Torpedo Fuses: The Bane of Classic German Automobiles

The photo below illustrates how the torpedo fuses BMW, Mercedes, Audi, Datsun, Porsche, and others used in the 1960s, 70s, and 80s have not stood the test of time. This post dives deep into automobile fuse technology… because I find it fascinating.

Automotive Fuse Types

We take fuses for granted, especially in our cars. The car you drive is likely to have several fuse boxes containing dozens of fuses. It’s rare owners of modern cars ever have to replace a fuse. In the 1960s, Robert Bosch GmbH, a supplier of automotive technology started selling German car manufacturers their Torpedo-style (also known as continental, bullet, European, or GBC type fuses):

Torpedo Fuses (also called Bosch or bullet fuses)
Torpedo Fuses

During the same time, in Japan, manufacturers such as Toyota used glass tube style fuses like these (my 1978 Toyota FJ40 Land Cruiser has these in it):

Glass tube style automotive fuses

The automotive engineers of the time clearly felt these fuses were suitable. However, as the electrical systems in cars became more and more sophisticated, better technology was needed. Sadly, as it turns out, they waited too long to update their designs to something better: The now-ubiquitous blade-style (also called plug-style, type 1081-C, ATO, or Littlefuse) that came along in the 1970s:

Blade style automotive fuses

Upgrading Classic BMWs from Torpedo Fuses to Blade Fuses

One of the cars I’m the most passionate about is the BMW E28 (learn about my favorite E28, Vlad, here). The E28 is the BMW “5-Series” built between 1982 and 1988. It has the highest Look Back Quotient (LBQ) of any car ever made. It also was the last BMW chassis to use Torpedo/Bosch-style fuses.

Vlad – My 1987 BMW 535is (E28)

Those of us who own E28s really, really wish BMW would have switched to blade-style fuses earlier, as the resulting electrical issues give us no end of grief. Interestingly, the BMW E23 (7-Series from ’77 to ’86) and E24 (6-Series from ’76-89) switched to using blade-style fuses mid-way through their production (September ’86). Why the heck didn’t they do the same thing with the E28?

It didn’t take me long after buying Vlad in 2013 to discover how troublesome the fusebox in E28s was. There’s a great community for these cars, centered around the mye28 forum, and the old-timers were quick to coach me on tips for mitigating the issues. The issues include:

  • Fuse holders not holding the fuses tightly, causing intermittent glitches.
  • Excessive heat and melting fuse box plastic (we call it “melty”) due to poor conductivity of old and dirty fuses.
  • Melting wires throughout the car (e.g. in the headlight switch assembly) caused by fuses that should have blown, but didn’t.

I set about trying to fix the problem at a fundamental level instead of just mitigating it. I wasn’t the first to try, but I’d not seen evidence anyone had succeeded. The leading idea was to retrofit a later model E23 or E24 blade-style fuse box into the E28. The wiring of the E23 & E24 is close to that of the E28 and this sort of upgrade can work. Even though I purchased a brand new E23 fuse box, I never got around to doing the work.

Then, a new guy on the forum posted a note about a prototype fuse box kit he had engineered. His concept was brilliant: Keep the original fuse box, rip out the torpedo-style holders, and slip in a new circuit board that holds blade-style fuses into place. As a bonus, he included LEDs that indicated whether a fuse was blown or not.

Holy Grail Labs’ BMW E28 Blade Fuse Upgrade Kit

I pounced, begging to be a test subject. I soon learned his name was Galahad. We became friends and agreed to become partners, forming a company to build and sell his kits. I’m Chief “Do The Stuff that Lets Engineers Do Magic” Officer and he’s Chief “Do Magic” Officer.

Given his name is Galahad, you probably now see why we named the company Holy Grail Labs. HGL currently sells kits that work on the BMW E12 (’72-81 5-Series), E21 (’75-83 3-Series), E23, E24, and E28 and are working on kits for more BMWs and classic Porsches.

Anyway, back to the topic of automotive fuse technology…

Automotive Fuse Technology Deep Dive

Before the introduction of ECUs, a fuse in a car only had to prevent wiring fires. For wire sizes commonly found in a car, it can take seconds at even very high currents to start a fire, meaning early automotive fuses did not need to be very fast or accurate – they only needed to break the current before the wiring failed. The general specifications for the torpedo fuse reflect this: the rating is a guideline for a minimum current that will pass indefinitely, and they take half a second or more to blow even at dozens of amps higher than the rating.

In practice, torpedo fuses have additional problems. Since the fusing element is completely exposed, it tends to oxidize and increase in resistance. Additionally, the fuse holder design used by BMW loses spring tension over time and the contacts oxidize too, both of which also lead to increased resistance in the fuse system. Increased resistance leads to increased heat, melting the fuse box and degrading the fuse – you can see the thermal discoloration at the end of the blue fuse pictured above along with the melted plastic. It’s very common to find the paper assembly card inside the fuse box significantly discolored around those two fuses even if the box itself hasn’t melted.

The two fuses that commonly have melting problems in E28s are both rated for 25A. However, there’s enough wiggle room in the spec that a fuse could survive 40A indefinitely and still be rated as a “25A” fuse. A dirty torpedo fuse can easily reach 50mOhm, and at 25A the fuse itself is a >30W heater – which goes up extremely quickly with higher than rated currents the fuse will still allow, nearly tripling by the time you reach 40A. Having the equivalent of an incandescent desk lamp for a fuse is dangerous for both your car and you, running counter to the point of the fuse in the first place.

Modern blade fuses were designed to preserve not just wiring but onboard computers, which require much faster and more accurate fuses to avoid damage. The specified tolerances are much tighter and they are specified as a maximum survivable current, not minimum. At 12A a 7.5A rated blade fuse will last a fraction of a second, while an “8A” torpedo fuse could last up to an hour!

In real-world use, blade fuses and their holders are made out of materials that do not oxidize as quickly – if at all – taking care of the main source of unwanted resistance for torpedo fuse. The holders have also been redesigned for both much higher clamping force and to sustain it better, tackling the other major problem area.

Modern fuses aren’t perfect – they don’t instantly trip once you’ve exceeded the limits – but they properly address the major problems inherent to torpedo fuses, and do so in a way that significantly increases system safety.

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