Alright, so I am trying to wire a switch that will manually turn on my electric fan when the XJ (97 4.0) is running hot. I followed Dan-H's advice and ran a second 12V source to the fan relay in the box. I got out my FSM and identified the control wire (blue with pink tracers) of the relay. Now the weirdness. When the ignition is off I can jump 12V from the battery to the control wire and trip the relay and drive the fan. However, I cannot get the relay to trip from the in-cab switch that I wired (gets power from ingition power source that I share with my CB). I used a voltmeter and verified that 12V is going to the relay control wire but I cannot get the fan to turn on. However when the ignition is on the relay is getting a steady 12V on the blue/pink wire that originates from the PCM harness. I also installed diodes to control the flow of 12V from my switch and from the PCM. Yes, they are pointing the right direction for flow as I have tested the downstream side with voltmeter.

I am baffled with this. There is something I am not familiar with in this wiring. I want to just run a second relay and take power from my battery to the relay and out to splice into the fan power cable. I can still switch the relay with the same 12V I am using now. My question is: if I drive 12V manually from my switched relay to the fan, what happens when the AC is turned on and trys to send 12V thru the exisiting relay to the fan that is already turned on? Ugggghhhh. Did you follow me? I am tearing out what little there is left to my hair.

Thanks

SeanP

It's because the 12v power lead that goes to the relay (not the switch/signal) lead is a ignition switched lead. It only provides power to the fan when ign is on no matter what the signal lead says.

you'd have to rewire the power lead on the relay also to be a non-ign power source. Make sure you put a fuse in it if you do.


:beer:

Hi Sean,

I am not at all sure I understand your question.

However, that said, here is how I would wire an electric fan in a vehicle with A/C.

There are two control relays,

Relay 1 is controlled by a signal from the A/C

Relay 2 is controlled from your Manual Switch inside your Jeep



Here is the diagram:

http://www.dana60.com/daless2/acfan.jpg
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Please notice the fan will receive operational “power” via the same fused circuit. This is important.
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You do not want two separate and fused power feeds going to your fan. This would cause parallel circuit, which would work fine normally, but in the event of a short in the fan, would make the fuses cumulative.

If both fuses where 20AMPs, in a parallel circuit, it would take 40 AMPS to blow a fuse.

The way I have these relays wired will address this for you.
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How it works:

When A/C is OFF:

Relay 1 is Off, or Not Energized.

Operational voltage flows from “C” - Common Contact (30) through the “NC” - Normally Closed (87a)

Power then flows through the wire to the “C” Common Contact (30) of Relay 2.

At this time, if you turn the fan control switch in your Jeep “On”, then Relay 2 will energize, causing the Common Contact (30) to close the circuit with the “NO” - Normally Open (87) contact, passing power to the Fan.
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When A/C is Turned On

Relay 1 turns on

This breaks the current flow via the Normally Closed (87a) contact to Relay 2, taking it out of the fan circuit.

Relay 1 Common Contact (30) will close with Relay 1 Normally Open Contact (87) and pass operational voltage to the Fan.

In each case, current is flowing through only one fuse (the same fuse) and via one path to the fan.
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Quick note on Diodes.

I am sure you know this, but here goes anyway.

The diode will have a ring painted on one side. The wire closest to the ring must be attached to the +12v side of the relay coil. In the diagram I made, this would be to connector #86.

If the diode is wired back words a direct short will exist “around the relay coil”, which will prevent the relay from ever latching closed.
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As I said Sean, I am not sure I have answered your question but thought I would offer this up for your consideration.

Hope it is useful.

Frank

I'm not sure I see the great advantage in having the elec fan run with the motor off.

If you just supply fused 12v to the switched lead on the existing relay it will make the fan come on as long as the key is on. plus the fan will run as it did from the factory.


If you wanted the fan to run without the ign I would just replace the existing 12v power lead to the relay that is based of an ign circut and replace it with a non-ign lead. The 12v switched lead into the relay from the a/c or temp sensor will still turn the fan on or so will the switch.

I think it is a big plus to run the fan with the ignition off. It helps recovery time and aid in cool down. I just ran a fused hot lead into the fans white wire at the radiator. I just did this temporarily for a trail ride. I was planning to get all fancy with relays but never got around to it. My fan worked great like this for 2 years. Its much easier building a CJ :beer: :beer: RK

Sean,

Here is an update that includes a temperature sensor to control the fan as well.

http://www.dana60.com/daless2/acfan2.jpg
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If you wire your fan exactly as shown it will behave like this.
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With Ignition Switch “ON”

Relay 1, 2, or 3 can control the fan.
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With Ignition System “OFF”

Relay 2 can control the fan “IF” the in cab switch is in the “on” position. The fan will RUN until the in cab switch is turned to the “off” position.

Relay 3 can control the fan :IF” the temperature sensor is “ON”. Once temperature sensor turns “off” the fan will stop running. (Most of not all vehicles with electric fans are wired this way.)

Originally posted by xj4rocks


If you just supply fused 12v to the switched lead on the existing relay it will make the fan come on as long as the key is on. plus the fan will run as it did from the factory.



In effect, this is what I did. However, I could not get the switched 12v from my manual switch to energize the existing relay.


The diode will have a ring painted on one side. The wire closest to the ring must be attached to the +12v side of the relay coil. In the diagram I made, this would be to connector #86.

Frank, thanks again for you help. Just to make clear on the diode issue, the ring should be closest to the relay on my switched 12v going into the relay control (86), right?

