Crank Fuse Nightmare - 99 Chevy Blazer
 

I would like to start by saying I have read just about every thread regarding the crank fuse, neutral safety switch, and starter installs I can find on here and I am still having issues. It all started after we had to do a crank and bearings in a Jasper engine. After the R & R, we started having issues. The crank fuse would blow after the engine was hot, but not when cold. We swapped the starter and it went away for a week. I then swapped the neutral safety switch and it was good for another week. I swapped the starter and relay and was good for another week. I also replaced the negative cable on the battery and cleaned all grounds. So we know the starter, starter relay, NSS, and battery cable should be good, yet I still keep blowing this fuse when hot. I pulled a wiring diagram off the net today from s10 forum and it is attached. If one was to look at this and was blowing the fuse, would you look at upstream components or downstream? I got into the fuse box and harness and had a issue. The wire coming into the box from the ignition was yellow as specified, but the wire leaving the fuse was supposed to be purple, but it was a double black wire. It was purple at the NSS and left me somewhat stumped. I verified the other pin outs on the fuse box for position and color. At this point, I am stumped. I dont know if I should look at the ignition switch, or the wiring in the harness between the crank fuse & NSS, or the NSS to the relay, or the relay to the starter solenoid. Any suggestions would be appreciated.

I would have to get into more diagrams and make sure that theres nothing else feeding off any of the other fuses, you could have an open on a other circuits power wire and its getting its feed from the crank fuse or another one on its circuit which means its carrying the load of multiple circuits, i would say you need to get an amperage measurement from the crank wire and start pulling fuses til the load comes down and then investigate that circuit (that doesnt sound like it makes any sense) ok how about this, ohm the crank circuit , mainly just check continuity, then unhook battery and check the crank terminal at fuse to the block for short to ground and check the crank terminal to positive battery cable for short to power. If you have continuity from fuse to starter relay and no shorts, then the wire should be good...maybe, next since the wire is "isolated" between crank fuse and starter relay with exception of the nss and we want to rule all of it out, check for continuity between the crank terminal in the fuse box and all the other fuses, this will tell you if the crank wire/circuit is shorted to any other circuits which would be overloading it. Another thing you could do is....i dont know, im fallin asleep but im really wondering what you have going on now....
Post back any new findings or thoughts, I'll post anything else that comes to mind.

If the problem started after the motor was pulled and then reinstalled, I would be concerned about a pinched wiring harness between the block and transmission. The time frame could be a coincidence and it really is caused by a particular bump or under certain acceleration/deceleration event.

So when something gets hot enough to call for power, it blows a fuse. What call for power when it gets hot. O2 sensors?

Temp sensor?

Here is the thing, if it starts/runs as normal when it is cold most of the time, checking wires for continuity and ohming them out my be useless until the point in time that it fails,thats when I really need to look. I have an idea on the amp draw. I know this is not suggested, but I put a 20amp fuse in once when it was acting up and it blew that. I also put a 30 amp in and was able to get it to start when it left me stranded. I know not the best of ideas, but it gives me an idea that it is drawing somewhere between 20-30 amps when acting up. 90% of the no start crank fuse blowing issues occur when it is hot after running, 10% have been when its cold. Just an observation. Now the one thing that is weird is I have been testing it over the past week in the driveway, without driving or hitting bumps to try to exclude that. While testing in the driveway without moving I start it, let it get to operating temp and shut it off and turn it on. Twice this week it will start 10-15 times without issue when warm and then I come back to it 5 minutes later and it blows that fuse. This is while hot. Thanks for the tips on troubleshooting the circuit. I will have to look into snagging a multimeter. Any other suggestions are appreciated.

Heres something to try, on my way to work this morning i started the engine and then pulled the crank fuse and drove to work, it didnt affect anything else in the vehicle, no check engine light or anything, try pulling the fuse once the engine is running and make sure that everything electrical is working, radio, wipers, lights etc... this will prove that there isnt anything operating off that circuit...but there shouldnt be any power at that fuse unless the key is turned to crank....
Question, is the fuse blowing when you try to crank when warm? I'm thinking swartlkk may be on the right track, i just assumed that would have been checked already, that since pinched wires are always a possibility with an engine or trans swap, also you may not be looking for a "pinched" wire but actually a pulled wire, if the harness was stretched during removal or install you could have wires that have insulation pulled apart and exposed copper wire, maybe when warm they relax and touch each other. I would start with the crank wire at the starter and work your way through the harness and verify that the wire is ok.

Interesting idea. From the diagram, the only time the fuse is used is while starting so that makes sense. It should not effect anything once started. Will try this evening once I get home after work. Yes, the fuse blows while trying to start when warm. Here is the thing, when it is continually blowing the fuse, it does so without the starter even cranking. I turn the key and all dash lights light up, but no cranking of the starter, just a blown fuse. I can and have gone through many boxes of fuses when this starts to happen. It will blow 10 in a row and that is when I usually just walk away and let it cool off. Most of the times it will start then, sometimes it wont.

What my plan of attack is at this point is isolating each leg of the circuit as there isn't very many of them. From the diagram you can see I have one yellow wire into the crank fuse from the ignition. From there it goes yellow wire to the crank fuse and out of the crank fuse with a purple wire to the NSS. Coming out of the NSS with a purple/white wire to the starter relay. Coming out of the relay with a large gauge purple wire to the starter solenoid. There isn't much too it which is making it even more frustrating. My question is if the crank fuse is blowing, should I be looking at the downstream circuit, or the upstream? Only thing upstream is the ignition switch. Any other suggestions are appreciated.

Ok i just looked at the diagram again, the crank fuse only powers the coil in the starter relay, look where the power from the nss comes to the start relay, the squiggly line represents the electromagnet coil that is used to pull the higher current contacts together on the other side of the relay, the crank fuse doesnt send power to the starter wire at all, i think you may have a starter relay thats coil is shorting out, try putting the headlamp relay in its place the next time it blows a fuse. If it still blows then you need look at wiring between the ignition switch and nss and between nss and starter relay, pretty much you need to look at harness going to the nss.

I would say downstream, theres another fuse to blow upstream If it was in the ignition switch.

Hopefully i made sense there..

Correct, the crank fuse has power from ignition during starting that goes through the NSS and then energizes the relay. If I was having problems with the starter, it would likely blow the 40 amp ignition fuse. I have it narrowed down to anywhere from the ignition switch to the relay. So far I have replaced 3 starters, 2 relay, and 2 NSS's. The only component I have not replaced is the ignition switch and I need to verify wires. My electrician at work seems to think it is not a wire issue, but a contact issue in the ignition switch. This has happened with the original seimens relay as well as with the aftermarket relay, so I don;t think it is in the relay, but with the wires coming and going in between. I guess we will see tonight.

Have you checked to see if it will blow a fuse with no starter relay in at all?

Im betting your gremlin is in the harness to the NSS