Issue is with my 2000 ZR2 -

Crank fuse has been blowing randomly when trying to start the truck. The first time it happened I suspected it was the neutral safety switch since I'd had that happen some years ago. Put in a new NSS, truck started right up. Drove it the following day and it started blowing fuses trying to start to drive back home. I pulled the plugs out of the NSS and replaced the fuse which blew immediately with the key in the start position. I pulled the starter relay and replaced the fuse which again blew when turning the key.

At this point I suspected damaged wiring insulation giving me an intermittent short. After moving around the wiring harness in the driver side front fenderwell and the harness leading down the driver side frame rail looking for obvious damage to the wiring, I replaced the NSS plugs, starter relay and Crank fuse and it cranked and started. Drove home with no issue, I didn't expect there would be one once it was running. Once I got home and shut it off, I tried to restart it and it fired right up. Since the temps in my garage were hitting 100-105 I had to put off troubleshooting this. I have a short finder kit I had bought from Napa about two decades ago (self-resetting circuit breaker and needle gauge) but it only works on a powered circuit that is actively shorted. Not ideal for a short tied to the ignition switch.

I found the Crank fuse blown again when checked yesterday so it appears it blew as it started up the last time since it hadn't been touched since then. I started trying to look for a short by testing continuity between each side of the crank fuse socket to ground, I got continuity on the right terminal. I tried moving around the harness as I had done when I got it to start and looked closely at the wiring insulation where the split loom harnes was cracked and broken from heat and age. I couldn't find anything obvious and started worrying I was doing this the hard way. Since I kept thinking it would be a useful tool anyhow, I bought a tone generator and probe kit hoping it will prove useful for this issue.

Does anyone have any advice for what else I should look at while trying to trace this? The ignition switch itself was replaced but it's been about eight or nine years since then. Haven't seen any unusual warning lights aside from the random ABS/Brake lights that remind me my ABS module has been twitchy for something like ten years.

 

Does the crank fuse only blow when you move to the start position or does it blow when you move to the run position but do not attempt to start?

George

 

It only blows when attempting to start. It’s fine in the run position. It also made the 35-ish mile trip home in 105° weather and did not blow the fuse until I turned it off and restarted it.

 

1. Check the fuse value
2. Disconnect both wires on the NSS and pull the start relay
3. See if the fuse blows
4. If not then reattach the ppl wire at the NSS and see if the fuse blows
5. If not reattach the ppl/wht wire at the NSS and see if the fuse blows
6. If not re-install the relay and see if the fuse blows
7. If so replace the relay with a known good equivalent and see if the fuse blows

George

 

10A fuse was installed when this issue began, diagram specifies 10A fuse in the CRANK fuse holder
Fuse did not blow with NSS and relay pulled.
I replaced the starter relay and reconnected the NSS. I traced the wire from the fuse holder to the NSS (the solid purple wire) with the tone generator but I could only locate it from the NSS up until it was bundled into a larger split loom and routed up the firewall. Due to interference from the tone generator itself I couldn't find which cable bundle contains that purple wire at the fuse block.
At this point I tried to start it with my short-finder installed in place of the 10A fuse (device is an auto-resetting 25A circuit breaker). Unfortunately the battery voltage had dipped down to about 10.5v and the starter would only rapid-fire click. It did click which indicates it did not trip the breaker and power was still sent through the starter relay to the starter itself.
I swapped the rear window heater relay into the starter relay socket (both have the same part number and the defroster relay is functional). I replaced the 10A fuse in its holder at this point. Same result - starter was powered and clicked Since the battery was still low I am not certain how conclusive this part of the testing was.
I pulled the cables off the battery, reinstalled the terminals and have it connected to my trickle charger to let it charge until tomorrow evening before I attempt to start it again.

 

Get your battery charged up and repeat steps 2-8 in post 4.

George

 

Battery was at 100% this evening so I tried starting it and it fired up on the first attempt and did not blow the crank fuse. I'll have to try it again later after taking a drive, I'm still suspecting damaged wiring insulating making an intermittent short.
Previous testing the last time it would not start it was blowing the Crank fuse with the NSS disconnected and starter relay pulled which seems to narrow down the portion of harness it could be in.

 

Which wire(s) were disconnected from the NSS?

George

 

I removed both of the weatherproof connectors from the NSS - so all wires were disconnected.

 

Then you most likely have a short in the wire from the fuse block to the VSS. It is probably corrosion under the fuse block or the wire in the loom going to the transmission. There are a couple of ways to do this. If it were me I would not unbolt the fuse block and bend it up unless I knew for sure that this is the problem area because there may be unintended consequences. If you go that route use a mirror and minimize the trauma on the wire loom.

I would open the loom just after the fuse block and try to locate that wire. Your tester might be able to help you here. If not then it’s probing same colored wires in resistance mode with the other meter lead down at the trans. It’s a big wire loom so that might be a challenge. The problem sounds intermittent so you may or may not be able to find it on any given day but I am 90% sure that this wire needs to be replaced. You will need proper soldering skills and shrink wrap tubing to do it right

George