Amplifier Power Cable Through The Firewall?

citroDane · Post by **citroDane** » 15 Jul 2008, 15:18

Hi everyone,
Just joined the club/forum. You're doing a great job ‘cause it seems to be very active!

I've had a 99 Xantia 2.0 HDI for four years and feel pretty reasonably treated by it. Got a couple maintenance questions I'll try to post soon. For now I'm installing an amplifier in the trunk and I'm wondering how to get the power cable through the firewall to get it to the battery. I looked under the factory carpet under the dashboard and didn't see any visible holes or wires going through (of course there's that rubber insulator between the carpet and the car floor/firewall). So before I spend a lot of time taking things out to look for a place to drill a hole, has anyone tried this and got a suggestion or any idea where there is a suitable hole to use for this?

Much appreciated,
Gabriel

AndersDK · Post by **AndersDK** » 15 Jul 2008, 16:19

Hi Gabriel -

A few words of advice :

1) remember that the high power running out to the amplifier in the positive cable from the battery plus terminal, have to return to the negative battery terminal. Thus you need a similar thick cable for the negative side, as the chassis body and existing earth points wont be a good quality return for such high currents.

2) using extra cables directly from the battery is ALWAYS imposing an extra fire hazard in your car. The battery voltage will not just drop by a shortcircuit. Instead the battery will dump all available current into the shortcircuited cable, instantly makng it white glowng hot and set off a fire.

3) you must then use a safety power fuse at the battery plus terminal. Low-loss high power fuses and fuseholders exactly for car-HiFi installations are available for this. Your MOT mechanic will love to see this fuse installed close to the battery plus terminal

4) anywhere the power cabling is feeded through chassis holes you MUST protect the cables with rubber grommets shielding off the sharp steelplate edges from the cables. Even a sharp chassis bend must be padded against the cables.

To look for available holes in the firewall - its best done loking from the engine bay.
But remember : a pair of bulky cables fiddled through a hole already crammed wih other cables - will most likey set off other problems. There is a danger that the existing cabling MAY be disturbed, damaged or disrupted.

I'd do a new hole in the pasenger side - where you dont have that much stress from the divers feets working on the pedalbox.

Good luck 8)

mpr1956 · Post by **mpr1956** » 15 Jul 2008, 17:08

Hi Gabriel.. I managed eventually to get a 30 amp power feed through the rubber sleeve where the bonnet release cable passes through the bulkhead. A straightened wire coat hangar helped greatly, pushing the cable through. It would also help if you have the body of a teenage russian gymnast -as vision and access from the drivers footwell (rhd) is quite difficult. I was convinced I would never walk upright again after finishing !! Good luck and best wishes, Martin

deian · Post by **deian** » 15 Jul 2008, 18:12

Remember not to get the power cable too close to a signal cable, they will affect each other... what i mean by signal is something sensitive that goes to/from the ecu. To overcome this you could shield the power cable. Or if you can't find an existing hole drill one in a good place for access but away from signals.

ICE installation Tip: get the power cables running along one side of the car, and the signal on the other (if your amp will be in the boot that is). Better like that to get a clean signal to the amp.

deian · Post by **deian** » 15 Jul 2008, 18:14

mpr1956 wrote:It would also help if you have the body of a teenage russian gymnast

Ronnie Wood may be able to help there, his new bird may have some fit gymnasts to help you.

citroDane · Post by **citroDane** » 16 Jul 2008, 11:01

Hi Anders, Martin, Dei,

Thanks a lot for your responses fellows. Ironically enough my computer got hit by a virus after I posted my question yesterday and I was fighting till now to get it fixed. It's my work computer so I was dependant on the IT guys to bring it back to life.

Anyway I just logged in and was dang happy to see responses so fast. Thanks a million again.

Well, you taught me a few things already:
1) Good idea to shield the power cable
2) Ought to run it on the opposite side of the car to the amp signal cables or somehow get it away from those
3) Not a good idea to use a hole already crammed with other car wires
4) The hood release cable hole is a good candidate to get through the firewall
5) I first need to practice gymnastics to do this myself

Some new questions:

1) Anders, a few years ago I bought a used bmw which had a hifi system installed already. That guy had the amp – cable (40 cm long) directly bolted to the body and the signal didn’t seem to suffer. What do you think, was that luck or maybe the amp was low power and not spitting out too much current?
2) If I drill a “homemade” hole in the firewall, won’t I risk having rain/splash water getting through it?
3) Dei, where is the hood release cable hole exactly? Admittedly I haven’t looked for it myself yet.

Cheers,
Gabriel

AndersDK · Post by **AndersDK** » 16 Jul 2008, 12:18

citroDane wrote: Some new questions:

1) Anders, a few years ago I bought a used bmw which had a hifi system installed already. That guy had the amp – cable (40 cm long) directly bolted to the body and the signal didn’t seem to suffer. What do you think, was that luck or maybe the amp was low power and not spitting out too much current?
2) If I drill a “homemade” hole in the firewall, won’t I risk having rain/splash water getting through it?

