Engine bay switched 12v power - 986 Forum - The Community for Porsche Boxster & Cayman Owners

jaykay · 06-17-2021, 10:24 AM

Has anybody identified a good source/location for switched 12v power in the engine bay? I am talking right in there to power a sensor

Stl-986 · 06-17-2021, 06:18 PM

well, the AC compressor could be considered switched...in a way.

believe everything in that harness is going to be dme controlled so I wouldn't advise using anything there. better to just run a new wire through the same grommet the main loom uses.

jaykay · 06-17-2021, 06:56 PM

Yes, it is a bit iffy in there. I was trying to completely avoid poking a hole through the firewall grommet. It looks to be a hassle to replace if one chops it up too much....if it still even available.

O2 sensors??

911monty · 06-18-2021, 11:36 AM

Alternator?

Stl-986 · 06-18-2021, 03:43 PM

They make tools for running a wire through a rubber grommet. Unless you really hack at it, you cant do much damage.

Besides the tool there is always a metal clothes hanger, tape wire to 1 end and pull through grommet. it's rubber after all

ike84 · 06-21-2021, 05:49 PM

You can tap into the phone connector in the front center dash, run under middle console, and through the shifter cable grommet. It will take less than an hour 30 min if you've removed the console before.

Sent from my POCOPHONE F1 using Tapatalk

jaykay · 07-10-2021, 06:59 PM

Thank you, yes I am aware of that plug. Its very handy but I think the circuit is only rated at 7.5 amps. I am no electrician but I would think connecting up two 5amp loads would give trouble on that circuit.

Inside the cockpit, I am considering pulling power off of a rarely use 30 amp circuit and then have individual 5 amp fuses for each load .....and others.

I was eyeing the defroster fuse which is 30 mps and should provide switched 12v to power a number of different loads. I believe one would stick a tester on to one fuse terminal with ignition on and that will do it. Then plug in a fuse tap and run that to your terminal block to power all of your stuff. I am not sure what would be a good available ground for the terminal block near the fuse box.

I never use the defroster but I guess there could be trouble if one accidentally hit the defrost button. Is there a better 30, 35 amp circuit out the fuse box that would be better?

The are unused slots but I doubt there is power there. I really need to get a good tester.

pilot4fn · 07-10-2021, 09:15 PM

Quote:

Originally Posted by jaykay

Thank you, yes I am aware of that plug. Its very handy but I think the circuit is only rated at 7.5 amps. I am no electrician but I would think connecting up two 5amp loads would give trouble on that circuit.

Inside the cockpit, I am considering pulling power off of a rarely use 30 amp circuit and then have individual 5 amp fuses for each load .....and others.

I was eyeing the defroster fuse which is 30 mps and should provide switched 12v to power a number of different loads. I believe one would stick a tester on to one fuse terminal with ignition on and that will do it. Then plug in a fuse tap and run that to your terminal block to power all of your stuff. I am not sure what would be a good available ground for the terminal block near the fuse box.

I never use the defroster but I guess there could be trouble if one accidentally hit the defrost button. Is there a better 30, 35 amp circuit out the fuse box that would be better?

The are unused slots but I doubt there is power there. I really need to get a good tester.

Sounds like you need quite a lot of power there.
Why not use a relay between. Take a the switched power from some easy spot to wake up the relay and the actual power to your 5 + 5 Amps directly from battery thru that relay?

Stl-986 · 07-11-2021, 12:07 AM

Couple things.
1st - Recommend getting the FSM as it will have the wiring diagram
Next - What exactly are you trying to do? Always helps to know what is trying to be accomplished and having the full picture.

jaykay · 07-16-2021, 07:09 PM

Yes, at present I am separated from a FSM (full service manual) access, but I do recall difficulty in using it to determine which leads are switched 12v. To an electrical neanderthal like myself this is pretty much a needle in a haystack but I will give it a go.

#1 power a number of loads in the cockpit via a terminal block. IIRC the block can handle 35 amps coming in:
-shift light;
-l and r fan switch latching relays;
-engine compartment fan latching relay;
-ipad charge;
-foot well lighting;
-machine guns;
-camera;
-phone charge;
-secret stuff;
-engine gauges.

I would think the 7.5 amp phone connector powering the terminal block would not work. The fuse would certainly pop as the draw from two of the above loads would kill it. Hence, the original question: how to best safely tap a 30 amp circuit from the footwell fuse box.....phew. And ground it....properly.

#2 Power an engine gauge sensor module to be located on the trunk firewall with ignition switched 12V power. The load is fused at 5 amps. Sensor wires will be routed through the trunk firewall somewhere. I think the engine bay itself will be too harsh for it.....

jaykay · 07-16-2021, 07:11 PM

Quote:

Originally Posted by pilot4fn

Sounds like you need quite a lot of power there.
Why not use a relay between. Take a the switched power from some easy spot to wake up the relay and the actual power to your 5 + 5 Amps directly from battery thru that relay?

