When diagnosing electrical issues in classic cars, the key is finding the problem before even thinking about a fix. Today, we’re diving into a 1978 Porsche 911 SC with an electrical mystery: after extended driving, especially in traffic, the owner smells something burning inside the car. That’s a red flag, and our job is to track down the source.
The first place to check is the fuse box. These older Porsche fuse boxes are notorious for being outdated and prone to connection problems. Since the burning smell suggests overheating somewhere in the system, we’re going to measure voltage drop across the fuses to see if we can spot a weak connection.
Voltage drop testing is a crucial diagnostic method to identify poor connections and excessive resistance in an electrical circuit. Here’s how we set it up:
Depending on the reference manual, a normal voltage drop should be 0.25 volts or less, but in my experience, anything above 0.1 or 0.2 volts is a problem. The closer to zero, the better.
Right away, we found something suspicious with the fuel pump fuse. It was hot—120°F when checked with a thermal camera. That kind of heat means resistance, and resistance means voltage drop.
Here’s what the readings showed:
These numbers confirm a serious voltage drop between the fuse and its connection points. A 0.1V drop just between two points in the same fuse block is not acceptable—it means there’s unnecessary resistance somewhere in the circuit. That resistance is what’s causing overheating.
From our readings, the main suspect is the connection between the brass block and the fuse tang. That’s where the voltage drop jumps, indicating a bad connection. This weak link is likely causing excessive amperage draw, leading to overheating.
To confirm, we need to pull the fuse box and inspect the back of the fuse block where the brass block and tang meet. If there’s visible corrosion, looseness, or damage, we’ll know for sure that we’ve found the culprit.
Beyond the overheating fuse, this Porsche also has a hard-start issue in the morning. Given the multiple voltage drops we see across circuits, the warm-up regulator may not get proper voltage. A bad connection anywhere in the system could be affecting multiple components.
This is why voltage drop testing is essential—it helps us find the problem, not just guess at it. Before we start replacing parts, we need to fully understand the issue. Next, we’ll remove the fuse block, inspect the connections, and determine whether tightening or replacing components is necessary.
Stay tuned as we continue tracking down the electrical gremlins in this 911 SC!
Porsche Voltage Drop Part 1
