Classic Retrofit origins: How we developed our fuse panels

Older Porsche electrics are far from infallible, and perhaps the weakest link in the chain is the original fuse panel. This uses older fuses with a huge potential for corrosion; the fuses were originally ceramic, but are nowadays made of plastic and used riveted contacts that deteriorate and introduce resistance into the system. Not the route to success!

When we first sat down to understand how we might re-engineer the original Porsche fuse panel, we applied the following criteria:
  • Plug and play: screw terminals as per original, simple installation, no wiring mods required
  • LED diagnostics for blown fuses
  • Integrated headlamp relays
  • 21 standard fuse ways
  • 3 spare fuse ways for additional circuits
  • An affordable target price to make the upgrade a complete no-brainer

Early research centred on parts sourcing. The first big problem was that there were no suppliers of what they call 'boxed' screw terminals in Europe: a connector that was similar to the original screw terminal that could offer a boxed design with a 30A rating and a PCB mount.

What we found in abundance were "barrier strips" - large plastic connectors with screw terminals. The trouble with these is that they are too large to fit in the original fuse panel. We ordered a number of samples and this mockup shows how they dwarfed the original fuse holders:


Ignoring the jumbo connectors, the picture does show the intention of how the blade fuse holder and diagnostic LED would eventually sit and also shows how an automotive relay fitted nicely in the space. Eventually, we found an American supplier of the right kind of terminal and ordered some samples.

Why we did not use crimp terminals

One early question from a fellow 911 owner was why we were not making the boards work with spade terminals and crimp connectors. This was due to the potential to add failures rather than prevent them.

A crimped installation would involve at least 42 spades on 42 wires. In many cases, you have to put more than one wire into one crimp which they are not designed for. The wires are all different gauges and if you have more than one of the thicker wires they won't even fit. You then have to work out how to replicate the hidden bus bars (which are on the back of the Porsche fuse blocks) by making up wiring bridges using yet more crimps.

Every time you add a crimp, you're introducing a potential failure point. Considering that the average user would probably buy their crimp tool in Halfords/Walmart, this seemed like asking for trouble. Ask yourself how many times you have managed to pull the wire out of a crimp connector having thought it was a good connection!

So we stayed with the screw terminal idea and tracking the bus bars on a circuit board that would be a direct replacement for the Porsche original. The circuit board could also integrate headlamp relays and diagnostic LEDs for fuse failure. In other words a plug and play integrated product, rather than a collection of parts. There would be no need for any additional wiring or modification to the existing loom. Making the upgrade doable in under an hour without having to tag any wires was a design goal.

Eventually our screw terminals arrived from the US - they were 30A rated and the hole was a similar size to the originals. They were very good quality and remain the terminals we use today.

Here's another early mock up: pretty much the same size as the original unit. Clearly, the blade fuses and holders are much taller, but the original fuse box lid still fitted with about 1mm to spare!

The switch to blade fuses

Several factors influenced the switch to blade fuses. One of the main factors was that it is almost impossible to find actual ceramic fuses today: the fuses are nearly always plastic. If the brass contact is not perfectly clean and making a good contact, the fuse heats up and the plastic melts. Over time, the fuse shrinks and goes loose in the holder. Last time this happened, I couldn't close the sunroof as I went on to the motorway, just as it started to rain. Others have been left stranded when the fuel pump fuse melted. So the readily available blade fuses were a no brainer. We chose the standard sized fuses, as they are easier to remove and insert than fiddly mini-fuses.

Having found our terminals and fuse blocks, the next job was to design the printed circuit board that the fuse holders, terminals and LEDs etc would be soldered to, with tracks providing connection between components. We had also decided to add automotive relays to power the headlamps - something Porsche did not do in period, but which is a serious safety concern nowadays. We used Panasonic relays, well overspecced at 40A and rated reliable to more than one million switching cycles.

In researching original fuse panel design by buying lots of old fuse panels, we found that some cars had additional terminals on the buses between the fuses, presumably as they had more equipment (AC etc) fitted at the factory

Porsche also externally linked some of the fuses with copper bridges, generally between the left two fuse blocks. As the bridging was different from car to car, we did not add these links to our boards but added suitable links in the kit.

Unlike the Porsche fuse blocks, our circuit board bridging is clearly marked, to include:

  • Always Live - straight from battery
  • Switched - ignition
  • Running - ignition + start

These groups have to feed multiple fuse ways, so Porsche connected some of them together on the back of the fuse blocks to save having to do wire links. Rather than redesigning the original blocks, they continued to add external brass blocks over time. We decided not to do this and made all of our connections on the board.

One thing we were struck by was the factory's method of putting all switches 'before' the fuse. This is poor electrical design as, were a switch to short to ground internally, the fuse would not blow to prevent disaster. Our advice: always have a fire extinguisher in the car.

We made four test boards and three guinea pigs were given the boards to try the installation process and put some miles on the parts. The testing went very well, so we finalised the board design and commissioned the first manufacturing run. Although PCBs can be made in most colours, we decided to stick with classy black with white silk screen printing. Our first test boards were a little taller than the original, but the fuse panel cover still fitted perfectly with more than enough room for the wiring.

Upgraded fuse panels made in the UK

Our fuse panels are made in the UK, not in China. One benefit of this production decision was revealed early on, when there were technical issues to go through with the screw terminals. A lot of heat is required to wave solder the terminals to the board so the machine needs a special setup. This was soon resolved. "It always amazes me how many things need to be considered to get things right," said Jonny. "I do think we are good at this stuff in the UK and I wonder if a Far Eastern manufacturer would have picked this up."  

We have now manufactured hundreds of these panels for many happy customers. Even if you don't think your car has electrical issues, they are cheap peace of mind, but the reality is that all 911s suffer from increased electrical resistance due to ageing fuse panels and every car benefits from this upgrade. As one customer with an early (pre-1973) car told us, "I had numerous electrical issues in the twelve months after buying this car but not had one in the three years since I fitted the fuse panel." This speaks for itself and echoes our own experience after eight years running the original test board in our 911 SC.

See the full range of Classic Retrofit fuse panels here. We ship worldwide!