87 Relays almost never fry. I have submerged, frozen, and melted them and they still work.

You run a 30 amp fuse from the batt or 12 volt power 10 gauge wire to the relay.. 30 amp to the 30 post. Works nice.

You run the 5/10 amp from the thermostat switch to the 86(signal)
You run a 2nd 5/10 from the over ride switch to the relay 86(signal) also

87 is your 12 volt output to the fan

85 is your 10 gauge ground

All the 86 is doing is connecting the 30 and 87 terminals so 86 can be a lighter gauge wire/fuse.

I like to run headlights and fan relays directly off a dedicated battery post using ring terminals and a wingnut on the battery post. It saves wiring and keeps things clean. I have the fuses inside weather proof holders. Located close to the relays yet out of sight like tucked into a inner fender hole behind a piece of trim or on the side of the fan cowl up high.

 

The 5A fuse is likely for the trigger side of the relay. 30A fuse would go on the high side of the relay.

I always like to install fuses as close to the source (battery) as possible. That way you have less chance of the wire accidentally grounding between the battery and fuse. When I do a custom wiring harness, I will try to use an under-hood fuse box so the power doesn't have to go far before being fused.

 

So, I put the 30a fuse inline before the #30 pin on the relay? BTW, the battery will be in the trunk and the relays are under the hood so fuse placement won't be anywhere near the battery.

 

Yes. The relay is only going to be on when the 86 trigger is on. So placing a fuse before the relay makes sense because the circuit is only completed when that trigger is on. This also allows for relays to be placed closer to the power sucker and the fuses to be placed in a group further towards your power post..

When you have a rear mounted battery you want to have a power(and ground) post under the hood.

You run your alternatiors sensing (S) wire to this post so that when you say turn on your headlights that alternatior will "sense" the drop and increase voltage faster then if it would if it had to go all the way back to the battery.

Ok now the point of relays is to reduce voltage drops through wiring. So you want to place them as close to the power suckers az possible..

I would run a 0000 off the battery up to the starter. Then have 4 from the starter up my firewall to my power post. Then 4 going to the alternatior and another 4 going to a mini 8 fuse pannel by the back of the headlight. These 8 fuses would then feed my front clip relays.
The power post would then have a 4 gauge going to a interior 16 or so fuse panel to power the rest of the car.
The alternatior sense wire would go to that power post.

I would also spend the coin to run a 0000 negative wire from the battery to the block then have 4 gauge running off of that to ground everything properly.

In the trunk I would have a 4 gauge going to a 8 fuse panel powering relays for my tail,turn,backup lights, fuel pump, and future stuff.


If say your fuel pump fuse goes you walk back to the trunk and check easily. If your fan fuse goes out you throw up the hood. If your radio goes out you look in the glovebox/center council.

 

Very good explanation! Thank you for that! The car in question is a '54 Customline with SBF. The only deviation I'll have is I'll be running my 1/0 welding cable (POS batt cable) from the trunk to a power post on the inner fenderwell. From there probably a 4ga cable to the starter solenoid on the inner fenderwell. Also, from the power post will be 10ga wires to power the fan and horn relays plus a couple of others. I'll fuse each power wire before each relay. The relay box is right next to the solenoid.

 

I run a 1 wire alternator . 30 amp fuse hooked to alternator feeds a relay about a foot from the alternator / 10 ga. Wire . Relay is mounted to the fan shroud & feeds the fan . Short wires , unobtrusive , has worked for 20 years & 50 k miles..

 

This might be a better match to your harness. Painless Performance Products 30103 Fan-Thom II Electric Fan Relay Kit W/Thermos | eBay

 

Thanks Jeff! I think I've got it all figured out now. I've decided to keep the main wiring harness and control harness separate from each other. The main harness will handle lights/gauges/accessories and the control harness will handle all engine electronics/fuel pump/fan/etc.