Help needed - new JVC head unit powering down w/ volume and bass

[muchmany]

Hey all, rookie trying to DIY a head unit upgrade and running into power issues.

Recently got an 84' Blazer S10 in pretty good shape, but the previous owner's no-name head unit wasn't Bluetooth so I wanted to upgrade. The old head unit was having power issues, cutting out power and restarting intermittently.

I installed a new JVC KD-SX38MBT unit and everything seemed to go well with the wiring. I cut out the old harness (which was actually two) and connected the old wires to the new harness with butt splice connectors. Sounded great for the minute that it was powering. It started playing music fine, but then it quickly turned off and restarted same as the old one when the music got loud. I turned the volume down and it stayed on, but could see the display lights on the head unit flickering when the bass hit. I turned on a podcast quietly and it didn't lose power. But any loud music causes it to restart.

Any ideas on a sequence of troubleshooting to zero in on the problem?

A couple other pieces of info:
-I don't think it's an alternator issue because the same thing happens whether running off just battery or with the engine on.
-Both the constant and keyed wires coming from the dash show 12.3v on my multimeter. Under year old battery shows 12.3v off and 14.3v while running. All the grounds that I could find under the hood seem fully intact. Willing to consider Big 3, but don't want to rely on that advice just yet
-One strange thing was when I cut out the old harness, I identified the black ground wire (which tests as ground). But there was another loose black wire that was not connected to the old harness that tests as a ground just the same as the other one. How should I treat these two grounds?
-The previous owner installed a 200v dual outlet inverter and it doesn't work when I plug a phone charger into it, so I'm curious if that's a clue about general power in the truck? Also auxiliary oil and water gauges. Wondering if the 40 year old electrical supply isn't enough for everything, but don't know how to tell.
-Don't know anything about the speakers, but they're some kind of after-market 4x6 front and 6x9 rear. Don't love them, might want to replace if that changes how I should go about things now.
-Truck feels a bit weak on the start and the dash indicator lights are a bit wonky (one turn signal indicator stays lit solid, high beam indicator will switch on and off for no reason).

Before

Before

After (don't mind the cut wires, this was mid undoing my work)

Thanks very much for any help,
Evan

Click to read more...

 

[HardofWhoring]

Well, the yellow wire that is cut, looks like your 12v/batt + wire.

IMO butt splices just aren't good. I would never use that for anything long term. Prime day deals are about to end, but if you're not going to add pins and use a harness, (which is the better option), at least pull those all off, recheck all the wires are connected right, and what about using these:
Amazon product ASIN B07GDDKJ1D

 

[HardofWhoring]

That harness to all black wire nonsense needs to go.

 

[muchmany]

Well, the yellow wire that is cut, looks like your 12v/batt + wire.

Maybe you missed it, but I wrote that that picture was from the middle of me removing the butt connectors. It was connected when it worked.

That harness to all black wire nonsense needs to go.

Can you explain a little more? The black wires were already there and match up to the 4 speakers (2 each). I'd really rather not rewire all the speakers, but if you think that's a problem then maybe?

 

[HardofWhoring]

I missed that it's just for the speakers, that's a little less worrying. Is that electrical cord they used?

You have red, yellow, and orange wires going to not yellow red and orange.
Yellow is generally 12v batt+
Red is generally accessory
Orange is generally illumination.

My guess is you have 12v power to the wrong one. It's getting enough current to turn it on, through the other wire, but once you turn it up the volume it's trying to draw more than the, not supposed to be power wire, can supply.

It looks like those wires aren't connected to where they should be. The main two that NEED to be right, are batt + and accessory. If you have a test light you can check em both on the vehicle, and verify the head unit. If you connect those two correctly, it should fix the head unit's power issue.

 

[keep_hope_alive Premium Member]

Well, the yellow wire that is cut, looks like your 12v/batt + wire.

IMO butt splices just aren't good. I would never use that for anything long term. Prime day deals are about to end, but if you're not going to add pins and use a harness, (which is the better option), at least pull those all off, recheck all the wires are connected right, and what about using these:
Amazon product ASIN B07GDDKJ1D

Actually, a proper crimp can last the life of the vehicle. What's important is using the correct die/crimper, and getting strain relief. All of the connections from the OEM are crimps, none are soldered. That said, OEM crimps are a 2-part assembly that includes an insulation crimp for strain relief. My newest ratcheting crimper includes both a conductor crimp and an insulation crimp.

What you linked is a terrible product. The solder never gets hot enough to melt before the plastic tears. And the conductors never get hot enough to wet-out properly.

 

[keep_hope_alive Premium Member]

If the unit shuts off when you turn the volume up, it's either voltage drop from a poor ground or a short at one of the speakers.

 

[HardofWhoring]

Actually, a proper crimp can last the life of the vehicle. What's important is using the correct die/crimper, and getting strain relief. All of the connections from the OEM are crimps, none are soldered. That said, OEM crimps are a 2-part assembly that includes an insulation crimp for strain relief. My newest ratcheting crimper includes both a conductor crimp and an insulation crimp.

What you linked is a terrible product. The solder never gets hot enough to melt before the plastic tears. And the conductors never get hot enough to wet-out properly.

I didn't say crimps, I said those butt crimps. Op wouldn't be here asking this question if they had a proper crimper, and read it again, I said pins and a harness (WHICH WOULD BE A PROPER CRIMPER) was the best option.

I've never had a problem with those self solder tubes, not sure why you can't figure them out.

 

[HardofWhoring]

If the unit shuts off when you turn the volume up, it's either voltage drop from a poor ground or a short at one of the speakers.

Wrong again. If it was a short at one of the speakers, it either wouldn't wait until the volume was turned up to short out, or it wouldn't effect the head unit.

OR the actual answer that I already gave; if you look at the wires, you'll see the reduction in wire size coming off the head unit's yellow wire (which is 12v batt+) after the butt connector.

 

[muchmany]

I missed that it's just for the speakers, that's a little less worrying. Is that electrical cord they used?

You have red, yellow, and orange wires going to not yellow red and orange.
Yellow is generally 12v batt+
Red is generally accessory
Orange is generally illumination.

My guess is you have 12v power to the wrong one. It's getting enough current to turn it on, through the other wire, but once you turn it up the volume it's trying to draw more than the, not supposed to be power wire, can supply.

It looks like those wires aren't connected to where they should be. The main two that NEED to be right, are batt + and accessory. If you have a test light you can check em both on the vehicle, and verify the head unit. If you connect those two correctly, it should fix the head unit's power issue.

Yes, the power wires' colors don't match from the dash to the new harness. However, I tested them and the wire I'm treating as the constant 12v is powered all the time and the wire I'm treating as the switched is only powered when keyed, so I'm good there. Also, there is one slightly thicker gauge wire (the constant) and that matches up with the only wire of that gauge on the harness. I think the PO just used weird colors.

If the unit shuts off when you turn the volume up, it's either voltage drop from a poor ground or a short at one of the speakers.

If it is indeed from a poor ground, how would I test for a voltage drop while the head unit is playing and I turn up the volume? What is the solution, running a new ground for the head unit or doing a big 3 ground upgrade under the hood or both?



I could really use some more advice from anyone else if they have ideas? Thanks