I'm only posting this because I thought it might be interesting for some of your car guys and some of you electrical guys to wonder about. It's not a real problem, and I've spent no time trying to figure it out other than in my head. Describing this has turned out really long winded, so please hang in there with me.

I have an old Jeep. In it I have installed a cheap Chinese bluetooth radio. (okay, case closed, THAT's the problem yes we know.)

It has only three wires connected to it. (it's an all in one unit, speakers built in, poor for music, fine for the occasional pod cast which is all I use it for. I really only installed it because I wanted a working clock) With no speakers attached, no amp on the RCA outs, no antenna hooked up, I only have three wires in use.

A ground. A constant hot (for the clock and presets). And a keyed hot, only powered when the key is on, to make the unit work.

The Jeep has a battery cut off switch on the negative side of the system. The disconnect cuts off everything going to the negative side of the battery. Everything but the ground for the radio. That's on the battery side of the cut off switch, so the clock keeps time.

Pretty simple right? Straight forward.

Here's where the puzzle comes in... when I throw the battery disconnect and cut off the negative side of the battery, the radio comes on. Display lights up as if the key was on. Push the power button, the unit turns on.



Even if I come up with some strange theory why the constant feed is now powering the keyed feed, that doesn't explain why the unrelated (as fas as the radio is concerned) cut off is causing it. Which it obviously is.

quote:

Display lights up as if the key was on. Push the power button, the unit turns on.

something is finding ground through the radio & causing 12V to be 'seen' on the switched hot. Depending on what else is on the hot/switched line, it could be a lot of things. 'cheap chinese' is probably the answer.
You defeated the purpose of the battery cut off by bypassing it. It's probably causing a ground loop too (the switch has a little more resistance than the direct line to batt).

My kids powerwheels has a bluetooth stereo that completely hoses my phone if I connect to it. Not sure if it's just buggy, or it's opening the gates of hell.

I can't help, but yeah, I did find the story interesting.

quote:

Originally posted by snidera:

You defeated the purpose of the battery cut off by bypassing it.

The purpose is so someone can't turn my lights, blower or anything else on while the doors are off most of the year. Adds a little anti theft as well.



I need to unhook the keyed hot and see what happens.

Steel dash Jeep? Radio grounding through its chassis, either by intentional design of the chassis to ground through chassis contact or use of a ground wire in case of a plastic dash.

Radio May have been designed for the use of either as a grounding point and or the ground wire as a redundant ground point. You probably need to change the battery cutoff to the positive side and run your constant hot off the battery.

Another thing, maybe the radio is grounding through the antenna cable?

Unit is only attached to the plastic parts of the dash (held in with RTV, lol.) The unit itself isn't deep like a traditional single din radio, it's only maybe 3" deep and very light.

There is no antenna or cable.

quote:

Originally posted by cas:
Unit is only attached to the plastic parts of the dash (held in with RTV, lol.) The unit itself isn't deep like a traditional single din radio, it's only maybe 3" deep and very light.

You sure the ground wire is on the correct side of the kill switch?

Isn't one.

Your disconnect breaks the ground, but you have introduced a second ground (clock) that becomes the ground loop when the relay (battery ground disconnector) is open.

So, you not only defeat your "disconnector", you are creating a "constant draw powered" circuit in the process.

I use similar "relays" for my classic cars, but they "break" positive and nothing is energized. I also eliminated the old "point wind" clocks.

I don't follow.

The constant on radio circuit is always that, I didn't create anything.

Actually with the disconnect off, that circuit becomes a totally isolated straight from the battery, and straight back. (theoretically anyway. Lord knows what goes on in the Chinese wonder box)

If you're saying what snidra said above, that some other item is possibly finding it's ground through the keyed side, that I understand. The ECU perhaps, that's about the only thing that would get constant power otherwise)


Again, I have not defeated the disconnect. With it off, none of the lights work, nor the blower motor, horns, wipers, engine will not crank. That's the intention.

