Hard wired
One of the common things we see when building a fresh interior is the rat’s nest of wiring behind the dash. Old, brittle wiring, and wiring going nowhere seem to plague many older cars. Ideally, the best time to tackle classic car wiring troubleshooting and restoration would be before the trim goes in. So, let’s look at wiring, shall we?
Father time
Time isn’t particularly kind to certain components, even on the best-kept vehicles. While there’s no denying that the electrical systems on classic cars were much simpler, they aren’t immune to problems. Uncoated steel rusts, rubbers perish, and electrical systems develop gremlins. Oh, the joy of owning an older vehicle and navigating classic car wiring restoration challenges.
The early days
The basic electrical components on the earliest motor cars were as simple as ignition systems and some lights. But the cloth-bound wires were essentially exposed, and therefore unreliable. Plastic insulation began to replace cloth insulation in the 1930s. This insulation offered superior resistance to abrasion and moisture. Introducing pre-wired harnesses improved vehicle dependability and streamlined the assembly process. Using standardized connectors enabled interchangeability and simplified assembly further. Classic car wiring troubleshooting remains a vital skill as these components age.
Harnessed power
As classics age, dirt and brittleness reduce the conductivity of electrical wires. The wiring harness and switches, often 50 years old or more, were not designed for such longevity. Distributors and coils cannot function efficiently without getting the full voltage they were designed for. Restoring classic car wiring systems often involves dealing with loose and corroded connections, blown fuses, or melted fusible links.
Batteries, Cables and Terminals
It makes sense to start by checking the battery’s condition. Make sure there’s no corrosion and that the terminals are in good shape. If you have an older flooded battery, ensure the water level covers the tops of the cells. Inspect the battery cables to ensure they fit tightly on the posts without any movement. Also, check the cables themselves; if they’re worn or damaged, replace them. Next, check the voltage using a multimeter.
Spaghetti wiring
When multiple electrical components fail, a fuse or ground connection is likely the culprit. The first to inspect would be the fuses to check nothing is blown. Look for any blown or discoloured fuses and replace them, ensuring the correct amperage. Blown fuses are often a sign of issues like a short circuit or circuit overload. Next, give as much of the wiring that is accessible a visual inspection, looking for obvious damage. Burnt, cut but not terminated, and frayed wires can be pretty common, especially near the fuse box. Exposed wires where the insulation has hardened and cracked are also common.
Staying grounded
Every old car needs a solid electrical ground from the engine to the frame and the engine to the firewall. Over time, these ground straps often become dirty or worn and may need cleaning or replacement. As you inspect your car, you might find several factory-installed ground straps—though some may have gone missing over the years.
In addition to the cable connecting the battery’s negative post to the engine block, you might find straps running from the engine to the firewall, the dash to the body, the body to the frame or battery negative post, and the frame to the engine block. Check that all ground connections are clean and in good condition. If you’re uncertain about their condition, replace them or add an extra one for good measure.
Restoring Aging Electrical Systems in Classic Cars
If individual systems or components are not working properly or at all, time to start troubleshooting. Isolate the circuit by staring at the fault and tracing the wiring back.
Check for wiring continuity using a multimeter. If continuity is broken and the switch tests correctly, there may be damage to the wire. Or a previous owner may have spliced something in poorly. If the wiring passes through the engine bay, repair activity or moving parts may have damaged the insulation or crushed a wire. Minor repairs can be carried out by cutting and rejoining the wire. If the wire isn’t long enough to rejoin, replacing the wire is best practice rather than splicing in a shorter length. If any connectors show signs of damage or corrosion, replace them using new, quality connectors to ensure a reliable connection.
Then test that power reaches the component. This will tell you if there’s a problem with the switch or wiring. If there’s power at the part, then the part is the problem, not the circuit.
Reviving Old Connections: Wiring Simplified
On a full restoration or ground-up build, the quicker, better alternative is to start from scratch with a new loom. For simple setups that don’t require an ECU or body computers, there are many wiring kits available on the aftermarket, so finding something suitable should be a matter of convenience. There’s no reason one of these kits wouldn’t be a DIY proposition. If you’re handy with the tools, can read a wiring diagram, and have a little patience that is. Otherwise, they give your auto electrician a hell of a head start, which of course means saving you dollars.
Modern Solutions for Classic Car Wiring
Once you start to add a bunch of additional circuits and modern technology, you’ll need to take a different approach. Using one of the above kits plus a kit to run the ECU is one method. Companies like Haltec have everything you need for this upgrade. But adding electric fuel pumps, thermo fans, and maybe a digital dash means a lot more wiring and a LOT more complexity. This is because the wiring loom connects high-current-draw devices to the battery through a relay, and then through a fuse.
Enter the Power Distribution Module. A PDM uses solid-state components to replace a conventional loom’s fuses, relays, and circuit breakers. There are fewer wires and connections, meaning a simpler installation with fewer points of failure. But while the wiring is simplified, set-up means another session with the computer guys to get things running. Newer versions like the NEXUS range are apparently easier to set up and definitely offer some interesting features. But for now, the PDM is a bit overkill for the average street-duty toughy.
Covered up
While we always recommend speaking with your trimmer early in a build, there is still an order that works best. With so many wires behind the dash, below the carpet, and behind the headliner, completing this work first makes sense. Not having to remove fresh trim and backtrack is a great way to save yourself some money.