1) depends on the power consumption really. If you install a standard system - say 4 x 40watts then no problem.
If you install a medium power system say 4 x 250watts then it will likely raise problems with feed back loops and noise signals, because of stray current paths in the chassis panels.
If you install a thunder system say 4x1000watts, you must be very stringent in your cabling to avoid feedback loops and noise.
Feedback loops causes self oscillation in the power amplifier units, which will howl and scream away your speakers (and any living creatures nearby ).
May also cause "motorboating" - i.e. very low frequency oscillaton - which will burn out the amplifier units.
The reason why you will use the chassis earth as the negative feed is because ithat is the standard approach for the cars standard consumers fitted.
These consumers wont take more than approx 20Amps and will not suffer from any minor power losses or noise signals in the chassisbody panels.
But a stereo will instantly suffer as any imperfecion is amplified immensly and then reaches the speakers as quite loud indicators.
Some of the worst problem makers are the alternator, blinkers relay, wash/wipe motors etc. as all of these devices uses the chassisbody for the negative earth.
For the same reasons, you NEVER use the chassisbody earth for speaker returns or any signal earths.

2) thats always a risc. But thats one more reason to use a proper rubber grommet. You can get suitable rubber grommets at your ICE (in-car-electronics) dealer.
A good blob of any standard silicone bathtub sealer is a good idea to seal off against water and engine fumes/smells.

reblack68 · Post by **reblack68** » 16 Jul 2008, 17:13

There's an easier way, which I used to fit the 12S socket on my Xantia.

If your car doesn't have heated seats there is a vacant 40A fuse position in the fusebox. You can connect your power cable there via a male spade terminal and protect it with an inline fuse.

Drilling through the bulkhead is risky. A member on here gave himself a whole lot of trouble because he nicked some wires behind the dash when he was drilling.

citroDane · Post by **citroDane** » 16 Jul 2008, 17:57

Hi fellas,

Anders, ok. You convinced me to go buy a (-) cable!

Thanks for all the info…very insightful.

I must say on the question of putting a hole through the firewall I’m scared to do damage to something as you pointed out Richard. Plus there’s that risk of taking in water or for the edge of the hole to start rusting up!

I was thinking of starting to look for the hood release cable hole as Martin suggested. But it didn’t sound like I could get two cables, (+) and (-), through it!? So I’m still a bit stuck it seems. Or am I?

Richard, I don’t have heated seats, so that’s great. Q: is the empty slot in the fuse box already connected to the car
circuitry (i.e. to the battery plus) on one side or it just an empty unused slot that needs wiring on both sides?

Cheers,
Gabriel

reblack68 · Post by **reblack68** » 16 Jul 2008, 18:15

You will only get the live feed from the fusebox, if you're running a negative wire you'll have to take that through the bulkhead.

Look at the fusebox section of your handbook to find out which empty fuse positions are rated at 40A. One terminal of this will be live.

This advice came from a caravanning forum. Others may have an opinion on whether it's suitable for an audio application.

Stempy · Post by **Stempy** » 16 Jul 2008, 20:50

You could always use a separate battery sited next to the amp, then all you'd need to do is run a much smaller charge cable to keep it topped up and your high current power cables would be kept very short and safe. You might find a motorcycle battery that would be up to the job, it'd be smaller than a full size car one.

jeremy · Post by **jeremy** » 16 Jul 2008, 20:54

Why not run your power feed underneath the car in trunking?

http://www.vehicle-wiring-products.eu/V ... mepage.php

http://www.autoelectricsupplies.co.uk/

http://www.maplin.co.uk/

citroDane · Post by **citroDane** » 16 Jul 2008, 23:51

Richard,
I was looking at the fuse box and didn’t see any empty slot in the section with the high amp ones. Just two rows with 40 and 80 amp fuses.
Anders was talking about low-loss high power fuses and fuseholders for car-HiFi installations but I think this is definitely worth trying before buying-installing the HiFi specific product.

Stempy,
Dunno much about motorcycle batteries…have you tried this solution yourself?

Jeremy,
Sounds like an interesting approach. I think I will have the same problem getting the cables out of the trunk and to the bottom of the car though (as with getting them through the firewall). Or is there a hole on the bottom of the trunk?

Cheers,
Gabriel

reblack68 · Post by **reblack68** » 17 Jul 2008, 06:25

On my car the space for the heated seat fuse was with the normal sized fuses in the main fusebox. It may be different in your market.

If you decide to run the cable under the car you will probably have to drill into the boot but access is a lot easier and safer than trying to go through the bulkhead. You might be lucky and find a rubber bung in a suitable place, I think there are some in the Xantia.

citroDane · Post by **citroDane** » 18 Jul 2008, 16:24

Hi Richard,
No luck finding that empty slot. But it’s ok those “small” fuse boxes made for car HiFi Installations aren’t too big a problem to install in the engine bay.

I think I’ll go with drilling a hole in the trunk some place and then run the cables under the car…it does sound like the least risky / demanding solution.

I’ll be back with updates on this once done.

Cheers,
Gabriel