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I have a 03 c4500, I just installed a brand new engine. When I went to turn the key to start for the first time I lost all power, nothing would turn on while cycling the key. Went to remove negative cable to inspect connections and noticed my wrench arc agains the metal bar holding down the batteries. I have 12 volts on the truck frame. I checked to make sure all my grounds were good and my cables had good ohms. I isolate each of the 3 fuse boxes and each has 12 volts when connected to the main power wire, I isolated each fuse box and pulled fuses was able to stop getting 12 volts on frame From only one fuse box when pulling a 60 amp fuse Batt/Haz. But the other 2 no matter how many fuses I pull I still can’t get the 12v to go away. What do I do next? I’ve already started to unplug things but still no change. It’s so frustrating I know it’s gonna be something stupid. Thanks for any help you can throw my way
 

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It's pretty clear that a bat+ cable (or wire) is attached to or touching the engine or frame. I have no idea how similar or dis-similar your 03 C4500 is to m;y 2008 pickup when it comes to wiring. I believe I would trly to find a power distribution diagram (search this website and others - I recall recently someone saying they have a pretty good library of wiring diagrams for 4500, 5500, 6500 trucks - and follow up up by comparing that to your truck.

Look here Info: - 1999-2006 GM Medium Duty Electrical Manuals

Try to narrow down the problem. You said if certain 60 amp battery fuse was pulled, it stopped the 12V on the frame. To me that's a starting point. If that fuses powers the frame, that circuit must be connected (or shorted) to the frame somehow, or something on that circuit is touching the frame. You're probably aware that the battery main grounds should go the the engine, from the engine to the frame, from the frame to the chassis (something close to this at least). I am assuming you have 2 batteries??? I would disconnect the alternator cable both ends and both terminals of one battery and see if anything changes; if no change, disconnect the other battery, alternator cable still disconnected, and reconnect the original battery. Any change??

If your truck has original cables only, your bat+ should be red and grounds black, but don't depend on it. Does your truck have that 175 amp mega fuse? Wiring there can be easily mis-attached. If you touch a test light from your battery ground to the engine block does it light up? Same test to the frame? You should have no light either test. Now do the same from the battery pos terminal. both should light up. Check from engine to frame. Just looking for something not correct, someplace to start, something you can correct to lead you to your solution.

From a thousand miles away, it seems like an impossible situation, but you have it. If you put a new engine in the truck, you are no stranger to them. Very hard to guess a precise solution.
 

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It's pretty clear that a bat+ cable (or wire) is attached to or touching the engine or frame. I have no idea how similar or dis-similar your 03 C4500 is to m;y 2008 pickup when it comes to wiring. I believe I would trly to find a power distribution diagram (search this website and others - I recall recently someone saying they have a pretty good library of wiring diagrams for 4500, 5500, 6500 trucks - and follow up up by comparing that to your truck.

Look here Info: - 1999-2006 GM Medium Duty Electrical Manuals

Try to narrow down the problem. You said if certain 60 amp battery fuse was pulled, it stopped the 12V on the frame. To me that's a starting point. If that fuses powers the frame, that circuit must be connected (or shorted) to the frame somehow, or something on that circuit is touching the frame. You're probably aware that the battery main grounds should go the the engine, from the engine to the frame, from the frame to the chassis (something close to this at least). I am assuming you have 2 batteries??? I would disconnect the alternator cable both ends and both terminals of one battery and see if anything changes; if no change, disconnect the other battery, alternator cable still disconnected, and reconnect the original battery. Any change??

If your truck has original cables only, your bat+ should be red and grounds black, but don't depend on it. Does your truck have that 175 amp mega fuse? Wiring there can be easily mis-attached. If you touch a test light from your battery ground to the engine block does it light up? Same test to the frame? You should have no light either test. Now do the same from the battery pos terminal. both should light up. Check from engine to frame. Just looking for something not correct, someplace to start, something you can correct to lead you to your solution.

From a thousand miles away, it seems like an impossible situation, but you have it. If you put a new engine in the truck, you are no stranger to them. Very hard to guess a precise solution.
Hey Ron thanks for the reply! I pulled damn near every cable of the truck isolate each fuse box, pulled fuses etc. riddle me this. With main positive cable to fuse box connected and negatives disconnected I get 12 volts on frame, I can physically see my cable and they’re not touching ground. With main positive and original ground hooked up I get about 11.6 volts on frame, however if I bypass my original ground cable with one I made the 12 volts goes away!, that’s cool and all but as soon as I disconnect my new ground cable and leave just the positive, my 12v comes back. This one is throwing me for a loop.
 

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Remember that 12V is only the electrical pressure; the other component is the volume or amps. How much amperage is there?? it could be just a couple of milli amps (.001 amp) and nothing of great concern. Without knowing your answer, there is still clearly a problem. The fact that one ground cable produces voltage on the frame and a different one doesn't says there's a problem with that cable. At least, if I understand what you are telling us.

Have you picked up the fuse box to see if it is grounding/shorting on the underneath side? Is it mounted on metal or plastic? SOMETHING is touching that frame that shouldn't be.

I wish I was there to see what you are dealing with.
 
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