Electrical wizards - question for ya

I want to do the upgrade that Craig has on his site at "Swiss Army Jeep" for adding a manual switch to the electric fan circuit. ( http://www.olypen.com/craigh/auxfan.htm ) I e-mailed him but haven't heard back, so here it is.

This would be real easy if I just added an after market switch, but I want to use a switch that I got from the yard so they all match. The schematic for the fog light switch shows power coming into the switch on a green wire and exiting to the foglight relay on the green w/tracer wire, and the black wire is grounded for the light to work in the switch. Green w/tracer and black are on the same side of the switch. So I bring power in on the green and tap the green /tracer into the orange on the fan relay. But if I ground the black so that the light will work, every time the thermo switch or A/C energizes the relay for the fan, won't the power feed back down the line and make the light work? Would this stop the relay from working or cause any problems? I don't have a problem with the light coming on as long as it doesn't mess things up. Wouldn't be a bad indicator to have really.

The light should only come on when the switch is on, it is a LED, "D" stands for Diode, these only let the current flow one direction.
As far as wiring the fan, I would suggest finding the switched wire to the fan itself. Some years used positive switching, some used negative. I believe 95 and earlier modles used positive, but check this with a meter first. Once you find the switched wire, cut it in two, connect the one from to the fan to #30 connect the other to #87a. Connect #87 to a fused +12v source for positive switching, ground it for negative. Ground #86, and run a wire from your switch to #85.
Doing it this way will not provide any "feedback" at all! It is also the safest way to do it!
Good Luck

whenever i can i replace wiring in my jeep especialy if you are going to use it for something you rely on like cooling. if it were me i would take the switch that matches everything and run new wires to a relay and then from the relay tie the wires into the ones alread coming out of the fan (does that make sense) that way you can switch on the fan when you want but it will also come on by it self if it needs to. i may be totaly off but i thought that aj's-armor makes a complete kit for this for like $30:laugh3:

By the way, the way I described will let it operate like normal, it just bypasses the stock system when you turn the fan on, turn it off and back to normal.

Hmmm . . . interesting. This isn't a "right now" project, just something I want to do for a chance to help with the cooling. If I read it right, both are suggesting to put a relay in line to trigger the fan just as a safety. Sounds smart and not that hard to do. Brandon, why would I spend $30 for a kit when I can do it myself? :laugh3: I don't see you spending the money for a pro welder for the front 44 or you'd have BEEN AT NACfest.

TRAILREADYXJ said:

By the way, the way I described will let it operate like normal, it just bypasses the stock system when you turn the fan on, turn it off and back to normal.

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I think I understand. When the relay isn't energized by the in cabin switch, the power is going through the relay from the stock system. When the switch is turned on and the relay energized, the stock system is interupted and the manual system takes over.

Grizzley said:

I think I understand. When the relay isn't energized by the in cabin switch, the power is going through the relay from the stock system. When the switch is turned on and the relay energized, the stock system is interupted and the manual system takes over.

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Exactly!

Grizzley said:

Hmmm . . . interesting. This isn't a "right now" project, just something I want to do for a chance to help with the cooling. If I read it right, both are suggesting to put a relay in line to trigger the fan just as a safety. Sounds smart and not that hard to do. Brandon, why would I spend $30 for a kit when I can do it myself? :laugh3: I don't see you spending the money for a pro welder for the front 44 or you'd have BEEN AT NACfest.

Click to expand...

Your aux fan is already controlled by a relay, and on an '88 the relay is controlled only by the temp switch in the radiator tank, not by the ECU. It's a stand-alone circuit. You don't need a relay to control a relay. Just put a parallel tap in the two wires coming off the switch in the radiator tank -- one wire to each leg of your switch. If you don't ground the ground terminal of the fog light switch, the indicator light is out of the equation.

Eagle said:

Your aux fan is already controlled by a relay, and on an '88 the relay is controlled only by the temp switch in the radiator tank, not by the ECU. It's a stand-alone circuit. You don't need a relay to control a relay. Just put a parallel tap in the two wires coming off the switch in the radiator tank -- one wire to each leg of your switch. If you don't ground the ground terminal of the fog light switch, the indicator light is out of the equation.

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This method would work, however using the relay the way i suggested will not let power go where it is not supposed to, like the sensor. I install 12V electronics for a living and have been MECP certified, the way I suggested is just the best way to go, not the easiest.

TRAILREADYXJ said:

This method would work, however using the relay the way i suggested will not let power go where it is not supposed to, like the sensor. I install 12V electronics for a living and have been MECP certified, the way I suggested is just the best way to go, not the easiest.

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Your way is better for the 91 and newer models where the aux fan is tied into the ECU. The Renix models don't use a "sensor." The thingie that screws into the radiator tank is nothing but an ON/OFF switch that controls a free-standing relay. Adding a second, manual switch in parallel to the switch in the radiator does not provide any opportunities for backfeeding any other circuits in the vehicle. Applying your solution to the Renix XJs accomplishes nothing other than adding a second relay to bypass the factory relay, which is connected to nothing other than the aux fan circuit.

Needless complexity. Don't make the mistake of extrapolating from your year XJ to assume that the same thing is correct for all other years.

Eagle said:

Needless complexity. Don't make the mistake of extrapolating from your year XJ to assume that the same thing is correct for all other years.

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Actually mine is a 90 with a renix. I guess I am in the habit of going a little overkill when it comes to electronics. I'm sure you've read up on re-wiring fog lights, headlights, etc... direct to the battery to make them brighter, this is basically the same principle, just supplying the most juice to the fan vs wire that is 15+ years old. But you're right, with that simple of a circuit, your way would be just fine.

Geez - all that needs to be done on the RENIX is to wire a toggle switch (of your choice - I didn't use OEM switches until I inherited a few) in parallel with the "radiator spark plug."

The RENIX fan is nearly isolated, but by paralleling the CTS you are simply mimicking the function of the OEM switch, so you won't be causing any trouble. I usually go to either a coverred toggle switch in a convenient location, or to a blank spot in the dashboard with an OEM rocker.

The later versions are only slightly more difficult - just figure out which relay feeds the fan and how it's tripped (provide ground or +12VDC) and duplicate that with a parallel circuit thru a switch. Rock solid and Mark-1 Mod-0 reliable.

Either way, you don't need the extra relay, since you're using one already in place. The current through the switch is negligible, so it should last nearly forever. The relay is doing all the work - and you didn't even have to add that!

Even when designing for reliability, the simplest way is usually best. The nice thing about automotive electrics is that the functions we need for things like this are already in place, and there's usually an easy way to make the system do what you want it to do with a mininimum of additional parts...

5-90

What happened to the swiss army jeep web site? He had some good info! Anybody know if it has been relocated?