I have a 99 blazer 4wd with 170k on it. im having a problem with the starting engaging while the truck is running when you shift between "D" and "3" . It has also happened a few times while just driving down the road without shifting. Also the shift indicator doesnt work properly, sometimes its lit and sometimes its not or it lights up neutral when its in park. not sure if it might be related but wanted to mention it. any help would be nice thanks

 

is your shifter pulling the keys with it?

 

There has to be a wire shorting out in the steering column or the shift linkage is touching wires outside or under truck. The only way the starter can engage is wires touching that go to the starter solenoid.

 

Sounds like you've got a sloppy ignition switch. Fairly cheap to replace and has been talked about how to replace on here more than a few times.

Do a search and you should find what you are looking for.

 

elgringo what do you mean by pulling the keys with it? i checked at the relay the there is one hot terminal and i checked that another terminal gets hot when it cranks with the key and the same terminal goes hot when shifted so i would think its in the column or somewhere before the junction box and not after on the starter side like 4x4blazerguy said. i checked the linkage under the truck and didnt see anything rubbing wires or anything. AJbert why do you think its the ignition switch? how common is that? thanks 4 the help

 

I am having the same issue with a full size 99 Gmc sierra as well automatic trans. It started one day when I started the truck and the starter stayed on after i released the switch, I then turned it off and swapped relays around in the box to see if it was a relay issue. Nope, now when I start the truck it wont do it till its been running for a good 15 minutes or so. I changed the ignition switch after some troubleshooting(not cheap I might add lol) 200 dollars at your local parts store for my truck. to my dismay I started it this morning and let it run a few minutes and bam it done it again... I cant see it being the starter either it doesnt do it when its not running. guess what im saying is I feel your pain lol. ughh

 

Neutral Starting switch bad?

 

check your starter relay mounted on the firewall near the trans dipstick .......i had a similar problem and found that relay to be bad. also check your neutral safety switch on the transmission for any cracks in the plastic near the connectors. a crack can let water in and cause a short.

 

Upon inspection clip on starter heatshield grounding on wire... Thanks guys for your help!

 

I have the exact same issue. I recently installed another 4.3 with a auto trans 2 wd.
the new engine and trans came from a 1/2 ton truck of the same year 98. I used my original harness. Everything works except the starter engages when I shift it in 3rd gear and when I shift it 3,2,1 on the display blacks out. I'm wondering if the 1/2 ton shift sensor on the trans may be different from the original on.my Blazer? I never had this issue until I changed the motor and trans.