There's a few reasons the to cut off is on the negative side, vs the positive side, but mostly my purpose changed. Anti theft was the original idea. I wanted all the dash lights to still work, I wanted the starter to crank, while keeping the engine grounds that control the fuel pump and other things from working. So it would seem normal, just not keep running. Over time I came to realize the real "threat" I was dealing with wasn't from theft, but with people messing with it, especially at work. Coming by and finding the lights on, or coming out at the end of the day and finding the battery dead. So eventually all the grounds got put on it.

Sounds very strange to me. I would get a volt meter and measure voltages. Disconnect all wires from radio. Measure hot always to ground with and without the ground battery disconnect. Do the same for the key switched wire to ground. See if there is something weird going on with your wiring or a strange defect in the radio. Hmmm?

If I understand you correctly, the radio is connected to constant 12 volt, and directly to the ground of the battery, so that it is not affected by your ground disconnect. And your switched/accessory is connected to some accessory feed, I imagine the one that normally fed the radio ignition.

What I think must be the case is that your accessory/ignition wire is being fed from a relay which is normally closed, and opens when given voltage (or ground) from your ignition switch. When you use your battery disconnect, the relay opens because it loses ground, and the accessory wire is fed, just as it would be if the relay was opened by voltage from the accessory/ignition switch.

Where I work we call that "back powering". Almost always unintentional, but ground loops happen a lot in cars because of the chassis and often long power and signal runs. And they cause weird shit at best, fires at worst. Thats where the term "gremlin" comes from WRT automobiles.

You have a negative ground system.

If you "break" the negative ground with your isolator, but have any other ground path present, all "positive" loaded devices having a negative path, will provide flow and negate (bypass) the disconnect.

It is not complicated, but it is complex.

Unless you disconnect (or isolate) the battery negative post completely (and you are not, as you state you have device(s) connected to that source to keep them working while the isolator is breaking the negative circuit), you are defeating the purpose of the isolator and your desired effect.

This is one of those things where my years of experience (nearly 50) in aviation, auto, residential and industrial, as well as low voltage, "helps me see" the issue and likely problem.

I am trying to help you look at it with "different eyes" and fix your problem.

If I were there, I could look at it and show you how it works.

(I am a electrical/mechanical whisperer, but find it difficult to teach folks how to whisper.)

What DoctorSolo, said.

It’s the same reason why, for example, on a semi truck trailer, the loss of a ground to the clearance lights will cause the turn signals to permanently illuminate. Current will find a path home.

Jeep electronics and Chinese electronics. There's your problem.

Radio chassis (housing) not grounded?

quote:

Originally posted by darthfuster:
Jeep electronics and Chinese electronics. There's your problem.

It’s been that way since the late 70’s and not just Chrysler/Daimler Chrysler/Cerberus/FCA/Stellantis. GM started in 78 with the non chassis ground radios, to the floating/reversing ground accessory switches, this being necessary with isolating components due to the computer control systems.

quote:

Originally posted by rizzle:
Radio chassis (housing) not grounded?

In the original description he said it is grounded, in fact the only thing that goes directly to ground is the radio, bypassing the ground cutoff switch. The positive side is still hooked up. That's really the root of the problem.
Ground on a 120v ac system isn't, or shouldn't be, the same thing as neutral.
On 12v dc, it's the same thing really. The ground wire on the radio is more to kill any static electricity build up in the radio chassis, not a safety to prevent electrocution like it is on 120v ac.
Somewhere inside the radio there is one or more pathways to the negative side wiring and the ground wired chassis of the radio.
Anytime there is any sort of draw for power in the car, like when you open the door, the interior lights need power, it gets it through the radio chassis ground wire instead of the black ground wire.
The surprising thing is that it hasn't fried something in the radio itself, to the point of making smoke.

Did FIAT buy LUCAS?