controller for cooling fan, relay wiring

I saw this thread here. And I read the information to the left on relays. I am just not quite smart enough to figure out my answers. Sorry for posting something so simple but I need some help. I decided to use a controller (Derale Part #16749) to controll my electric fans on my 1995 Lexus SC300. The controller uses a coolant temperature probe and has some kind of electronic board with a replaceable relay attached to it. the wiring diagram it comes with looks like this: My goal here is to control one fan with the controller and use a relay to activate another fan at the same time. I can think of two ways to do this. I am sure there are probably other ways too. A B Which is the best way? Is there a better way I am unaware of? Moving on, my controller requires a switched +12v ign source(which I can get from the ignitor), and an a/c clutch trigger wire. I can think of a few ways to do this also. A1 B1 C1 Which would cause the least impact on the factory system? Sorry again for posting something so simple. I really just cant figure out the proper way to utilize a relay.

Sorry Silver, I looked for Derale 16749 wiring info and found so much bullsh ... unbelievable! I even found one diagram using TWO relays where the coils were wired in series (hence needing 6V coils or a 24V supply!), and the second relay did nothing anyhow - you could bypass it entirely. And some of your circuits don't makes sense... some will be on all the time.... All you have to do is wire the unit as per its wiring diagram. Its relay is rated for 30A. If the total of both fans is under 30A, you can use the relay for both. If both fans are over 30A, you need another relay or two. If one fan is under 30A but the total exceeds 30A, you also need another relay. In all cases, I suggest you use the original relay. If you need extra relay(s), the original 30A relay is used to turn them on (eg, 30A relay's #87 to new relay's #86 & its #85 to ground). You will have to decide if you want one relay for both fans, or one relay each. I recommend one relay per fan so that if one relay fails, you still have one fan (ie, the other). It also means you can easily modify for 1 or 2 fan operation later. The relay contacts must be rated for the fan currents (plus a bit extra). Another consideration is that the power wire to fans should be fused at/near the battery - ie, battery +12V thru fuse to relay #30 and out via #87 to fan +12V. (The fuses integrated with those relays really only protect the relay and downstream wiring; they do not protect the power source (battery/alternator) to the relay. They are great in distribution or fan-out applications - eg, a master fusible link of (say) 80A at the battery going to 2 or 4 30A or 20A or 15A "fused relays" for headlights.) And you need the wire gauge to suit. So wire it up as per Derale instructions with diode connection from the AC etc. Whether you have the Derale relay power 2 relays, or one relay, or a fan & relay, or 2 fans depends on the fan current draw and your desires. And match the Derale relay's fuse to suit. FYI - I saw cute wiring whereby the fan's +12V went to a SPDT relay's #30 & #87 (normally open) to +12V as expected, but #87a going to ground. I thought this had the potential of minimising alleged voltage spikes (from the fans), but I figured it would cause regenerative braking - ie, brake the fan, and prevent its "freewheeling" rotation at higher vehicle speeds. That could reduce airflow etc. (And some here would be seriously concerned over the extra drag that would slow the car down.) I'm happy with my SPST relay. But my fan is only about 8 Amps.

"I recommend one relay per fan so that if one relay fails, you still have one fan (ie, the other). It also means you can easily modify for 1 or 2 fan operation later." Ya I want to wire it so that one fan or fan relay could fail and the other would still run. I am using two Perma-Cool #19008 fans that draw ~7.5 amps. would this accomplish what I want? also I dont understand what this means "with diode connection from the AC etc. " I know alternators have diodes but that is the extent of my knowledge. Also I am interested in that last thing you mentioned. Could two fans that draw ~7.5amps cause a significant spike? & Thank you so much for the help!

Still wrong - you have connected the relay solenoids in series. (And you should have a fuse at the battery +12 unless the relays are close to the battery - hence the battery to relay wire is/are physically secure with negligible chance of shorting etc.)    Since they are only 7.5A hence hence 15A total have under the 30A relay capacity and wiring, just connect both fans to the 30A relay. You can modify later if you need to. There are stand-alone spade terminal that fit on a male terminal (relay etc) and provide two male terminals so that separate wires can be conveniently connected. The diode(s) should be shown as part of the Derale kit's wiring. Diodes are often used to connect multiple inputs and prevent an input from powering or triggering another. Spike protection is an issue in itself since the fans probably generate voltage when in free-spin (and probably negative).... Normally it would be done in the supply side - not on the fan side as is the case with relay-coil spike quenching. I'll be off-air for a few days.... Good luck.

Ive been really busy so I havent tried the install yet but I did come up with a new diagram for wiring it: will that work? will it allow one fan/relay to work if one fan/relay fails?

Only if the controller can drive BOTH relays. If not, the controller might blow. And you may as well connect your #86s to their #87s. That way the #86 is also fused.