Why did Porsche allow unfused circuits?
 

Hello all, hopefully my first foray into this forum goes well.

I am in the process of adding air conditioning and the Safeguard system into my '77 911 and just had an electrical problem when I finally got all the parts in and reconnected the battery.

In poring through the wiring diagrams, it seems like Porsche uses unfused circuits in many parts of the car (e.g. taking power off of the top of the fuse blocks) and I cannot for the life of me understand why they would do that.

I get the need to have switched/unswitched power, but why did they not put an appropriate fuse on every circuit or circuit groupings? They seem to have compensated a few spots by sprinkling single fuse holders here and there (e.g. the footwell fan blowers on the later cars).

Not being an electrical engineer, can someone enlighten me as to whether there are good and valid reasons why, electrically speaking, one should not set up all the electricals being fused.

My reason is that if there is no good reason, I will run an additional fuse block in the car (perhaps 8 fuses) and relocate all unfused circuits to that block. I suspect I will have to jumper it a bit to allow some of the fuses to be unswitched and some to be switched but that is a trivial issue.

Thanks

Douwe

Rijswijk, NL

A better question is "Why do they have all the circuits run from the battery, to the switches and then to the load instead of fusing it as close to the positive battery lead as possible?"

The whole of the electrical design is insane.

I think what happened is all the enthusiastic German engineers wanted to work on the exciting stuff: Body design, Engine design, Suspension design, eveb Transmission design.
There were only a couple low grade wannabe German engineers to work on the boring wiring harness, so to fill out their ranks they hired a few engineers that had been let go from Lucas...
... But that is just a guess. :p

Ain't THAT the truth...back arsed wiring....

Best,

Doyle

I am not totally familiar with the design convention Porsche or any automaker used, but I can't think of any reasons other than cost, and /or a high level of confidence in their design reliability, or just it was how they did it 'back then'.

Although, and this is a quick answer, some items (motors) draw a large amount of current (in rush), and then settle to a lower level. If you fuse to this operating level, you run the risk of blowing the sucker with the in-rush, but if you size to the higher in-rush, you might as well not have a fuse in there. Slo-blow fuses sometimes help, but have to be selected properly.

Me - I don't like un-switched power, and I think a few more fuses here and there could have been used.

if they made everything perfect, to last forever, they would never sell any parts. :0

I believe most vehicles typically don't fuse specific circuits (e.g. ignition, starting). Please correct if I'm wrong.

As for other unprotected circuits (footwell fan motors, etc.), I can't say other than oversight.

What other 911 circuits are unfused?

Sherwood

In early days, fuel pumps weren't fused. Dashboard lights don't seem to be.

Excellence was expected, but not always achieved.

My explorations into the electrical system of a BMW 2002 is too dated to recall, and I never messed with a Benz I used to own, but VW also has some poor electrical design. I suspect the Germans are just "not good with electricity" (or were, as late as the mid-1980s).

I think the theory is that small wattage lights (interior, ashtray, etc.) do not draw much wattage and act effectively as their own fuse. The light bulb can not draw more current than when it is in operation (full wattage) and it is already basically shorted to ground on the other end.

Mark

bad theory -- if the hot wire touches ground it will draw as much current as the wire can hold, melting the insulation and likely starting a fire

Think about what you just offered. Using your logic, there could be no wires coming off the battery. If the wire leading from the battery to the fuse touches ground it will draw...

There has to be a provision to get the battery current to the fuse along a length of wire, even if it has to run the length of the car back to the rear fuse blocks. In the case of interior lighting Porsche decided that the length of unfused wire was short enough to not require a fuse.

How many fires have there been from unfused circuits? I know of some in the headlight circuit and a few from the front blower (that was fixed by Porsche) and some close to the battery. Interior light wiring fires? Never heard of any.

Any wire can be pinched. Can a car maker cover ever instance of a pinched wire, probably not. The wiring harnesses in our cars are very good and Porsche used good components.

