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Need help troubleshooting voltage drop

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Wondering if someone with some expertise/experience with wiring might be able to help me troubleshoot an issue I'm having. I thought I was starting to get good with this stuff but last night I wasted MANY hours and at the moment I'm stumped. I'll try to explain...

I've been working on re-wiring some things to add a third amp (for a sub) and an equalizer. I've done lots of reading on it all and concluded that I should use a relay for 12V turn-on signal for the amps and equalizer. I also added a hard-wired bluetooth receiver which has no turn-on signal so I thought I'd use a second relay that's switched off of the ignition (accessory power) for the power to the equalizer and the bluetooth receiver.

All 3 amps are located in the observer's compartment with the batteries. I have an amp rack with a distribution block (with positive and negative connections in one) that's powering all of that directly from the house battery - no issues with all of that. In order to run the equalizer, bluetooth, and receiver off of that same house battery I ran 4ga positive and negative leads (through a 40A fuse) from the distribution block over to the compartment underneath the steering wheel; where I hooked up a Blue Sea fuse panel. That's where I'm having the issues.

I suppose I could go into all of the details on how I wired the relays off of that fuse panel, but in troubleshooting it all I've disconnected all of that to try and just run positive and negative wires from/to the fuse panel. I've done Here's what I'm finding:

- If I check the positive and negative leads with my MM, I get around 14V.

- If I connect positive & negative wires (and fuses) and check the voltage at the ends of the leads, I get about the same 14V.

- As soon as I connect those leads to any component I'm reading a significant voltage drop. When I connected them to the Equalizer it drops to around 5V. When I tried just the little bluetooth receiver it was around 1.2V.

- If I hook up a second positive lead to another fused connection on the fuse panel, It reads the same low voltage as the other wire (although I believe it was higher).

- If I hook up the 12V turn-on lead to the EQ directly from the head unit, that connection reads 12V as it should (I tried running the head unit off of the same fuse panel, but I hooked it back up in it's 'original configuration' during troubleshooting so it's running off of the dual-battery switch at the moment like everything else).

It took me a LOT of troubleshooting to get this much figured out and I certainly have more to do. Hoping someone might be able to point me in a certain direction on what to check. There's been a lot of re-wiring involved to get here so I'm assuming it's some sort of bad ground or perhaps something is grounded that shouldn't be. Basically I'm trying to figure out exactly WHAT would cause the voltage to drop like this as soon as a 'load' is applied to figure out where I should be looking. Everything seems perfect at the fuse box and at the end of any wires I hook up to it until I hook them up to something.

Thanks so much for nay help!!!

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Post the schematic of how you are wiring the relays. And can you indicate where exactly are you measuring the voltage drop.

Edited by MLA

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Do what MLA said.

Also, a poor ground potential has the same effect on voltage as does a poor B+ wire or connection. So, take a known ground where you were getting good voltage and use that as a reference to the various B+ points where you had a significant voltage drop. If you get a different result, then you know what ground link or connections to focus on. You can also do the inverse. Could save you some time.

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Thanks you both so much for chiming in. This was really driving me crazy last night, but I went out and gave it another try after I posted this and was able to figure it out.

Sadly, the issue was simply a bad fuse on the 4ga power cable that I'd run to the Blue Sea fuse box. I hadn't suspected that since it was reading almost 14V when nothing was connected to it and continuing to read some voltage when it was.

To figure it out, I took the little bluetooth module, a bunch of alligator clips, and basically worked my way from the distribution block towards the fuse box. Maybe I'll invest in a test light for the next time. With the new fuse, everything was perfect and I was able to wire my relays back in, etc. Wish I hadn't wasted so much time trying to figure it out, but I'm glad it wasn't something that I'd wired improperly and I'm thrilled to to have it sorted out.

On to setting my amp gains. Thanks again!

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