Auxilary Cooling Fan

I would like to hook up a switch to my auxilary fan so that I can turn it on whenever I feel the engine is running too warm, not the computer. But, I also want to maintain when the fan currently comes on - with the AC, when the temp is too high - for when I am not driving it, etc. Will running a wire and switch from the battery to the power wire of the fan cause any damage to the computer? What gauge wire and what size fuse would be needed for this?

Thanks

OK,

You will need two relays rated for the power output you want and one switch.

The coil on relay #1 will be powered by the existing power going to the fan. Disconnect the power lead to the fan and run it to the relay #1 coil. The coil on relay #2 will be powered by the switch.

Next, parallel contacts from each relay together. Run a power lead to on side of the contacts and the other end to the hot side of the fan. The negative side of the fan needs to run to ground.

--- existing hot lead that goes to fan ------------Coil on relay #1 ---------ground---

----12vdc---------your switch------------coil on relay #2 -------ground---


------12vdc--------normally closed contact relay #1----------to fan

------12vdc--------normally closed contact relay #2----------to fan

This way, when the computer tells the fan to turn on, the contacts on relay #1 will close giving power to the fan and if you turn on the switch, relay #2 will close giving power to the fan. It will not matter if both the switch or the computer is calling for power as the relays isolate the power.

Note: the 12vdc going to the contacts can be from a single source and not divided as the drawing shows.

Tom Dennis

That's a very neat solution for preventing backfeed into the chassis circuitry.

Not that this is only necessary on '91 and newer XJs. The older ones use a thermal switch in the radiator tank, which in turn activates a relay to send power to the fan. On the older ones it is acceptable to wire the manual switch directly to the same relay that the radiator switch feeds, since it is a stand-alone circuit and doesn't connect to anything "electronic."

WILL THIS WORK FOR A 1996 CHEROKEE AS WELL?

ALSO, WHAT TYPE OF SWITCH DO I NEED?

SINGLE, DOUBLE POLE?

SINGLE, DOUBLE THROW?

I KNOW JUST ENOUGH TO BE DANGEROUS AND DON'T WANT TO FRY ANYTHING OR MAKE MULTIPLE TRIPS TO THE STORE.

THANK YOU.

I believe that you would need a single pole double throw switch and relays.

You only need one set of contacts for the relays and switch. You need to wire to the normally open contacts. Make sure that the relays have N.O. contacts (normally open). The switch does not matter as toggle it one way and the contacs are closed and the other way and the contacts are open.

Be sure that the contacts are rated for the amperage of the fan.

I like using relays as the switch does not need to handle the heavy power load so you can run lower sized wire into the console and leave the heavy duty wireing inside of the engine compartment.

Eagle - thanks for the compliment. We always use this technique in our control panels as we often deal with different power sources.

I also used this method for my Hella E. Code head lights. The high beam switch triggered one relay and the low beam switch triggered the other relay. Works great.

Tom Dennis

note: if you use the double relay technique, it does not matter what year of vehicle you use.

thank you for the reply. I have a couple more questions if you don't mind.?

1) when you say "existing hot lead to the fan" do you mean on the PDC/EDU side of the plug or the fan side of the plug?

2) do you connect the ground wire from the PDC to anything?

3) are you bypassing the fan connection, splicing into it, or ?

4) I think you wuold use a SPST switch. with the switch turned on you are controlling the fan. with the switch off the computer is in control. or if you use the double throw is one position for the computer and one for your control?

Thank you for the help and patience.

Yeah, it sounds to me like the switch only needs to be SPST (single pole single throw ... i.e. "ON-OFF")

Thanks for the help. I am barely electronics literate, but this should help out a lot.

One more question - how do I determine what the amperage of the fan is for the contacts?

Thanks,

you would need to use an ammeter or take it to an auto electric place or have somebody knowledgeable meter it for you.

you might be able to get it from the service manual or from a service guy at your local stealership.

Check the rating on the ckt that powers the fan. Also the fan motor itself may have its v/a rating on it, don't know for sure but most motors do. According to my 89's FSM, pg 8W-10-2, the fuse in the PDC, #5, is 40Amps. Thats one hefty ckt, 480Watts.

Rich, I am looking in my FSM in the electrical section and cannot find a specified rating for the relays.

If I read your post right PDC fuse #5 is rated at 40 amps. Is that all that fuse is connected to?

I agree that is a large wattage draw. I was thinking more along the 20 amp at most range for the fan.

My reference is for a 1996.

aux fan

Typo in my firs post, I have a 98XJ NOT 89 . Thats all I could find hooked to it in the FSM, seems to be on it's own and at 40amp I'm not suprised.

Anthropy said:

OK,


--- existing hot lead that goes to fan ------------Coil on relay #1 ---------ground---

----12vdc---------your switch------------coil on relay #2 -------ground---


------12vdc--------normally OPEN contact relay #1----------to fan

------12vdc--------normally OPEN contact relay #2----------to fan

Tom Dennis

Click to expand...

Sorry guys - I noticed a fubar in my origional drawing - the contacts on the relays should be normally OPEN.

You can run small wiring inside of the vehicle through the switch as the coil does not take a lot of current to activate. That way, you can run you high amperage wires just to the contacts on the relays and keep the big stuff in the engine compartment.

If you guys still need it, I can manually draw up a proper schematic and then scan it - it would be in a jpg format.

Tom Dennis

could you do the schematic?
I think that would help clear up most of the questions.
Thank you.

I have a drawing done - currently it is in a MS Word format. Also, I used 'ladder' logic for the wiring as it is what I am most use to using.

I will see about getting it up here. If someone wants a copy right a way, email me a [email protected] and I will send you a copy.

Tom Dennis

I found a way to convert it to a pdf format now so anyone can view it.


http://home.mchsi.com/~dennistr/FanRewire.pdf

Eagle said:

That's a very neat solution for preventing backfeed into the chassis circuitry.

Not that this is only necessary on '91 and newer XJs. The older ones use a thermal switch in the radiator tank, which in turn activates a relay to send power to the fan. On the older ones it is acceptable to wire the manual switch directly to the same relay that the radiator switch feeds, since it is a stand-alone circuit and doesn't connect to anything "electronic."

Click to expand...

So this would be a good solution for pre '91 vehicles:

http://home.mchsi.com/~dennistr/FanAltWire.pdf

And this one for '91 and up:

http://home.mchsi.com/~dennistr/FanRewire.pdf

edit:
The indicated relays in the drawings are not the existing relays in the vehicle.


Tom Dennis

You know, I don't know if this would make any since, but my 2nd jeep came with a tow package, as well as a pull behind the ole RV package. I striped all the crap off for the RV package and found 2 little boxes (1"x1"x.25") and they look like an amp for a stereo. They were for the rear break lights hooked through the RV, and stay stock with the jeep. It allowed for 2 seperate power inputs, and has one output. Allowing the RV to power the break lights, or the jeep. Something like that would be eazy to do.

It may be bridge rectifier using diods that would prevent back feed. You could use that, but I think the fan would pull a lot more power than the lights so you may end up frying the beastie. Check to see what the amerage rating is on it.

Also, the use of a relay is nice when having the switch as then you don't need to run the large wires into the cab at the switch.

By the way, did both links work for you. A guy emailed me and is having trouble viewing the info.

Tom Dennis

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