Odd Ground? Issue??

[JonV]

I have a weird one. On my 57 Thunderbird, 312 I run a battery disconnect on the negative terminal. Simply a green knob you unscrew. I've had it this way for years.

Was tinkering around and set my test light on the valve cover, with the negative battery disconnect, disconnected, I have power at the valve cover. This is with the test light grounded on the negative terminal. Which now that I type that, with the disconnect, disconnected, I guess this is not the ideal ground.

If I put the disconnect in to complete the ground, I have no power at the valve cover. If I use a different ground, other than the negative battery terminal, I have no power at the valve cover.

I did check and clean both the battery to firewall ground and the motor to firewall ground. No change.

I did recently add a new battery, the old one did last over 12 years.

I did change the solenoid, also with no change.

I am leaning toward the switch, but am a little baffled.

Any help would be greatly appreciated.

Jon

[redstangbob]

I would suggest you have a small current draw somewhere in your car, perhaps the reason for the disconnect? JMO, Bob C

[Peddller]

Your test light is becoming the ground for the car. Any draw at all even the clock would cause a current flow through the test light and cause the bulb to glow. If you were to turn on the headlights it would burn out the bulb in the test light.

[voltron]

Move the battery disconnect switch to the positive terminal and your test light won't light when you connect it to the negative battery terminal and touch the valve cover with it. Placing the battery switch on the positive terminal will still prevent the current draw others have mentioned from draining the battery.

[Kneetoe]

Hello,
Saw this thread and wonder if I have a similar issue? Any suggestions most welcome!
Background
I have fitted a new battery as a result of repeated cranking due to a bad condenser (really!) The starter motor has been well used!
I have the green disconnect on the negative terminal and disconnect to trickle charge.

Problem
The trickle charger NEVER goes 'green' in fact it stays red and gets really hot!
I disconnect the Positive terminal too (so battery totally disconnected) AND ALL IS WELL!

I touch the Positive battery lead to the Positive battery terminal (NEG still disconnected) and the trickle charger immediately goes red.

So it would appear that even with the NEG disconnected, I'm getting a significant current draw!
Right now, I have the battery totally disconnected and on charge - ALL GOOD.

Any ideas? I wonder if it could be started motor related? I read somewhere about voltage regular terminals being closed?

Thanks!
John

[CSPIDY]

Any corrosion on the post or the cable end will draw additional current from the charger.

[Alan H. Tast]

Any corrosion on the post or the cable end will draw additional current from the charger.

And/or corrosion inside the cable. When were battery cables last replaced?

[55blacktie]

What battery lasted 12 years? Surely you kept it on a charger when not in use.

[ICON 1956]

I always keep the trickle charger on my T Bird, I have one of those non acid electric type with the period correct battery . so I have had it four 4 years now with no issues.... Not cheap .

[paul2748]

As stated above - connecting the test light to the ground (minus) battery terminal and the valve cover, you are providing a ground so the light will go on. You are not getting power to the valve cover - just getting a complete circuit.

Disconnect should be connected to the ground side of the battery. Safest method.

Personally, I don't like those disconnects (zinc ?? lead???) with yellow coating and green knob). The contact point can corrode with turning it on/off and you will find yourself with no connection and wonder what is wrong. Happened to me twice - first time didn't realize what it was, second time found the corrosion when thoroughly examining it..-

[DynoDan’55]

Disconnect should be connected to the ground side of the battery. Safest method.

Personally, I don't like those disconnects (zinc ?? lead???) with yellow coating and green knob). The contact point can corrode with turning it on/off and you will find yourself with no connection and wonder what is wrong. Happened to me twice - first time didn't realize what it was, second time found the corrosion when thoroughly examining it..-

I agree. Just something else to corrode/add resistance. Only advantage is, if you have a short/fire and need to disconnect the battery quickly and don’t have a wrench handy. I keep a spare 1/2” socket near, but have a circuit breaker in the hot feed wire and thus don’t really worry about electrical fires. Only concern is wearing the clock out (I usually leave the cable on while charging as I drive regularly year round), though constantly resetting the time could likely be just as hard on it.

[T-Bird Bob]

Guys, here are two things in effect:
1) This is electricity 101. Every permanently connected circuit like your clock and interior light (if your door is open) are just a resistance to ground. It is several hundred Ohms or even kOhms so it will only allow a small current to be drawn when everything is working normally.
Now, if you disconnect the negative battery terminal, these connections will put the whole car on the same potential as your battery positive side. Of course you will measure a 12V (or 6V for 1955) between any part on the car and your battery's negative terminal.

2) The green knob-style disconnects didn't work for me either. When you unscrew the knob it will only release the contacts to pressed together. If you have a lateral force on the contact from a heavy battery cable (or from it being rigid) it might not fully disconnect.
I use this one and it works fine for me.
https://www.homedepot.com/p/Battery-Doc ... /202931972

Good luck.

Bob

[Alan H. Tast]

Any corrosion on the post or the cable end will draw additional current from the charger.

And/or corrosion inside the cable. When were battery cables last replaced?

To circle back as to why I'm asking how old the cable is and when was it last replaced, we had a get-together on Saturday with members from the local
CTCI chapter and a group of 10 'Birds that drove in from the Kansas City chapter. In visiting with one of the people attending, he was commenting about an electrical 'gremlin' that had his '65 Landau acting erratically for about 4 years. After chasing down the problem of dim lights and low charging levels by changing light switches, alternator and cleaning/tightening ground connections, one day he was tracing down resistance in wiring with an ohm meter and accidentally moved the ground cable going from the engine block to the battery. The meter jumped, indicating resistance was significantly lowered IIRC (I'm not an electrical guru by any stretch of the imagination, folks). Wiggling the cable and testing both resistance and voltage going through it pointed to the ground cable being bad internally at the terminal. He swapped out the cable and all his lights, instruments, etc. came back to life, plus the ammeter pegged 'C' for the first time since he got the car.

Based on what I'm seeing here and the anecdote I heard over the weekend, I'd recommend replacing the battery-to-ground cable. They're not that expensive - even the repop ones. And you may want to check your old cable against a new one with a multimeter to find out how much resistance it has. I'd love to hear what the comparison is between the two.

[Kneetoe]

Thanks for the suggestion!
I'll take off the ground lead and check it!
Cheers