Well I have a number of loads. Which easy spot do you recommend?

pilot4fn · 07-16-2021, 09:34 PM

Quote:

Originally Posted by jaykay

Well I have a number of loads. Which easy spot do you recommend?

This is based on not knowledge but trial and error from one of my first project cars something like 30-years ago... and I'm defenately not electrician

Here's the idea: Take the swiched power from a 'easy location' for your application that provide only the wake up current to the relay (could be done for example by wire jumper or cutting the original wire and soldering new wire and both old cable ends together) - this shoud be very minimal current for wake up the relay on your application, since the relay should be similar size as there already is in the car.

Stl-986 · 07-17-2021, 09:46 AM

Need to really know what it is you are trying to do & install. This is something usually there isnt a generic answer as it depends on the loads of each thing, it's purpose, etc. For example you could tie into the radio fuse, but if the radio & whatever you are going to hook up draw more power then the fuse, you will blow the fuse at a minimum.

A generic answer would be to wire up to the power distribution panel in the driver kick panel, install correct size fuses for each item and relays as necessary.

What is common to do is to run relays directly off the battery and then use the ignition to "turn on" the relay. Do this all the time with Jeep's and aftermarket items so they aren't on all the time but only when the ignition is on.

If all this is unfamiliar I would look through wiring relays at the12volt.com.

As far as the original question, there isn't anything in the engine compartment I would connect to for wiring anything without knowing what it is, it's load, how many "things", purpose, etc.

Stl-986 · 07-17-2021, 09:55 AM

Quote:

Originally Posted by jaykay

Yes, at present I am separated from a FSM (full service manual) access, but I do recall difficulty in using it to determine which leads are switched 12v. To an electrical neanderthal like myself this is pretty much a needle in a haystack but I will give it a go.

#1 power a number of loads in the cockpit via a terminal block. IIRC the block can handle 35 amps coming in:
-shift light;
-l and r fan switch latching relays; Press the AC button, it will turn on the fans. Also finding a 30 amp latching relay is very difficult and not cheap when you do find one.
-engine compartment fan latching relay; Why? DME is going to turn this on when it wants to
-ipad charge; get this and wire to radio switched/ground https://www.amazon.com/gp/product/B08K271H6W
-foot well lighting; Use LED's and wire into existing wires
-machine guns; Thinking a 100amp will be the minimum here, more if you want an ejection seat
-camera;
-phone charge; Plug into the same above. also another one you could hide somewhere https://www.amazon.com/gp/product/B072JC1DQF
-secret stuff; Can't help with this since it is a secret
-engine gauges. These usually dont require power unless they are lighted, which would just wire into existing interior lights such as at the radio.

I would think the 7.5 amp phone connector powering the terminal block would not work. The fuse would certainly pop as the draw from two of the above loads would kill it. Hence, the original question: how to best safely tap a 30 amp circuit from the footwell fuse box.....phew. And ground it....properly.

#2 Power an engine gauge sensor module to be located on the trunk firewall with ignition switched 12V power. The load is fused at 5 amps. Sensor wires will be routed through the trunk firewall somewhere. I think the engine bay itself will be too harsh for it.....

Just saw this...see above. None of these are things I would wire to a single thing, they are all individual items that each have to be looked at by themselves, not together.

jaykay · 07-17-2021, 11:17 AM

okay what about just #2?

Stl-986 · 07-17-2021, 12:22 PM

what gauge? Again, most gauges dont require power other then a backlight which just get wired to your dash lights.

What sensor?

Really have to be specific on this. every sensor is going to have it's own requirements which you have to find out what they are.

jaykay · 07-17-2021, 01:03 PM

just a sensor module itself: 12V power, 5 amps.

placed in the trunk. So picture one box that can work on 9 to 25V and will draw 5 amps.

Dont worry about the gauges. Where would there be a suitable power connection.

Stl-986 · 07-17-2021, 02:28 PM

run a wire to the battery, connect it to a relay and then trigger the relay by ignition. Put a 5amp fuse in line from the battery. Output from the relay goes to your sensor and then connect sensor & relay to ground.

Without knowing what exactly it is I wont recommend anything else cause the risk of causing other issues can be too much.

jaykay · 07-17-2021, 05:10 PM

https://drive.google.com/file/d/1S79VSwt4LsVELBgBW-aCW3dJ9PFssLkT/view

here is an example of the module(s). I am not sure how it would help. The power requirements are as I have stated.

running a wire from the battery would be out of the question. we need a source in the trunk or engine bay

Stl-986 · 07-17-2021, 06:11 PM

Actually it does.

Why not install that in the front trunk or in the car such as under the pass seat?

If you are dead set on trunk then you will need to get the wiring diagram and see what you could tap into in the relays behind the driver side wheel.

Only thing in the engine bay is going to be the alternator but I dont recommend that.