I HAVE A 1989 GMC SIERRA ,I WAS DRIVING DOWN THE ROAD AND HEARD A LOUD POP . MY TRUCK DIED RIGHT AFTER THAT . WHAT COULD HAVE HAPPENED ?

The video may help?

https://www.google.com/search?q=1999+GMC+Sierra+Classic+3500+4x4+shift+down+while+driving+down+the+road

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Did you open the hood? It could be a popped serpentine belt or a broken timing chain or timing belt.

yup it can in the distributor you have a white plastic coil .very common for these to just die. you have to pull the distributor to change . when you begin make sure number 1 cylinder is at top dead center . it makes it easie to put everything back together . your rotor will point to the number 1 cylinder on reassmebly then set timing ........ You have turned it over with a spark wire off right to see if it sparkes. do this first !

Have you checked the fuel pressure? I think that the fuel pressure is between 55 or 60 pounds. They won't run under 52 pounds.

6 or 8 cyl ? Were you driving down the road when it happened ?

Usually the ignition module is inside the dist and that fires the coil. The coil is either in the cap or mounted on the intake manifold.
If the truck has over 100,000 miles the timing chain could be off. A timing light would tell you.

well i was driving and my truck just died ,no starter and lights radio works .the main starter fuse was blown and fuel pump dead ...replace all and back on the road now ...hope this helps serge

look out the code.. and check the speed sensors and the harness......check also the switch maybe you use the auto 4 wheel...thars why it engage its self automatically depending on the road,

It may be that the connecting rod popped out of its socket. Remove the cowling and take a look at the assembly..

Sounds like a fuel pump issue. Confirm readings with a fuel pump gauge. Post the psi readings so I can see them. I'm away from my reference material right now, but I think there is either a TSB or a recall out on the problem you are describing. Check with a dealer to be sure. Have the dealer run yourVIN to see if you're part of the recall or TSB.

there is an ignition module located rite beside the distributor cap on the distributor base these are well known for getting weak and heating up thus causing your engine to stall.