I've been reading up a lot on electronics, relays, diodes, types of switches, etc... I just installed an electric fan for my 258/4.0 head/ MPI engine on the outside of the radiator to assist the stock fan. I wired in a regular on/off switch and an adjustable temp. activated switch. I'd like the fan to stay on after I turn off the engine, but not be able to keep me from running my battery down. I was thinking about putting in a dual batery set up and how I would keep the fan from draining both batteries. I remember about CJDave's oil presure switch; I even printed out the whole thread on the dual battery setup so that I'd have it if I needed it in the future. I had thought that, instead of using an oil pressure switch to close the circuit between the batteries, why not set up a circuit of sorts to run off the ignition switch. When the igniniton is on the circuit is closed, when the ingnigion is off, switch is open. I was thinking to use a relay, but you'd have to use a pretty big relay for that. Is there any other way that anyone can think of to do this by the ignition instead of using an oil switch?[img]/wwwthreads_images/icons/tongue.gif[/img]



Robert87yj[img]/wwwthreads_images/icons/tongue.gif[/img]
4.2 w/<font color=blue>(4.0 head & MPI)</font color=blue>
Process of a moonguy after beer
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Aside from the obvious , why do you want to keep the fan running after you shut the engine down ? And I assume it runs until the temp switch kills the circuit ? My buddy does this on his `50 coupe and he is not absolutely sure there is a big benefit to this or not .

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Happy Owner of a 1979 CJ5
Almost finished [img]/wwwthreads_images/icons/tongue.gif[/img] ( Ya Right ! )</font color=red>

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[img]/wwwthreads_images/icons/smile.gif[/img] Robert, I can see that your Jeepchick is getting a lot for her money in her choice of guys.[img]/wwwthreads_images/icons/smile.gif[/img] You are definitely thinking.....a lost art in some areas of society.[img]/wwwthreads_images/icons/wink.gif[/img] The IDEA of having the auxillary battery connected through an oil-pressure-activated constant-hold-rated solenoid is so that your little cousins, Leroy and Duane cannot fool around in your jeep and run the cranking battery down from playing with Jeepchick's hair dryer, the PIAA lights, and leaving the hot dog warmer on. Using the ignition "RUN" pole would indeed accomplish the same thing, BUT the reason that we did not DO that is that we EXPECT the second battery to be on it's butt from supplying auxillary loads, and we would just rather not have it connected to our fresh/main battery when we are trying to use the fresh/main battery to crank up and go. So that's the justification for the oil pressure switch. YES....many auto manufactirers DO leave the fan running after engine shut-down as long as the temp switch is calling for it. There IS some opportunity for thermo-siphoning to take place that causes coolant to circulate in SOME engines as they sit there, but probably not all. It really depends on the layout. You DO know that many early autos and especially farm tractors did not have water pumps, but operated on the thermo-siphon principle just as a percolator coffee pot does. We are going to have to make this decision as well on our CJ since we have 95% decided to go totally electric fan. We would recommend that if you do have the after-shutdown fan operation, that it be; if at all possible; on the auxillary battery side of the system. [img]/wwwthreads_images/icons/smile.gif[/img]

CJDave
Quadra-Tracs modified While-U-Wait by the crack moonguy[img]/wwwthreads_images/icons/wink.gif[/img] Quadra-Trac Team.[img]/wwwthreads_images/icons/tongue.gif[/img][img]/wwwthreads_images/icons/smile.gif[/img]

My liquid cooled Polaris ATV actually has the electric fan run with the engine shut off but the key in the "on" position - I would assume it is reading off of the temp sensor in some way... The first time after getting the ATV it did this to me I just about freaked out for a second - not used to such high tech gagetry on toys[img]/wwwthreads_images/icons/tongue.gif[/img]

Dave,
Is there a special type of relay that would handle the amperage between the batteries. The highest amperage rated relay that I have been able to find is rated at 70 amps. Lets say I decided to run a relay between the batteries whose coil is charged by the ignition switch being on (wire from ignition into relay coil and then to ground). How large of a relay would I be looking at,...or should I incorporate some type of sylinoid(spelling?) into the circuit? I haven't been able to read up much about sylinoids(spelling?) yet. If my thinking serves me right, I would want a relay larger than a 70 amp relay, right?



