Same fuse keeps blowing - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Finnegan · 08-17-2018, 10:19 AM

Hi All,

The fuse for the radio has blown twice this month, and four times over the last year or so. Any ideas what's going on? The radio is aftermarket, installed by the PO. I've had the car for around 4 years, and this is relatively new behavior. 2004 base, if that makes any difference.

Thanks!

particlewave · 08-17-2018, 10:28 AM

What size are the blown fuses and which stereo?

Finnegan · 08-17-2018, 10:32 AM

Stock, it calls for a 15. I'll check the model of the stereo in a bit.

particlewave · 08-17-2018, 10:39 AM

15 amps will give you 180 watts. If the stereo's max power rating is more than 45 watts x 4 channels, you might try a 20 amp fuse.

.

MWS · 08-17-2018, 01:20 PM

The the unit was installed by a PO, it's possible they didn't do the best job. Try pulling out the unit and checking the connections on both the constant and switched power, additionally, if there is an aftermarket amp, check the switched power lead from the head unit to the amp for a bad connection. Also, check the wiring carefully from the head unit as I've seen some people just jam the unit in and wiring can be damaged on the metal sleeve (insert) that holds the unit.

Reason #654 why I hate PO mods...you never know the quality of the installation.

thstone · 08-17-2018, 04:26 PM

Quote:

Originally Posted by Finnegan

Any ideas what's going on? !

The unit is either pulling too much power for the circuit or there is a short.

Finnegan · 08-18-2018, 03:09 PM

The intermittent nature of the problem is what's puzzling me. It has gone years without blowing this fuse; then a couple times a year, and now a couple times in a month. I'll give a higher amp fuse a try next.

rexcramer · 08-18-2018, 03:45 PM

As thstone has pointed out, you are developing a short somewhere. The point of the fuse is to prevent an electrical fire. If you continue upping the size of the fuse, you get to a point that is referred to as a 'smoke test'. It will be obvious where the short is. It will be where the smoke and or fire is coming from. I suggest that there is a wire that has been chaffing against something that is getting progressively worse over time. Wires are usually bundled together and you risk melting adjacent wires with a larger fuse. Thus creating additional electrical problems. Start looking at wires coming through the firewall/bulkhead.

paulofto · 08-18-2018, 04:40 PM

Quote:

Originally Posted by rexcramer

As thstone has pointed out, you are developing a short somewhere. The point of the fuse is to prevent an electrical fire. If you continue upping the size of the fuse, you get to a point that is referred to as a 'smoke test'. It will be obvious where the short is. It will be where the smoke and or fire is coming from. I suggest that there is a wire that has been chaffing against something that is getting progressively worse over time. Wires are usually bundled together and you risk melting adjacent wires with a larger fuse. Thus creating additional electrical problems. Start looking at wires coming through the firewall/bulkhead.

+100 to what he said

Like putting a penny in an old 60amp 1920's home fuse box. Ahh most of you have never seen a home fuse box. I'm old.

particlewave · 08-18-2018, 05:40 PM

START by calculating and installing the appropriate fuse size for the stereo, as I suggested (if the 15 amp fuse wasn't already adequate). If that blows, then you have a problem.

There is no sense in searching for a short until you're sure you have one.

Quote:

Originally Posted by rexcramer

As thstone has pointed out, you are developing a short somewhere. The point of the fuse is to prevent an electrical fire. If you continue upping the size of the fuse, you get to a point that is referred to as a 'smoke test'. It will be obvious where the short is. It will be where the smoke and or fire is coming from. I suggest that there is a wire that has been chaffing against something that is getting progressively worse over time. Wires are usually bundled together and you risk melting adjacent wires with a larger fuse. Thus creating additional electrical problems. Start looking at wires coming through the firewall/bulkhead.

marsheng · 08-18-2018, 09:32 PM

See here

https://www.pecj.co.jp/en/fuse/outline/p3.html

An automotive fuse will blow if it is subjected to 15 amps continuously for a 100 hours. That is it rating. (Radios wont do that)

If you want it to blow in instantaneously (0.02 sec) , you need a current 600% higher than its rating.

There are may different fuses so the figures my not be exact but the principle is there.

Sometimes the inrush current when switching a device on can be many times higher than operational. However most amps these days have soft starts. If you are old enough, you will remember speakers giving out a huge thud when the amp was turned on. (My old hifi at home actually dipped the ceiling lights when it was turned on)

I'll put my money on short.

On constructive note, if the fuse wire is splattered all over the inside of the fuse holder, definitively a short. This may not always be true, if the wires from the battery are not that large, (as in my 986) the resistance in them may not allow the large current flow, so it will just blow it 'normally'

Starter986 · 08-19-2018, 03:58 AM

Quote:

Originally Posted by paulofto

+100 to what he said

Like putting a penny in an old 60amp 1920's home fuse box. Ahh most of you have never seen a home fuse box. I'm old.

I remember that box in our garage. Screw in fuses. Even as a young kid I remember thinking the setup looked frightening.

Oh... we're not that old.

Finnegan · 10-12-2018, 10:36 AM

Update: I used a 20a fuse last time, and it blew out yesterday. So approximately two months since the last one.

thstone · 10-12-2018, 11:04 AM

Nothing has changed. Its either pulling too much power or you have an intermittent short.

Most likely, your wiring has developed an intermittent short over the years.

marsheng · 10-13-2018, 01:14 PM

Quote:

Originally Posted by thstone

Most likely, your wiring has developed an intermittent short over the years.

Totally agree. By upping the fuse, you are allowing a higher current to flow through the wires that have been designed for and potentially you are making for the possibility of starting a fire or making smoke signals. I know its hard to find a short, but it is what is needed. I have found the wiring not the best in the Boxster with no real covering in the wiring looms. Any vibration/rubbing against the chassis goes through the current carrying wires and not the covering sheath.

Cheers Wallace

jcb986 · 10-13-2018, 01:35 PM

It’s a short which is causing a fault. If you have the original radio, you could swap them out. Then wait and see if it blows a fuse. Of course, if it doesn’t your radio is the problem.

Finnegan · 10-15-2018, 09:36 AM

I do not have the original radio, so I can't test it that way.

Can someone tell me if that fuse is powering anything aside from the radio?

Finnegan · 10-22-2018, 09:41 AM

I spoke with the indie shop I go to for routine things. They said they could 'try' to hunt down this fault.. If this were your car, would you go to

Usual indie shop savvy about Porsches
Auto electric service shop, not necessarily savvy about Porsches
Reputable car stereo installer

particlewave · 10-22-2018, 10:14 AM

Electric service shop. There's nothing special about these cars and an electrics specialist will be able to chase it down easier/faster.

Finnegan · 10-31-2018, 11:55 AM

Update:

I took it to a very good auto electric shop. The guy says that the fuse itself is not blown, and can't find anything amiss in the wiring back there. He suggests the problem lies with the radio itself. Maybe something has come undone in the radio housing and is intermittently shorting?

Any ideas?