Tracing electrical faults on a fifty-year-old harness

Old vehicle wiring gets replaced far more often than it needs to be. The copper is usually fine. What fails is every place two pieces of metal are supposed to touch.

01Understand the failure mode first

A fifty-year-old circuit does not usually fail open. It develops resistance — at a connector, at a ground, at a switch contact, at a splice. Resistance in a circuit produces a voltage drop, and voltage drop is why the symptom is almost always "works, but weakly, and sometimes not at all". Dim headlights, slow wipers, a starter that clicks, a gauge that reads low. All the same underlying problem in different places. This is why continuity testing misleads you. A corroded ground can show continuity on a meter — a few ohms — and still be completely inadequate under a 10 amp load. You must test under load, which means voltage drop testing, not continuity.

02Voltage drop testing, which is the whole technique

With the circuit on and drawing current, put your meter across the two ends of a section of the circuit. You are measuring the voltage being lost in that section. Rules of thumb for a 12 V system:

• Under 0.1 V across a ground connection. More than 0.2 V and it needs cleaning.
• Under 0.5 V total across the whole supply side of a circuit.
• Under 0.3 V across any single connector or switch.

This finds bad connections that every other test declares fine. It is the single most useful skill in old-vehicle electrics and it takes ten minutes to learn.

03Grounds, first, always

The majority of faults are grounds. They corrode, they are attached to painted surfaces, and they were often marginal from the factory. Check, and clean to bare metal:

• Battery to engine block. The big one.
• Engine to body or frame. Frequently missing entirely, or relying on the throttle linkage, which is not a ground strap.
• Body to frame, on anything with a separate frame and rubber mounts.
• Individual light and gauge grounds, which are usually a screw into painted sheet metal.

Adding grounds is cheap and fixes an enormous proportion of odd symptoms. If the symptom is bizarre — the indicators flash when you press the brake, the fuel gauge moves with the headlights — the answer is a missing ground, and the current is finding its way back through a path it should not be using.

04Then the bulkhead connector

Where the harness passes through the firewall. It carries almost everything, it lives in the worst environment on the vehicle, and its terminals corrode. Symptoms of a bad bulkhead connector: multiple unrelated circuits misbehaving at once, things that work when you push on the connector, and heat damage around specific pins. It usually cleans up. Disconnect the battery, take it apart, clean each terminal with a small brush and contact cleaner, check for spread female terminals, and apply dielectric grease on reassembly. Heat damage — melted or discoloured plastic around a pin — means that circuit was drawing more than the connector could carry. Cleaning it is treating the symptom. Find out why.

05Common specific faults

Headlights dim. Almost always the ground and the switch, not the bulbs. All the headlight current on many of these vehicles passes through the headlight switch and the dimmer switch, both of which develop resistance. A relay conversion — using the original switch to trigger relays that carry the load — is a genuine improvement and is invisible. Gauges read low or erratic. Ground first. Then the instrument voltage regulator, if fitted. Many older vehicles use a small mechanical regulator that supplies a pulsed reduced voltage to the gauges; when it fails, all the gauges read wrong together, which is a useful diagnostic signature. Charging problems. Test the voltage drop between the alternator output and the battery positive under load. A corroded main charge wire produces "the alternator is not charging" with a perfectly good alternator. Also check the ground path from the alternator case back to the battery. Intermittent everything, worse over bumps. A connector or a chafed wire. Look where the harness passes through a bulkhead, over a sharp edge, or near an exhaust. Wiggle sections with the circuit live and watch for the symptom. Blows a fuse occasionally. Chafe, and it is worth finding rather than fitting a bigger fuse. A wire rubbing through against a bracket is the usual answer, and the fuse is doing its job.

06When replacement is genuinely right

Replace the harness when:

• Insulation has gone hard and is cracking off when moved. Common in the engine bay, particularly near the exhaust manifolds.
• There has been a fire, or evidence of one.
• Somebody has spliced into it repeatedly with scotch locks and tape. At that point the harness is unknowable and unknowable is not safe.
• The vehicle is being restored and it will never be easier to get to.

A new correct-colour harness is not expensive relative to the time spent chasing faults, and correct colours mean the next person can use a wiring diagram.

07Safety, which is not optional here

• Disconnect the battery before working on anything. Negative first.
• Fuse every circuit you add, at the source.
• Never bypass a fuse to test something. If you must, use a low-value fuse and watch it.
• A shorted battery cable will start a fire in seconds. Old vehicles have burned to the ground because somebody left a spanner across a terminal.