Automotive Switch Question

[TallPaul]

I notice that the overdrive cutout on automatics will revert to on for the next time you start up the car. My wife's Mountaineer has a rear view mirror feature you can turn off, but will be on again next time you start the car.

What I am looking for is the reverse, a switch that if you turn it on, it will revert to the off position when you shut the power (key) off. It seems they make switches called "normally off" but I think that is the same as "momentary on" which is not good for my purposes because I don't want to hold it on constantly, just push it and it stays on but turns off when the power is cut.

 

[Kestas]

I'm guessing it weas designed this way so the driver doesn't forget to leave it in "OD cutoff" mode. Your request is the exception. Imagine if you forget to return the feature to OD ON and you (or the next driver) drive all the way to Florida that way!

Why the aversion to OD? The automatic transmission is designed to pick the optimal gear for your speed and acceleration requirements. The only time it should be switched off is if your driving conditions cause excessive shifting back and forth between D and OD.

My cars doesn't have OD cutoff. I simply shift from OD to D to cut out overdrive.

 

[TallPaul]

On, not for circumventing the OD. That was just an example.

I am looking for a switch for an aftermarket gauge lamp. The 2001 Ranger does not have a fuze in the interior that controls anthing switched from the lights. My other alternative is a toggle that at least it will shut off with the key off, but will remain on if I forget to turn the switch off. I'll use a lighted on (light on when switch on) for a reminder anyway. Can't get a wire under the dash unless I can shrink down to the size of a Rhesus monkey. Maybe pull a lead in from under hood.

 

[Rick in PA]

You could do that with a relay. You wire it so that the relay's normally open contact energizes the relay's coil. Then you have a momentary switch wired so that it can also energize the coil.

Once you energize the coil, the normally open contact feeds power to the coil and keeps the relay energized (the connection that energizes the coil also powers the lights).

When power to the relay is interrupted, the normally open contacts break and it stays off... until the momentary switch is pressed again.

It's a fairly standard circuit used for electrical motors where you don't want a motor to restart after a power interruption.

 

[HerrStig]

Is there a secondary fuse box under the dash you can put a fuse tap on? Fuse tap = a holder with a fuse in it and a wire sticking out you can draw power from. It plugs into the fuse socket.

 

[TallPaul]

I don't believe there is a secondary fuse box under dash, it's on the left side of the dash by the door. Another fuse box is under the hood and likely has the lighting fuse.

 

[TallPaul]

Originally Posted By: Rick in PA
You could do that with a relay. You wire it so that the relay's normally open contact energizes the relay's coil. Then you have a momentary switch wired so that it can also energize the coil.

Once you energize the coil, the normally open contact feeds power to the coil and keeps the relay energized (the connection that energizes the coil also powers the lights).

When power to the relay is interrupted, the normally open contacts break and it stays off... until the momentary switch is pressed again.

It's a fairly standard circuit used for electrical motors where you don't want a motor to restart after a power interruption.
Seems complicated but maybe I can do it. Is it like the diagram in the seventh post at this site?

 

[Rick in PA]

Here's the circuit:

It could be made with pretty much any 12V relay.
Hot = +12 volts (fused, of course)
Load = the lead that goes to the lights
Com = vehicle ground

Once I remembered "self-latching relay circuit", it was the first one in a Google image search. That's actually a pretty nice diagram.

 

[TallPaul]

Thanks. That looks pretty easy. I'll drop by Radio Shack and pick up the Momentarily On switch and a relay.

Question: Do we really need to connect COM to vehicle ground? Won't it be sufficient that the lamp being powered is grounded?

 

[yonyon]

In Rick's diagram, yes, pin 86 must go to ground. Without power flowing between pins 85 and 86 the relay will open and you're back to a simple momentary switch.

 

[Rick in PA]

Originally Posted By: yonyon
In Rick's diagram, yes, pin 86 must go to ground. Without power flowing between pins 85 and 86 the relay will open and you're back to a simple momentary switch.


Yup, another way to look at it is that vehicle ground is the negative (-) side of the 12 volt circuit.

 

[TallPaul]

Ah, I see, the coil has to be energized in order for the relay to make contact. Thanks.

 

[TallPaul]

Well I went a different and simpler route. I installed a lighted switch hooked to an unused fuse slot that is controlled by the ignition key. I will actually hit the switch less this way as I can turn it on for the evening and leave it on, then in the morning turn it off. Maximum it will be switched on and off is once per day that I drive in the dark. I won't forget and leave it on because the light, a small bright red (LED maybe)is rather bold. It is mounted below the dash and behind the shifter so I will see it when I start out if it is on, but I won't see it driving at night.

Thanks for the other info anyway, it never hurts to learn something new.