Mark

Germans and the Brits never were the best when it came to electrical circuits in their cars. Anyone that has owned or worked on any is well aware of this. And before someone bashes me for talking about foreigners.....I'm german.

If you guys think these P cars are bad....you should work on a few 60-70-80's vintage mercedes.

Really?

The europeans (incl germans) are known for their electrical gremlins across all makes and models. Even as late as the Mid 2000's they (european models) get poor marks for electrical performance.

Honda, Toyota and Nissan have never had these gremlins. Not even in the 70's and 80's. Yet VW, Porsche, BMW, MB, Jag, Land Rover... across the board had issues...

I think the europeans used poor quality components in their electrical systems, designed them poorly, and across all eurpoean nameplates was probably their biggest snafu affecting so many different cars to overcome spanning 30 years... One case where the asians had it well sorted in comparison... I def wouldnt say the electrical system was made of good components. perhaps you havent chased an electrical gremlin yet...

wires coming off the battery should have a fusible link on them as soon as possible

of course some wires are much more likely to be futzed with than others, like the wires behind the dash that routinely cause hell for 911 owners

As a owner of Porsches, Mercedes Benz, and MGs, I can tell you the European automotive industry was not to blame as much as the manufacturers such as Bosch and Lucas for component products that were not engineered for simplicity, ease of repair, or compatibility. In many cases, the car makers had to use what was available from their country by law.

The Japanese (and to some extent the American) car companies had the benefit of competition to drive quality products and state of the art design in the components they used. The Japanese source the best components, the Americans just make better components here in the USA, at least up until very recently.

Brad - I meant that Porsche used good components IN THE WIRING HARNESS (wire, insulation, connectors, and terminations). You took a stretch if you thought I meant that Porsche used good components in the entire electrical system; that is not what I wrote.

OK, thanks for the responses, and yes the general trend makes sense to me.

First thing is to make sure that any "hot" wires coming off of the battery get some sort of fuse early. Interesting as my 911 have about 5 wires coming off of the positive and near as I can tell, there are no fusible links other than what the fuse blocks contain. I puzzle again why they did just not run a single large wire to the fuse box (plus a big one to the starter) for unswitched power.

Anyway, I have had several near fires mainly caused by wiring failing and then shorting to ground....I melted my headlight switch and most of that wiring as the heater lever illumination light wire shorted....

So, about my idea of running an additional block....think it will be that simple? I figure it is easy to find the unfused wires (they are at the top of the power panel), determine if they need switched or unswitched power and then just put them on the new block....

D.

Good one... Reading from post #1 and on, I was going to post; "Could be worse, it could be British :eek:"

I dont disagree. I read your comment wrong. Thought you were saying that this wasnt a problem for Porsche... in fact whether it was a design problem or a parts problem, general electrical performance was a problem for all european manufacturers... funny how it took them so long to identify and fix it. even today if you mention "electrical gremins" and "modern automobiles" youd probablyt guess it to be British or German...

to the OP. I did a whole new 8 block fuse with ATE style fuses in the engine bay. I have EFI, MSD, and didnt want to splice all these switched and unwswitched lines into one... It wasnt too hard to do and really cleaned up that area. There are a few who have done it up front as well. I havent been quite that ambitious...

Indeed it is annoying but with the help of Gerry aka 86 911 Targa, I have retro fitted the more important hot leads:

1. Dash Lights at the Headlight Switch
2. Foot Blowers
3. Front Condensor Motor
4. Lo Beams, Hi Beams
5. Updated Summgler's Box relay to a fused one from our Host

Thanks again Gerry!

Much Thanks PAG for making my weekend :rolleyes:

i have, and almost had one. the power wire for the ashtray light was so old the casing was brittle and started flaking off. one day after starting the car the cabin filled with so much smoke i could not see out the window and the culprit was the ashtray lighter wire grounding out. a simple fuse would have prevented the smoke, and my almost heart attach, from happening.