Tracking Down a 928 S4 Starting Fault — and Finding Decades of Hidden Wiring “Upgrades”

Tracking Down a 928 S4 Starting Fault — and Finding Decades of Hidden Wiring “Upgrades”

 

🔧 Tracking Down a 928 S4 Starting Fault — and Finding Decades of Hidden Wiring “Upgrades”

Every classic car has its mysteries, but few can test your patience quite like a Porsche 928’s electrical system. These cars were ahead of their time — complex, beautifully engineered, and absolutely unforgiving of poor wiring repairs.

This story starts like many others: with an engine that cranked, smoked, and then refused to start.

💨 The Symptom That Started It All

While cranking the 5.0-litre V8 one afternoon, I spotted smoke rising from the fuse box. Never a good sign. The source was quickly traced to a black/red wire — the ignition-switched feed for the fuel pump and DME relay circuit (known in the wiring diagrams as Q12 on the blue plug at the Central Electric (CE) panel).

Closer inspection revealed the wire had burned its insulation clean through the main loom, all the way to the alarm/immobiliser module under the driver’s seat.

So began a full deep dive into the 928’s intricate electrical system.

⚙️ Understanding the Circuit

The black/red wire in question is crucial. It carries the ignition-switched 12V supply (Terminal 15) that powers:

  • The DME relay coil (which in turn runs the fuel pump and ECU),

  • The engine management system, and

  • The alarm/immobiliser control circuit that allows the car to start.

On the 928 S4, that feed leaves the CE fuse board at the blue Q connector (pin 12) and travels through the loom to the alarm module under the driver’s seat. From there, it returns power to the DME relay when the alarm is disarmed.

When that wire burned, it essentially shorted inside the sealed fuse board and cooked everything in its path — taking out the ignition feed to the fuel system and immobilising the car completely.

🔍 The Investigation

At first, the obvious suspect was the factory alarm module (Porsche part number 928 618 260 00). A replacement was sourced and fitted.

Immediately, some signs of life returned:

  • The fuel pump primed with the ignition on.

  • The dash lights came on normally.

  • But when turning the key to start, the starter motor stayed silent.

The ignition switch was confirmed good — it was clearly sending its “start” signal (Terminal 50), because the DME relay and fuel pump were responding. Yet the starter wasn’t engaging.

That led us to dig even deeper.

🧩 The Breakthrough

With the wiring diagram spread across the workbench and the seat out of the car, we traced the yellow starter-trigger wire. It should have run cleanly from the ignition switch, through the alarm module, to the CE fuse panel and out to the starter relay.

But instead, we discovered something else entirely.

At some point in its past life, this 928 had been fitted with an aftermarket alarm. The installer had spliced directly into the yellow starter wire near the CE panel — adding an extra immobiliser cut.

That crude join had corroded over time, creating a weak connection that carried enough voltage to trigger the fuel pump and DME relay, but not enough current to pull in the starter solenoid.

The result? Perfect ignition behaviour, perfect fuel system response, but a dead starter motor.

🔧 The Repair

The fix was as simple as it was satisfying.

We removed all traces of the aftermarket alarm wiring, cut out the oxidised section of the loom, and properly rejoined the yellow starter wire with solder and heat-shrink.
The burned black/red ignition feed was already replaced with fresh 1.5mm² wire and a new fused feed from the ignition-switched bus.

Once everything was reassembled and the CE panel refitted — the car started instantly.
The V8 barked back to life as if nothing had ever been wrong.

💡 Lessons Learned

This repair became a masterclass in classic Porsche diagnostics. Here’s what we took away:

  1. Smoke means stop immediately. The black/red (Q12) wire overheating was a clear sign of resistance and heat buildup — ignoring it could have caused a fire.

  2. Always start with what humans have changed. The 928’s factory wiring is remarkably reliable. It’s usually old alarm systems, radios, or immobilisers that introduce faults.

  3. Follow the circuit logic. The DME relay, fuel pump, and starter are all interlinked through the ignition switch, CE panel, and alarm module. Understanding that flow made diagnosis much faster.

  4. Trust your multimeter, not assumptions. Testing continuity and voltage on every suspect wire avoided wild goose chases.

  5. Document everything. Future you (or the next owner) will thank you when a fault reappears.

🚗 The 928’s Electrical Reputation — Undeserved?

Many call the 928’s wiring “complicated.” In truth, it’s just comprehensive — Porsche engineered redundancy and modular logic decades ahead of its time.

What confuses most people is the layer of human interference added later: alarms, immobilisers, trackers, stereos, and splices.
Once those are removed and the factory circuits are restored, the 928’s electrics are logical, robust, and beautifully laid out.

🧰 Final Thoughts

In our case, everything we learned along the way was correct — every test, every reading, every suspicion of a burnt ignition feed or alarm relay.
The real culprit was simply a bad connection, buried deep in the loom, left behind by someone who thought they could “improve” on Stuttgart engineering.

After all, Porsche never built a bad circuit — but plenty of people have made a  modified one. 😉


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