I think that running the 87a (I always wondered what that 5th pin was for) to a second relay is the way to go. Of course the pins are buried in the black plastic power distribution box. Did you find an easy way to get inside this box on your TJ?

Thanks again,

SeanP

Hi Sean,

Yes you got it, the diode end with the strip round it should go to the switched +12 volts connecting to the relay coil. This is pin 86.

Please note, you DO NOT need to add a diode to the A/C relay already in your Jeep's Power Distribution Box. It has a diode built into the relay.

You should add the relay to any you add to your Jeep, but in truth, most (Bosch Type) relays will work just fine without one. To be save use a 30 amp relay. Like this one for under $2.

http://www.partsexpress.com/pe/showdetl.cfm?DID=7&Product_ID=9682

If you want, you can buy relay plugs that already have the diode built into them, like this one,

http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=330-076&Did=7

What year is your TJ?

Mine is a 97. If you give me a few hours, (I have to run out right now) I will take a cut at designing the circuit and telling you exactly what wire off the A/C circuit you should tap into to control the fan.

Do you also have a temperature sensor to turn the fan on?

Let me know, so I can get this right for you.

Please tell me exactly how you want the fan to operate.

Frank

Frank, I want to be able to turn a switch on and run the fan all the time. When the AC turns on the fan will pulse (a few seconds on, a few seconds off). I only have heating issues when in 4Low and crawling. I do have the temp sensor, but it doesn't kick the fan on until it reads 217 at the thermostat housing, and by that time it is too late and the coolant is pushing past the rad cap into the overflow bottle. If I just turn the fan on when I see the temp creep up I should be fine. I do want to allow the normal operation of the ac/temp triggering of the fan when I don't turn my manual switch on. Your first diagram should allow me to do just that: run from 87A to 30 on my new relay, run my 12V from my manual switch that I ran last night to 86, run from 87 on the new relay to via 10 or 12 gauge wire to splice into the existing wire that powers the fan and then tap into a ground for 85 on the new relay. I just need to pop into that black box to get at the wiring and plugs on the bottom of the relays. I have a 97 XJ 4.0 which seems to have the same wiring harness as the TJ's . I also have the FSM, which helps me track down the wiring.

Thanks

SeanP

Sean,

Here is the wiring I believe you need to do.

If you do it this way you will not have to play around with the underside of the Power Distribution center at all.


Pull the + 12v power that will go to the relay chain directly from the + (positive) terminal of your battery.

Make sure you use the appropriate size fuse and fuse holder as close to the battery as possible.


http://www.dana60.com/daless2/acfan3.jpg


Go buy three Bosch Type Relays (30 amps) (any auto store, driving light relays), and wire then as shown.

You will also need one wiretap. There is a single BLACK wire that goes to the AC Compressor Clutch.

When the AC unit calls for the compressor to runs, +12 volts are on this wire. This same 12 volts will turn your fan on through Relay 1.

Don’t worry that the fan circuit will turn off when the compressor clutch turns off. It will turn on again via the temperature circuit relay if needed.

Let me know if I need to clarify any of my thoughts.

Good Luck!

Frank


PS: Sean. what type of temp sensor do you have?

If it is the type that slips into the radiator hose, (from under the hose and clamp) you might want to try installing it inside the engine gooseneck up on top where the water is warmer. Just a thought.

Frank, the temp sensor is fit into the t-stat housing at front of head. Wiring from temp goes to the PCM which sends the signal to 86 (blue/pink wire from my orginal post) to turn on the fan when the PCM reads 217f from the temp sensor. There is an existing AC relay in the power dist center that will send power to the control (again 86) of the fan relay.

Thanks for the updated schematic.

Sean

The pcm controlls the ground side of the relay. Why run all of these fancy circuits and relays, when all you have to do is switch the ground side of your existing relay? You should always use the ground side of the relay for control anyway. This assures that if any wires short, no wires will burn and fuses fry. Just simply turn on the relay. One simple wire in series with a switch will work.

my friend was having the same hot run prob on his 90 xj. I just by passed the harness and ran a fused switch to the fan. Now when it gets warm (a/c on or not) he can just turn it on and off as he pleases. less thinking that way :D I like the ideas you guys are doing but damn I hate wires :D

Both the A/C and the temp sensor signal the computer which then signals the relay. That's already built in (1 relay).

What you need now is constant 12v power (not signal..you need power) to the relay. You would do this by providing a new fused 12v source to the power post. Then wire you're switch like you have it to the signal post. (should not be the same wire.) the signal post takes VERY little power. I ran all the power for the fan through the switch once out of laziness and roasted the switch. light gauge wire for the switched signal lead and then heavier gauge which provides the constant 12v power to the power post on the relay.

From now on when the a/c or temp sensor or your manual switch activate the relay it will be drawing the power off of your new constant 12v lead you just installed.

:beer:

OK, I got it done. I used Frank's first diagram above and ran a second relay. I got into the bottom of the power distribution box and pulled the hot from terminal 87A and ran it to term 30 on my new relay. Then ran out of 87 a short 10 gage wire that was spliced into the power to the fan. Switched 12V from my in cab switch and ground to a bolt on the body. What screwed me up was that the ground from the factory fan relay goes back to the PCM and the terminal 30 is always hot with 12V. When the PCM interrupts the ground to the relay, this causes the relay to trigger and start the fan. Let me tell you that when I got it to work tonight I jumped up and down like I solved world hunger. I am an idiot sometimes.

Thanks to all fer your help. Especially Frank, I bow down to your electrical knowledge. I cannot tell you how cool of you it was to post those diagrams.

See ya on the trails.

SeanP