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Old January 11th, 2011, 16:07
fyrfytr1717 fyrfytr1717 is offline
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Join Date: Jul 2008
Location: Turlock, CA
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Re: Radiator Fan Override Switch (The Easy Way)

Originally Posted by Mudderoy View Post
The site pretty much runs itself, so I've been spending more time setting up for a live ustream show and podcast.
Wow, sounds like you're keeping busy with the online stuff. Great that you're doing so much to keep the XJ community alive. Figure it needs all the help it can get seeing as they've been discontinued for a decade now...

Originally Posted by Mudderoy View Post
As for the 10 amp fuse, well since I have running the ground wire to the switch when I bypass the relay and turn the fan on, grounding it, I am running the full current of the electric fan through that ground wire and the switch.
I think you're a little confused as to what my write up (and your version of it) actually accomplishes... The whole point of this mod is to override the factory system. You are not activating the fan when you flip the switch, you are activating the factory relay which in turn activates the fan. That DB/PK wire does not connect to the fan, it connects to the relay that the PCM is supposed to trigger to turn the fan on. If it did, a 10amp fuse would blow every time the fan came on as the factory fan circuit uses a 40amp fuse. Regardless, you should always place a fuse as close to the power source as possible. It can only protect whatever components are downstream from it.

Originally Posted by Mudderoy View Post
I didn't want to hook the relay up to the 12 volts directly but having it connected to a 12volt source that was there and not there seemed to be telling the ECM that there was a problem, therefore the check engine light would come on. After a few weeks of staring at that light and thinking about what my OBD II scanner was telling me it dawned on me I needed a constant 12 volt source going to the ECM. I can only assume that I am correct since providing that constant source resolved the check engine light issue.
The PCM can only activate the fan relay when the ignition is on. The DB/WT wire (so long as you tapped into the right one) is only on whenever the ignition is on. The PCM would never see it as "there and not". If you look at the FSM based wiring diagram I provided in the first post, you'll see that the factory relay is powered by a ST-RUN source. Using the DB/WT wire (which is also a ST-RUN source) exactly mimics the factory set up. Again, a constant 12V+ source will work fine, it just opens up the possibility of draining your battery if you forget to turn the switch off.

As for your fix solving the CEL issue, I can only figure that either you were using the wrong DB/WT wire (which incidentally is "there and not" while the ignition is on) or the integrated diode in my relay is preventing the CEL. I'll have to try putting in a normal relay and see if it causes a CEL to find out for sure. To make sure you were using the correct DB/WT wire, perform a continuity test with your DVM between it and Pin A2 on Connector 1 in the PCM as detailed in the write up.

Originally Posted by Mudderoy View Post
I probably need to install another relay and let all the current be there under the hood. I was concerned about the wire running to my over head switch box becoming light bulb filament and glowing a bright white just prior to the headliner fire.
No need to worry about fireworks in your headliner, as I mentioned in my earlier post a standard Bosch style automotive relay only draws about 0.16amps. That plus power for the indicator light in your switch (if so equipped) is all the current that is running through that wire. Shoot, I'm only using 24g wire for my switch and that still provides a significant safety factor. Fortunately for you (and me), that whole circuit is already protected by the fuse protecting the factory fan relay. The 10amp fuse you have in there won't hurt anything, it just isn't going to do anything. You should still consider fusing the 12V+ side of your relay coil though.
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