looking for 12v power when vehicle is running only.

[Maniak]

Can anyone think of a 12v power source that is active ONLY when the truck is running.

I have a 12V aux fan for the tranny cooler.. I have it set to only run when the key is in the on position and when the torque converter is not locked. Once locked it will wait 5 seconds before turning on the fan.

The problem is.. When I turn the key to start the truck, the aux fan starts (since the converter hasn't been locked yet, it doesn't wait 5 seconds before starting).. The fan pulls 8 amps (12 amps at startup). With one fan its not too bad.. But I'm going to install another aux fan (one on each tranny cooler) and 16 amp draw while I'm trying to start the truck just doesn't sound like a good idea (and thats a 24 amp peak draw at fan startup)..

I want to change my 12v switched power to a 12v source that is on only when the truck is running, not just when the key is in the on position...

~Mark

p.s. I know someone will ask how I keep the fan from starting right away.. I'm using a 38,000 microfarad (MFD) capacitor to keep the n/o relay energized.. This means once the converter is locked it energizes the relay (and cap).. When the converter unlocks the relay is now powered by the capacitor which keep it closed for about 5 seconds (until the voltage drops to 3 volts)..

 

[410Fortune]

I am not sure there is a power feed you can tap into that works in the running position, only 12v+ constant and 12V+ switched (Acc on)

I vote for put a switch on your new aux fan, it does no good at highway speeds anyways and you also dont always need it. OR wire in a temp controlled fan relay so the fan only comes on once the coolant or ATF reaches your set temp.

 

[sn0border88]

what you can do is to tap into the wire that is selected when the key is in the run position. That wire dosent have enough power to run the fan but you can splice that to run into a relay, therefore when the relay gets power from that wire it will close the circut and send power through to the fan. But I think that instead of doing all of that it would be easier to just run a switched source with a toggle and have the relay mounted close to the battery.

 

[1IronMike]

How about tapping into the alternator line (the one that charges your battery)and using it to pull in a relay (while running) that will close and make a circut from a better supply?

 

[sn0border88]

certainly not a bad idea.

 

[Jason94sport]

I'd put a switch in too. If it doesn't draw more then 15 amps you can use one of these...
http://cgi.ebay.com/ebaymotors/12V-...Z6763QQitemZ8024367131QQrdZ1QQsspagenameZWDVW
I have one mounted in my glove box for my siren since it draws a load even when not in use. With the switch I'm sure it won't drian my battery.

 

[sn0border88]

well if you want something that you can have assurance that it wont be left on when the truck is off then get a racing style switch with a cover and add a power light to it, so it will light up when in use. And put it somewhere that you can see it. for a switch like that, radioshack or walmart will have it, dont bother with ebay.

 

[DeRocha]

I side with Fortune.. I would use a temperature controlled switch as well as a toggle overide.. That way if you're wheeling and the ATF is above the temperature cut on value you can use the toggle switch to overide the aux fan and shut it off and allow full power to the starter. If you want to get fancy there are various 12v IC controllers that you can buy/build that allow variable delay on/off... BUT IMO they just add unnessary complexity to your vehicle. A simple design with minimal failure points is the way to go.

 

[Maniak]

Tapping into a wire when its in the "run" position is just switched power. I already have that one..

Currently, the wire I'm tapped into is a switch 12v that does not get power when the key is in the start position.. So its not quite as bad, but as I'm turning the key to start the truck, there is a 12 amp spike, then an 8 amp draw (while the key is in the on position) that is stopped right as I try to start the truck (when the key goes to the start position).

As for it not running when on the freeway.. It doesn't run when the torque converter is locked up, which already turns it off when on the freeway (or when I'm doing 50 in town)..

The alternator wire is connected to the battery, so there is always 12 volts there (I'm assuming your talking about the larger wire in the back of the alternator)..

Even with a temperature switch I will still run into the same problem. If its hot out the fan will start with the engine off when then key is turned to the on(run) position. On a hot day in Az the pan temp can be 130F even before I start the truck for the first time that day.

I am using relays and a capacitor and even a diode in my current setup. This way everything stays isolated so I'm not worried about drawing too much power from which ever wire I can find to use.

The diode keeps the power from feeding backwards, the relays (I use 3) issolate and reverse the signal from the converter and the capacitor keeps the fan from turning on right away once the converter unlocks which keeps the fan from turning on/off/on/off/on when I'm lightly on the brakes on the freeway.

I'm going to look into one of the signal wires on the alternator and see if any of those are only powered when the engine is running..

Any other ideas of where to find one that is only powered when running? Maybe something off the coil pack?

~Mark

 

[sn0border88]

coil pack was my first guess but I didnt think there would be any safe way to draw a current from the pack. if you can figure it out thats probably your best bet, easy to get to and everything. I thought that the smaller wire on the alt only had power when the alt was running.. maybe thats the signal wire your talking of?

 

[DeRocha]

How about the tach wire? Most remote starters use it to determine when the engine is running so the starter can stop cranking etc..

 

[flgatorz10]

fuel pump relay

turns on for a brief second when you flip the key to on, but then shuts off

 

[DeRocha]

fuel pump relay

turns on for a brief second when you flip the key to on, but then shuts off

No way.. First off the fuel pump circuit doesn't work that way. It will stay at 12v for 2 secs after loosing a tach signal. Even if it worked as you suggested it would be exactly what he is trying to avoid (a big electrical drain prior to starting the vehicle).

 

[buggman]

Digging up an old thread... but has anyone ever determined what is a good "run only" 12v power source that doesn't get power in the "acc'y" position?
I'm looking for something in my 03 Explorer to tap into that will only be hot when the truck is running, not in the accy position. I'm going to trigger a relay from that power source to control a battery isolator solenoid to keep my 2 batteries charging when it's running, but separate them when it's not running or when I just have the key on for the radio.
The only thing I've found is the alternator "exciter" wire or whatever it's called, but I believe it's PCM controlled & I didn't want to cause any issues there, even though I'd run a diode & fuse on the relay trigger wire.

 

[Mbrooks420]

I know this is old and Maniak has surely figured something out for this, but in both cases I’d use something like this:

https://www.amazon.com/Undervoltage...525698161&sr=8-10&keywords=Low+voltage+cutoff

Maniak also could have used a cheap ice cube relay.

 

[CDW6212R]

Use the oil pressure sensor signal. There's one wire there which only passes current when the oil pressure is high enough to turn the dash light off. I did that in my 99 Explorer to turn on my automatic headlight circuit. So my headlights only come on when the engine is running(I also covered the light sensor with RTV).

Older cars and the 1st gen's use a real oil pressure sensor, which may not cut power fully with ignition on but engine off. Test that, if it doesn't do what you need, you can still install a later sensor in a "T" fitting, and that will work.