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Old February 23rd, 2010, 12:54
fyrfytr1717 fyrfytr1717 is offline
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Join Date: Jul 2008
Location: Turlock, CA
Posts: 1,675
Re: Radiator Fan Override Switch (The Easy Way)

Thanks for the compliment kastein. I always appreciate your feedback given your electrical background. All the wiring will get even neater once the rest of the relays are wired up. Everything will be in loom and the exposed part of the harness will get covered back up again. I'm just waiting to get everything in place before I seal it all up.

As for the diode wiring, thanks for the clarification. I am in fact using a relay with an integral diode and I did have an issue with wiring it up the first time around. I assume you were alluding to that as I believe you posted up what ended up being the fix in my "Did I fry my ECM?" thread last week. I don't think it's absolutely necessary to use a diode protected relay for this mod, but I figured it's probably a good idea seeing as the PCM could theoretically be damaged by the voltage spike when the coil's magnetic field collapses upon deactivation.

My write up makes the assumption that all automotive style relays with an integral diode have the cathode connected to pin #86. Maybe this is not the case? This is however why I made sure to point out that the wire coming from the PCM (-) needs to be connected to Pin 85 and the ST-RUN wire (+) needs to be connected to Pin 86. I didn't provide the explanation as to why, I just figured if folks followed my instructions they would get it right without even having to think about it. If there are alternate configurations of this type of relay, it would be important for the installer to make sure they get the polarity right when hooking up their wiring. Here's a pic of the relay I used (Hella HL87104) with the schematic printed on the side of it.

As I mentioned earlier in my write up, it's not necessary to use a SPDT throw relay (87 & 87a Pins), I just bought my relays that way to leave myself some extra flexibility in wiring options.
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