It's not a PASSLOCK issue either because I'm not seeing the Security light coming on after the first crank.

I thought for sure that it was the fuel pump earlier, because I started having problems with it a few weeks ago (getting some weird problems with it sort of "surging" while driving, engine stalling in a turn, difficulty starting it on the first crank). I'm on a pretty tight budget, if I had to focus on one area, would it be the ignition switch or the fuel pump?

 

sounds like a primary power problem to me. Could be more than one problem but if lights are affected, it points to that.

Here's the starter circuit {for 99 but should be close} - attached and a map of power harnesses here:
Light Truck Chevrolet 2002*BLAZER 4WD*CHASSIS WIRING-LAMPS*2000-2002 ST BATTERY CABLES & RELATED PARTS (L35/4.3W)* Car parts for your Buick, Cadillac, Chevrolet, Hummer, Light Truck, Oldsmobile, Pontiac, and more!

When it wont start, I suggest you check voltage from batt neg to both frame and block, should read zero or less than 1v of course.
Also check from batt pos to any point in power dist block. Should also be none or very low. significant volt reading means bad connection between there and batt terminal

 

I have been troubleshooting the same issue. I ended up changing the Crank Position Sensor today. The resistance was higher than should be. I did get the Blazer running. I noticed that the fuel gauge bounces with the turn signal. I also had trouble shifting into gear. The kicker is that it also would not come out of 4 wheel high. So I thought I had the problem licked it looks like I am just beginning. Ignition switch is new but the key gets stuck and wont come out. I really dont know where to go next. But this site has great info wish I found it sooner!

 

No power then underhood light came on.

Change the battery ends and make sure to clean the wires up before you put them in the new ends.

It will cost you $4 total.

 

rman - after changing the crank position sensor were you able to consistently start the Blazer?

 

VOM shows no voltage issues pettyfog. At this point, jump starting it won't start it up now.

 

Will an ODB II scan provide a better idea on the problem?