924Board.org

[ideola]

I want to wire in an aux fuse block, one that will take modern spade fuses. I want the fuse block to be powered with ignition on (i.e. NOT a simple always-on feed from the battery. I will be using the fuse block to run all of the add'l items I've added to the 941 (PLX AFR controller + gauge, PLX boost controller + gauge, volt meter, water/meth injection kit, knocksense controller, etc.).

My research indicates the best way to do this is to run a relay before the aux fuse block. The relay would have: 12V+ input from the battery; 12V+ switched ignition; power feed to the aux fuse block; and ground. When ignition is switched on, the relay would close and send 12V+ to the fuse block.

My question is this: where is the cleanest and most direct place to tap into switched ignition? I know that there are a couple of wires coming out of the ignition switch harness, and I can trace what pin they go to on the main wiring harness, but I was hoping to find an empty receptacle to tap into. No luck so far, based on what my multi meter is telling me. I could use the key-on line that powers the radio circuit and rewire the radio as well, but just wondering if anyone else has done this, or if anyone has a better idea of where to tap for 12V+ switched ignition. Also, should this line be fused, and if so, how do I know how heavy the fuse should be?
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erstwhile owner of just about every 924 variant ever made

[Paul]

You need to tap into circuit 15 (hot with key on). Lots of places to do that. Just use a 3M tap and splice into any wire that's attached to a terminal labeled 15. Fuel pump relay, horn relay and several more. Or use a fuse tap on the hot side of fuse 8 in the main board (find hot side by removing the fuse and turning on the ignition)

Adding up the total load of your new panel will determine the size of the fuse, relay, and wire guage that you need to tap into circuit 30 (always hot).
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[!tom]

Also, put the fuse before the relay. That way you're protecting a lot more potential failures.

Make sure the entire circuit you are running to the new fuse panel has heavy enough wire and connections so that it is capable of blowing the main fuse that you install prior to overheating and melting anything too.
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78 924 NA
5-lug

[ideola]

Yeah, I knew about circuit 15, I was just trying to source it cleanly. I think what I may do is just take the radio circuit because it work with key-in (as opposed to the ACC position on the switch), which is what I want for all of these accessories. I'll have to re-wire the radio, but that's not too big of a deal, and I'll probably have to upgrade the fuse in the OEM fuse block, but seems like this will be the easiest method...in fact, the relay is already there!
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erstwhile owner of just about every 924 variant ever made

[musicalannette]

I dont have a book with me so i cant advise where to tap into, but the amount or current the relay you decide to use will be determined by the coil resistance in the relay (it far from isnt, but for simplicities sake see it as just a resistor).

The current in the wire can be worked out quite easiliy, and doesnt need to be that high, the coil resistance can vary, it is designed to create the magnetic field, some need 0.25 af an amp, some need as little as 0.04 of an amp. to make the contacts close that will carry the main power feed to your new fuse box.

Current (I) = Volts (V)/Resistance (R)

not easy to show on here, i know you no idiot, but just to clarify its current is equal to volts divided by resistance.

so current in wire feed to relay coil and new fuse you will pickis; (I) = Volts (V) and remember it might be upto around 14 when charging/Resistance of the relay coil (R).

Some coil resistances are up in the 600 ohm range, some are 50 ohms so it depends what you pick or can find at the right price. but the current draw and fuse will probably be less than 0.33 of an amp and as the current is more or less constant (assuming no coil heating effects etc)it shouldnt play havock with any thing else on the circuit. (I would advise not connecting it to the radio supply though if your going to have a long lead to the relay coil and/or ground as it might act as a pickup and introduce interferance to the radio power supply (inside the radio, the power supply circuitry inside can be crap at decoupling rf pickup, it should filter it out, sometimes it doest, best if its not there and it doesnt have to deal with it) because if its a long lead there you might be very unlucky have poorly designed power supply and hear background humm.


hope this helps a bit
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....

[ideola]

Good points on the relay sizing.

Here is the schematic I planned to copy, although I will double check the math on the distance and sizing of the wiring, and I don't plan to use a master ground block:


The connection from the original fuse block to the relay on pole 86 will be keyed ignition...
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erstwhile owner of just about every 924 variant ever made

[musicalannette]

ok, take your multimeter, set it to ohms and connect it between 86 and 85 and it will give you your coil resistance.

it looks as if is a fairly run of the mill automotive relay and i would think it will probably be around 50 - 150 ohms. you can then try and find a suitable fuse and wire.
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....

[musicalannette]

soory, that is assuming you want to add a fuse inline with the relay coil feed aswell as the the fuse inbetween the main contacts and the new terminal block.
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....