Chasing electrical short - help needed

[FF6602]

My "resurrection" build of 6602 (Mark 3.1) is progressing to the point where I'm sorting out the wiring installed by the previous owner. Everything related to starting the car -- starter motor, fuel pump, MSD box, etc. -- seems to work properly. The car starts and runs. Electric gauges work.

What doesn't work is anything that goes through the Blue Sea Systems 12-circuit fuse box. Lights, horn, wipers, etc. There are six circuits. I disconnected each of the six wires, one at a time, and tested each circuit and each one shows a fault in continuity testing.

I did discover two blown fuses and imagine those are related to a short somewhere. One other thing that appears odd to me is that there is a ground wire running from the 12V DC Negative fuse box post to an isolated grounding post on the dashboard. Do fuse boxes need to be grounded?

I'm at the limit of my electrical knowledge here. What else should I be looking at? Do entire fuse boxes ever go bad?

Thanks in advance!

[edwardb]

Pretty hard to diagnose with the information given. Anything's possible, but I'd rate it unlikely that an entire fuse panel is defective. Especially Blue Seas Systems. Generally high quality. Yes, the fuse panel has to have a power feed and a ground feed. Removing and testing continuity isn't exactly the right test. Does your panel look something like this? http://assets.bluesea.com/files/resources/instructions/Wiring-Diagram-5026_5031.pdf? You should be able to put VOM probes measuring DC voltage on the DC positive and DC negative leads and have battery voltage. Then with your negative probe still on the negative post, probe each load +12V terminals (by the fuses) and confirm you have +12V at each. Assuming the fuse isn't blown. If that all checks out OK, then the problem is elsewhere. I'd do a deeper dive on the specific devices that are blowing fuses. Can't comment any further without more details. Hope that gets you started.

[FF6602]

Pretty hard to diagnose with the information given. Anything's possible, but I'd rate it unlikely that an entire fuse panel is defective. Especially Blue Seas Systems. Generally high quality. Yes, the fuse panel has to have a power feed and a ground feed. Removing and testing continuity isn't exactly the right test. Does your panel look something like this? http://assets.bluesea.com/files/resources/instructions/Wiring-Diagram-5026_5031.pdf? You should be able to put VOM probes measuring DC voltage on the DC positive and DC negative leads and have battery voltage. Then with your negative probe still on the negative post, probe each load +12V terminals (by the fuses) and confirm you have +12V at each. Assuming the fuse isn't blown. If that all checks out OK, then the problem is elsewhere. I'd do a deeper dive on the specific devices that are blowing fuses. Can't comment any further without more details. Hope that gets you started.

The Blue Sea box you referenced is exactly the one I have.
112077

Thanks for the diagnostic advice. I will run the checks that you suggested and respond ASAP.

[Big Blocker]

The Blue Sea "distribution" fuse box that Paul referenced does need a ground wire [of substantial size] to act as a common ground to everything you have connected to the positive side. Without it, you would need to ground every circuit individually - this gives you a common ground point.

When you state that you checked ( "disconnected each of the six wires, one at a time, and tested each circuit and each one shows a fault in continuity testing" ) the circuits for continuity, the items you said were connected to the Blue Sea block should have continuity as each one is grounded after the device IF the activating switch is operated. Example: fused power to horn button, button to horn, other side of horn to ground . . . so reading for continuity at the power lead would show an open circuit until you pressed the horn button, completing the circuit. Same with wipers, lights (thru light switch), anything you have connected thru a switch to operate it . . .

Any chance that the master fuse feeding the Blue Sea fuse box is blown from excessive current draw when something shorted? If the picture you supplied is your Blue Sea fuse box, your ground wire might be just a bit undersized. You show 60 amps worth of fuses . . . if everything is powered up at once you might have close to that amount being drawn from your battery / charging system. The ground wire seems small for a 60 amp draw - and - what size is the master fuse that feeds the Blue Sea??

HTH

Doc

[FF6602]

Good news! I found the problem!

Thanks to Paul and BigBlocker for your interest in my problem and for your guidance.

Following Paul's advice, I checked for voltage through the fuse box by putting the VOM probes on the POS and NEG lugs. Nothing! So, I began to trace each wire to verify it against the hand-drawn wiring diagram I got with the car. To make a long story short, the ground wire for the ignition relay was disconnected but hidden by other wires. I took the connector apart, re-inserted the ground wire and plugged it back together. WooHoo! Power where it was supposed to go! Lights and horn now work.

Following up on BigBlocker's questions:

1.) I will increase the size of the ground wire to the fuse box. Do you have a suggested gauge?

2.) Thanks for the tip about checking continuity on switched circuits after the switch has been engaged. I did not know that and will keep in mind for the future.

3.) You mentioned the Master Fuse for the feed to the fuse box. Blue Sea's diagram references "main circuit protection of not more than 125A rating should be provided ahead of this device with an interrupt current rating consistent with the battery capacity". I don't recall seeing such a fuse anywhere in the wiring. Where would it normally be located? Between the master disconnect switch and the fuse box, I assume?

With appreciation for your interest and expertise,

[Big Blocker]

1.) I will increase the size of the ground wire to the fuse box. Do you have a suggested gauge?
Whatever your power wire is, your ground should match or be no less than one gauge smaller (Eg. #10 power - #12 ground). For what that "terminal block" is capable of (125 amps) , I'd probably use a #8 and master fuse the block with 100 amp master fuse.

3.) Where would it normally be located? Between the master disconnect switch and the fuse box, I assume?
Yes, between the battery source feeding the fuse block and the fuse block..
The individual fuses will protect your devices and the master fuse will protect everything in case of a total failure.

Doc

[Just puttering]

It looks like You dont need the ground wire if you do not use the ground terminals shown at the top by the ground wire!

My assumption is its hard to ground things on a wood or fiberglass boat, so the negative is brought back to the fuse panel.

On a metal car, all you need is things grounded to the chassis.

There are large circuit breakers for 12v systems so you would not need a spare fuse if you are in the design stages.

Also, the master fuse is incase the main wire gets shorted, if it gets worn on a sharp surface, etc. so the main fuse should be close to the battery, if you put it just before the fuse block it realy isnt doing anything