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This is a fresh swap with only a few minutes of idle time and few trips around the block..
A buddy of mine came over to check out the LS swap on Friday, so I fire up the engine. The engine fired up fine but when I revved it up everything suddenly shut off. I lost all power. I checked my battery ground cable and it was loose so I figured that was the problem. I went out to the garage today to look over everything and I found a few problems..
I noticed that the fuel pump no longer does a quick prime when turning the key to accessories, but its clearly working because it feeds the engine at idle.
Then the car wouldnt shut off when I turned the key. Ive never had this issue before. I looked on the Chevelle forums and they mentioned a problem with HEI, the voltage regulator, and the alternator backfeeding the system. It could also be the ignition switch but I feel that is unlikely. I dont have an HEI or voltage regulator. The wiring harness is an aftermarket American Autowire.
I pulled the wire from the alternator and it still kept running.
I looked over all the wires and I couldnt find anything out of the ordinary. Fuses look good.
What could cause my Chevelle to suddenly do this? It ran fine 20 times before.
Its a stock 6.0 with stock truck alternator. 1972 Chevelle.
Sounds like ignition hot (pink wires) are now acting as a constant hot. I would place bets it's the ignition switch. It's easy to check, Take a volt meter and check your pink wires that are supposed to receive 12 volts when ignition is turned on. Probably easiest to pull those fuses and check for voltage. You should have no voltage there until you turn the key.
I just checked the pink ignition wire that I use to power the LS. It is the "fuel" circuit on my American Autowire fuse panel and my test light is showing hot without the key in the ignition, only battery connected. Interestingly, this fuse was blown after the incident I described above.
So, I'm sure my problem is somewhere in this circuit. What would cause this ignition wire to go hot all the time? I traced the wires and nothing seems out of the ordinary.
I'm not familiar with the body harness, but If you are hot coming off the fuse box on a spot that should be ignition hot then you will need to go in front of the fuse box to find the problem. The body harness fuse box should have a main power line and an ignition power line. Find out where the fuse box is getting it's ignition hot power and see if it's always hot.
Are you running your entire engine harness (pink wires) from one fuse location? Normally you should have at least 2 fuses for those wires alone. I usually split them up into four, right bank, left bank, coils and sensors.
Yes there is only one +12v wire and one ignition wire coming off of the auxillary fuse box for the LS. I didn't rework the harness myself, I paid someone to do it which I now realize was a big mistake. Wiring is not my strong suit.
the pink ignition wire from my auxillary fuse box is tied into the fuel fuse on my Chevelles fuse box which was blown.
Right now all of my pink ignition wires are showing hot.
Ok, you do have an engine fuse box as well. That's good.
So the engine fuse box is tied to the chevelle fuse box Fuel fuse. Does your chevelle fuse box also have a fuel relay? You may have the relay stuck. The fuel fuse should not be hot at all times it either gets it hot from the ignition switch or from a relay triggered by the ignition switch.
Personally I would take that ignition hot pink wire and remove it from the chevelle fuse box and run it directly off your ignition switch or a triggered ignition switch relay.
I'm not sure if there is a relay on the Chevelle fuse panel, but there is one on the engine fuse panel. I was also thinking that might be stuck because my fuel pump doesn't kick on like it should. They are mini Delphi relays. Do you know of any place in the valley that has them? I'm in Phoenix.
The engine harness relay is probably working properly. The problem is the pink wires on the fuel fuse location. This should not be hot until the key is on. Do you have an installation sheet on the autowire car harness you installed? Check the wire that feeds the ignition hot fuse locations. Most car fuse boxes have a very large red wire that hooks directly to the battery or starter battery terminal for constant hot and then another wire that connects to ignition switch hot. (it's sometimes also pink in some harnesses)
What's the part number of the car harness you are using? I'll see if I can find the diagram.
So I traced down all of the wires on this circuit. All connections look good. One of the pink wires on this circuit is what supplies my auxillary fuse box for the LS.
I can't see any damaged wires so I just started unplugging things to see if it will turn off my test light. No luck of course.
I cut the ignition feed wire to the LS fuse box and that finally shut my light off. So it's safe to assume the short is somewhere on the engine side of the harness and not on the Chevelle harness.
I think your confusing yourself. As you said earlier, your ignition feed wire was hot at all times so cutting it loose will of course shut the PCM off. Your ignition feed wire should not be hot at all times. Your engine harness is more then likely fine. You need to remove the LS ignition hot and get that wire on a true ignition hot location. Your easiest fix would be to locate it off your ignition switch itself.
It's not shutting off the PCM, it's shutting off my test light when I cut the ignition feed to the LS fuse box. I have a test light in series from the ground cable to the negative battery post. If I touch the test light to the negative battery post and it lights up then there's a short.
So when that feed wire is cut, the light turns off, which would mean the short is somewhere in the engine harness. Am I way off on my thinking?
I'm having a hard time understanding your description now.
Your test light is between the battery negative terminal and the negative cable? All this will tell you is if something is turned on or not. ie current is flowing.
With your test light grounded to a good chassis ground check your ignition feed wire to LS harness (pink wires). This feed should only be hot when key is on. If it's hot with key off then cut the feed to the harness and read both sides of your open to ground with the test light. The LS side should be dead while the other side is hot (which it should not be unless key is on).
Please excuse my quick sketch. LOL I'm an electronics technician not an artist.
The pink wire supplying your LS fuse box should turn on and off with key. This is how the computer knows you are wanting to start or turn off the motor. If that pink wire is hot at all times then you need to find out why or move your pink supply wire to another location that is not a constant hot.
The wire going to the fuse box definitely has power when it shouldn't. There is also another pink ignition wire coming off of the bulkhead connector in the engine compartment and that one has power too. So you are right about it being on the car harness and not the engine harness.
I can't connect the LS ignition wire to another source because all of the ignition source wires are energized when they shouldn't be. I would think that I would find something significant under the dash like some melted wires or something but I can't see anything out or the ordinary. I have unbolted the dash for easy access to all the wiring and it's pretty clear that there isn't anything melted. Should I check each wires continuity? I have my buddies nice fluke at the house so I can check everything.
Have you checked the ignition hot wires at the ignition switch? The switch is what makes all those wires go hot when turned. I would read there for voltage to see if it's hot before you turn the key. If it's hot then remove the harness from the key switch and read it again.
If it's hot with the key switch removed then you do have a wire shorted to constant hot somewhere.
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Engine Suddenly Stays Running With Key Off 
