Issues with the 12v head unit supply

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1994 fjz-

I'm waiting for parts to come in to do the knuckle rebuild and I decided to update the tape deck head unit in the truck. When I pulled the dash I noticed that one of the previous owners had removed the factory wiring harness....lovely. I couldn't find one at the scrap yard, so I ended up just running longer wires and putting blade connectors on the end of each which worked pretty well. I got the head unit in and all of the speakers connected up and it fired right up. After turning the truck off and on again nothing happened. I checked my 12v connection and it's showing me 1.3v. After some digging around I realized I must have blown the dome fuse because the clock is out too. I took it out and it looked fine, but I replaced it anyway....again nothing. I disconnected the wire from the 12v supply and the original harness wire (solid grey) was also showing 1.3v, so it wasn't my wiring. I checked the fuse block and the dome fuse is at 12.5. After pulling fuses out and putting them back in just to see if anything else was blown (they all looked good) I noticed I had my clock back so I checked the 12v line...this time it was 11.6v. Back in business....I reconnected the radio with the truck off and it booted up. I turned it off again right after it booted and when I turned it back off I didn't have power or my clock. I know these are all on the same fuse, but the fuse is new and isn't blown. Is there anything I'm missing? I'm reticent to just run another 12v from the block because 1. I don't like the idea of a random hot line floating around in there, and 2 ....I really don't want to take the dash apart to fish a single wire through there.

This is what I get for assuming it was going to be a 2 hours job. Any thoughts?
 

LandLocked93

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Well I understand you reticence, however...
...a good, isolated power wire can go a long way in t-shooting the issue.
Is it power related? or ground related?
My first thought is to employ voodoo upon the particular PO who in their infantile ignorance chopped the OEM harness. Pins in the eye is a good start.
After that tho, get stand alone power to the unit and go from there. Isolate power to the unit from all other OE power demands.
Forget getting indirect responses simply by turning the key.
Are there spurious wires clearly not part of the OE canon?
You know the radio is good and functioning properly?
 
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Well I understand you reticence, however...
...a good, isolated power wire can go a long way in t-shooting the issue.
Is it power related? or ground related?
My first thought is to employ voodoo upon the particular PO who in their infantile ignorance chopped the OEM harness. Pins in the eye is a good start.
After that tho, get stand alone power to the unit and go from there. Isolate power to the unit from all other OE power demands.
Forget getting indirect responses simply by turning the key.
Are there spurious wires clearly not part of the OE canon?
You know the radio is good and functioning properly?
The radio has worked in the brief times that my vehicle has supplied it with 12v, so yeah I think so. The issue is that I'm seeing the toggle in voltage and it's so random I have no idea where to start. I ordered some piggy-back fuses that I've used in the past and like, but this is a truck that I'd like to daily at some point and I'd much prefer fixing what's up. My intuition was that a really was stuck closed, but there really aren't any that would affect this. If the dome fuse was not getting proper voltage It would be easier to figure....it's got to be a frayed wire some place right?
 

ToyotaMatt

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The radio has worked in the brief times that my vehicle has supplied it with 12v, so yeah I think so. The issue is that I'm seeing the toggle in voltage and it's so random I have no idea where to start. I ordered some piggy-back fuses that I've used in the past and like, but this is a truck that I'd like to daily at some point and I'd much prefer fixing what's up. My intuition was that a really was stuck closed, but there really aren't any that would affect this. If the dome fuse was not getting proper voltage It would be easier to figure....it's got to be a frayed wire some place right?


can you post pics of the back side of your radio please


i carry a wide array of audio oem repair plugs connector kits .

matt
 
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It's a real head scratcher that you're seeing extremely low voltage but not blowing any fuses. It's also weird that it seemed to go back to normal temporarily after pulling and replacing the fuse(s). Makes me wonder if you have a bad fuse or corroded fuse panel connectors... When the voltage is low on the circuit, check the voltage on each side of the fuse.

Otherwise, I think I would start by pulling all the "easy" stuff like the lower panel in front of the driver, glove box, both dash trim pieces, kick panels, and then do a visual inspection for damage or modifications. Bad electrical modifications are usually done by people putting in minimal effort, so I wouldn't expect them to be buried too deep. Maybe check for any damage where the harness passes through the firewall, too.

Screenshot_20220731-102558_Adobe Acrobat.jpg
 
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Just two side notes that may not be your issue but worth postulating .. 1 factory radio didn't pull that much current and newer units might need a bit more juice and 2 i think all after market units i have installed also had internal fuses that could blow... I know neither is likely your main issue but they either could be in addition to your low voltage issue.

If the fuse is not blown and you tested continuity from your power wire to your fuse box and it is good, then you might check before the fuse box back to the battery. What is your voltage at battery and what is the voltage you have on the radio/dome fuse?

If you have 12 v on the fuse how much does it drop or change when you turn on the dome light or have the door lights on while the door is open?
 
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It's a real head scratcher that you're seeing extremely low voltage but not blowing any fuses. It's also weird that it seemed to go back to normal temporarily after pulling and replacing the fuse(s). Makes me wonder if you have a bad fuse or corroded fuse panel connectors... When the voltage is low on the circuit, check the voltage on each side of the fuse.

Otherwise, I think I would start by pulling all the "easy" stuff like the lower panel in front of the driver, glove box, both dash trim pieces, kick panels, and then do a visual inspection for damage or modifications. Bad electrical modifications are usually done by people putting in minimal effort, so I wouldn't expect them to be buried too deep. Maybe check for any damage where the harness passes through the firewall, too.

View attachment 3074050

Just two side notes that may not be your issue but worth postulating .. 1 factory radio didn't pull that much current and newer units might need a bit more juice and 2 i think all after market units i have installed also had internal fuses that could blow... I know neither is likely your main issue but they either could be in addition to your low voltage issue.

If the fuse is not blown and you tested continuity from your power wire to your fuse box and it is good, then you might check before the fuse box back to the battery. What is your voltage at battery and what is the voltage you have on the radio/dome fuse?

If you have 12 v on the fuse how much does it drop or change when you turn on the dome light or have the door lights on while the door is open?
Thanks guys. I took the dash apart as far as I wanted to yesterday and saw absolutely no evidence of wire corrosion or fraying. The battery is pulling 12.4 with the engine off and around 14.8 with the engine on. The dome fuse in the box is pulling 0.0 (obviously) with the key off and around 11.7 with the ignition on. The dome light - that's another story...It didn't even occur to me until reading this that it doesn't work (hasn't since I bought the truck about a month ago) I didn't have a spare bulb handy yesterday so I didn't bother swapping it, but I probably should have tested to see if the socket was getting power....I haven't. I pulled and reseated every fuse in the truck again yesterday and of course the clock came back and the power wire was showing 11.8v...... I decided I'm just going to piggyback the dome fuse so I fished a new 12v through the firewall/dash as long as I had it apart and just put a wire nut and tape around the 12v ignition and constants from the radio. I had to order a few piggyback fuses because I only had micros, but it should work in the short term. I hate half assing stuff like this, but there's a laundry list of things I want to do to the truck before the snow flies and limited time to do them, so I'm gonna have to leave this one as well-enough provided the new wire supplies the correct power and doesn't do whatever the old one was doing.
 
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Used a piggyback and everything is just as it should be. I snipped and taped the existing 12v so hopefully nothing bad comes of it. I don't think it deserves its' own thread, but does anyone know the firewall grommet size? I ran a power line for a sub that I'd like to install and noticed that the grommet is just missing. Driver's side firewall directly behind the battery.
 

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