Robert87yj[img]/wwwthreads_images/icons/tongue.gif[/img]
4.2 w/<font color=blue>(4.0 head & MPI)</font color=blue>
Process of a moonguy after beer
[img]/wwwthreads_images/icons/mad.gif[/img] [img]/wwwthreads_images/icons/frown.gif[/img] [img]/wwwthreads_images/icons/crazy.gif[/img] [img]/wwwthreads_images/icons/wink.gif[/img] [img]/wwwthreads_images/icons/smile.gif[/img] [img]/wwwthreads_images/icons/blush.gif[/img]

[img]/wwwthreads_images/icons/smile.gif[/img] Robert....no need to re-invent the wheel. There is a SOLENOID just for this very job. It is called a continuous-hold solenoid, and will handle cranking current no problem if need be. Again, let me caution you about re-inventing the wheel. Use a HOBBS oil pressure switch to pull in the coil on the solenoid. The Hobbs would be fed from the ignition switch.[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
Quadra-Tracs modified While-U-Wait by the crack moonguy[img]/wwwthreads_images/icons/wink.gif[/img] Quadra-Trac Team.[img]/wwwthreads_images/icons/tongue.gif[/img][img]/wwwthreads_images/icons/smile.gif[/img]

You want the same effect of the Japanese cars, right. The Hondas fans stay on after engine shutdown. Chad

1985 CJ-7 258/TR upgrade/4"superlift/2" bodylift 33x12.50 BFG mt/Ramsey Platinum 9000,Tomkin tire combo,etc,etc.

You have more than a couple of choices here...

Any accessory relay will handle fans, so you can use most any kind of switch you want to.

Use a thermostat switch that kicks in about 130 or 140 degrees.
If you don't want to trust just the thermostat switch, then wire in a delay relay that allows the accessories to stay on for ten minutes after the ignition switch is turned off, like Cadillac cars have.
You turn the ignition switch off, and all accessories stay on for ten minutes or until the dome light comes on (like when you open a door and get out).

You can also put in a 'Battery Saver'.
It's a device that goes between your battery positive terminal and any loads.
When the battery voltage drops below around 12 volts, it shuts the source off until you push a button.
They are sold a number of places, and are safe and reliable.



"I Have The Body Of A God... Buddha"

You got some pretty good answers here. I personally like the TR delay relay idea for it's simplicity. However I wouldn't go to all that trouble. If the fan kept on working after the engine stopped, only the cooling liquid in the radiator will be cooled since the water pump stopped turning. The engine will not cool any faster. Besides, you risk discharging your battery if something goes wrong in the circuit. Not really worth it.

If you're having problems overheating and you're sure your cooling circuit is trouble free, you might want to switch to a lower temperature thermostat. I use one that opens at 160 degrees. This might be too low for you, I wheel in very hot temperatures and after a few minutes the engines ends up operating at it's optimum temperature.

I also have an extra electric fan behind the grill to aid the engine fan. I however hooked it to the AC compressors clutch using 2 relays so that it operates only the AC compressor engages. This improved the AC performance as well as lowered the engine temperature a bit. You might want to wire yours similarly only to make it work when the coolant flows through the radiator. Any ideas on how to wire it anybody?

A.B.

If you didn't get stuck, you didn't wheel hard enough...[img]/wwwthreads_images/icons/cool.gif[/img]
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[img]/wwwthreads_images/icons/smile.gif[/img] I think in this case, AB, that the engineers were looking for a way to reduce the HEAT SOAK that is so severe when you dead-stop a vehicle after running the interstate at high speed on a nice 112 degree day. By leaving the fan running the heat is puked out from under the front of the car, and all over the hapless bystanders, but the end result is that you DID get rid of some heat. There DOES exist some oportunity for minimal cooling system circulation in some vheicles. As I pointed out, thermo-siphon action is a well-known phenomena in liquid cooled engines. We are connecting out electric fan to the A/C clutch circuit as well. I notice that our Pontiac is that way from the factoreeeee.[img]/wwwthreads_images/icons/crazy.gif[/img]

CJDave
Quadra-Tracs modified While-U-Wait by the crack moonguy[img]/wwwthreads_images/icons/wink.gif[/img] Quadra-Trac Team.[img]/wwwthreads_images/icons/tongue.gif[/img][img]/wwwthreads_images/icons/smile.gif[/img]