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I spent 6 hours tracing a low beam fault in my Alfa Romeo Spider

10th May 2025, 12:13 by Utkarsh Chaudhary

Initially, it appeared as if the horn and the headlight's low beam functions were both powering the same relay.

BHPian Jeroen recently shared this with other enthusiasts:

During our Alfa Romeo Spider trip to France, we noticed my left headlight was not working anymore on low beam.

So we quickly checked the bulb and the fuse.

We quickly established it was neither the bulb nor the fuse. Cleaned all the fuses and fuse holders as they looked a bit corroded. But no, it seemed more like a wiring problem. I also noticed a peculiar situation. It appeared as if the horn and the low beam functions were both powering the same relay. Very odd, but if true, that would be a good point to start troubleshooting.

We also checked the voltage at the bulb connector. We measured 12 V! We put the bulb back in and measured the voltage at the connector again. 0V!

So that means that we have voltage, but as soon as the circuit gets loaded (i.e. the bulb inserted into the connector), the voltage drops to 0 V. This tends to be an excellent indication of having a high resistance somewhere in the circuit.

I could not be bothered fixing this in France. These sorts of electrical problems often take many hours to find, so I decided to have a go at it back home. We would not be driving during the evening/night, so not a problem.

Back home, the first thing I checked was in which wire the resistance occurred. Which is easy, because the mass wire goes directly from the connector to the headlight mounting ring. I measured, and it appeared fine.

But when I checked the positive wire for the low beam, I got a very different reading. Well over 200 ohms of resistance.

So far, I managed to pinpoint the problem to one particular wire in the low beam circuit. Note that these are H4 headlights, so high and low beams are in one light bulb, and there are three wires to the bulb connector. Mass, positive for low beam and positive for high beam.

Car electrical schemes can be a bit of a nightmare to read. So much information crammed into tiny diagrams. I am a member of an international (USA) Alfa Romeo Forum. One of the members took the trouble to colour in the various diagrams of Spiders. I got a set of these diagrams, had them printed on A3 size paper and had them plasticised. They are hanging in my workshop.

The wiring from the connectors runs to a connector on the back of the fuse box. Next comes the fuses, and subsequently the light stalk on the steering column.

Low and high beam wiring is wired into identical but separate connectors at the back of the fuse box. For good measure, I swapped the two connectors, just to observe what happened. Sure enough, the problem with the left low beam not working moved to the high beam position. And the high beam on both lights worked fine with the stalk in low beam position. So not a problem with the fuses, fuse box or stalk switch.

But what about this relay? I kept hearing clicking away every time I switched on the low beam. When going to high beam, the relay was de-energised. And what about the interference with the horn?

After some careful checking with my mechanical stethoscope, I concluded that the horn operated a different relay from the one I heard clicking when going to low beam. Because they are mounted very close together, it appeared as if the noise and the clicking feeling came from the same relay.

I pulled the relay, but the lights and everything keep working. It is an official Alfa Romeo Relais, but I could not find what it does! It is also not mentioned in the owner’s manual, all other relais are! So I suspect some sort of modification. But what puzzles me: why would it be energised on low beam only? The only thing I could imagine is that a low beam is required to operate the rear fog lights. But even without the relay, the rear fog lights still work. So the relay is a bit of a mystery, but for now, I decided to just leave it and work on fixing the low beam problem.

I took the fusebox out as far as it would go, lots of wiring of course.

In all honesty, I am always a bit wary of messing around with wiring in old cars. Everything has become brittle with age. So you might fix one problem to introduce two new ones!

I checked the wire and the connectors several times. Pulled them off, re-installed them. Pulled and tugged on the wire and its attachment on both connectors. At one point, the left low beam lit up! But then it died again!

I decided to try and follow the wiring loom as best as I could and see if had been damaged anywhere. Took the complete headlight assembly and mount off.

I could not find anything obvious. But after all this fiddling, the net result was that the low beam was working once again!! I had just decided to pull a new wire from the headlight connector to the fuse box. I put that wire in, but I have not connected it yet. We will see how long the low beam keeps working!

I spent about 5-6 hours on this low-beam problem. Tracing electrical problems on a car can be very time-consuming. Fixing the actual problem is usually very easy.

So after all that time, I am still not a hundred percent sure what caused the low beam problem, but it appears fixed!

One other small job, I made a little air scoop for the air inlet. I noticed the engine was not heating up properly at low ambient temperatures. As it so happens, we had the same problem on Bianca’s Coda Tronca. There, the solution was easy, we put the air filter intake in its winter position. Amazingly, that increased her coolant temperature by 10-deg C. That seems a lot, but I’m willing to give it a try before replacing the thermostat.