Headlight Fault on a Ford Capri
- Ryan Brown
- Oct 14, 2025
- 5 min read
Today, we investigate and repair a headlight fault on a classic Ford Capri.
The Problem
This classic Capri was brought to us with various lighting issues, primarily concerning the limited functionality of the headlights and rear lamps.
For example, the headlights turned on dim, but would not flash or stay on full beam.
The Investigation
The first test was to check whether the lamps had blown. By unplugging each lamp, we were able to power the main beams using our own test supply, verifying that the lamp unit itself was not defective.
As both lighting elements in the lamp share the same ground, this further indicates that the issue was due to a missing supply.
The Main Beam Supply
The switching of main/dip activation is facilitated by a headlight control relay unit within the engine bay. This device is mounted on the bulkhead and is responsible for distributing power to these lamps, directed by the stalk switches on the steering column.
We noticed that a variety of wires have been cut in the past. This was obvious from the colour changes in the wiring harness, with wires joined by "chocolate block" style connectors. Corrosion seeping into uninsulated wiring joints can cause a multitude of symptoms, so this was our next point of concern.
This unit outputs a separate feed to each headlight cluster, one for each side. By unplugging the headlight module, we were able to see if a short circuit was present on the wire. With no short found, we next supplied our own test feed at the socket, and found we were able to illuminate the lamp. This ruled out a broken wire between the two components.
Advertisements provided by Google AdSense
The Headlight Control Unit
In addition to housing pins for the power supply and headlight wiring plugs, the headlight control unit sneakily conceals the fuses that protect the headlight circuits. These fuses are a manufactured weak point, so if an over-current condition occurs, they would fail and purposefully cut off power to the rest of the circuit. For this reason, these were the first points to check when investigating this missing headlamp output supply.
It is visible that the two sets of fuses are connected as pairs, due to one side of each set of fuses being joined together. These fuses protect the two main beams and two dip beam outputs.
As the output voltage to the lamp is missing, these fuses were investigated. When testing each side of the fuse terminals with a multimeter, no continuity was detected. Under close observation, the fuses were not actually blown, but signs of corrosion were present.
The corrosion prevented a suitable connection between the fuse and the terminal on the fuse holder, blocking the current and causing the same symptoms as a blown fuse.
The old fuses were removed, with the terminals of the control unit carefully cleaned. Since new fuses are cheap and there is no benefit to cleaning up the old ones, the old fuses were replaced with more modern alternatives of the same rating.
When fuses get corroded, this can weaken the fuse strip, which is calibrated to blow when the current exceeds a set limit. The corrosion eating into this strip could have affected its properties, meaning that even after cleaning, the fuse will not be reliable. Even though one set of fuses was functioning properly, they appeared almost as corroded as the other side, and were exchanged anyway as a precaution.
With the new fuses fitted and the headlamp control unit reinstalled, the main beams were tested again. This time, they shone as bright as new, demonstrating that it was the fuses at fault. Some further tests were conducted to ensure the repair would prove reliable before the next point of concern was addressed.
The Brake Lamps
Uncovering the wiring to the tail lamp units revealed signs of previous repairs, including a familiar combination of aftermarket joiners, added wires and colour changes.
When determining which wires were for the brake light feed to test if a supply was present, one of the wires became loose and fell out of its terminal. Unsurprisingly, this was the wire to the brake light bulb. With little force, the other terminal slid off the wire also.
The terminals used on this wire were blue space terminals and badly crimped. The colour of the terminals indicates the gauge of wire it is intended for. In this case, blue terminals were designed for wires with a much thicker conductor, which is likely the main reason they failed to stay attached to the wire. Additionally, the crimp did not look healthy, as it was likely over-squeezed to make the wire stay. This theory was further supported by the overly flat appearance of the crimp, rather than the squeezed oval shape you would typically expect.
In this circumstance, there is no benefit to replacing these connections with new disconnectable terminals. In order to ensure a similar issue does not happen again, a more permanent solution was chosen. By soldering the wires together, a stronger and longer-lasting connection was created. The joint was then insulated, further protecting the connection from corrosion and minimising the risk of an exposed wire coming loose, as we saw in this example.
We also discovered a wire that had been added to connect one brake light feed to the other. We also found more bad terminals, as seen on the previous lamp.
We suspected that one brake light feed may be bad, and this was a quick fix. However, after testing the feed, no issue was detected. The problem we found was that the lamp has a bad ground connection, not a bad live.
The lamp units are grounded through bolts going into the body. However, where the car was repainted, this earth point had been sprayed over, preventing it from supplying a strong ground connection. This hindered ground was strong enough to illuminate some lower-wattage bulbs, but when more power-demanding bulbs, such as the brake lights, were activated, not enough current was able to flow.
The bad joints and the extra wire were removed, and a stronger solder joint was made. The ground connection was carefully cleaned up, and the lamp was tested. We found that now all the lamps were working. We loaded up the rear lamps, turning on one additional bulb at a time until every bulb was illuminated. This was to stress test the lamp unit, ensuring that none of the bulbs go dim when a new one was added - a key sign of a weak common live or ground connection.
And with this, the Capri now had full working lamp functionality front and back.
Advertisements provided by Google AdSense
