Daytime running light failure -- one side only

Question

I have a 1991 Miata. It's a Canadian vehicle, and so is equipped with daytime running lights. Recently, I've noticed that the passenger side light is not working as a DRL. The driver's side light works fine. A multi-meter shows that no current is being delivered to the passenger-side bulb, while the driver's side get 14 V.

The turn signal bulb is used as the daylight running light. The passenger side light works in all other respects. When indicating a right-hand turn, it blinks. When the main lights are on, it's on. When the driving lights are on, it's on.

I suspect that the relay responsible for turning on the daytime running lights is failing, but before I order the part, I thought I'd check here if there are other possible explanations for this behaviour.

UPDATE

Some more information in response to questions in the comments:

1. The bulb for the DRL is dual filament. The brighter filament is for the DRL and the turn signal. The less bright filament lights when the headlights are on.

2. When DRL are on, the turn signal sequence is off-on-off-on because the DRL and turn signal use the same filament.

3. The wires to the connector are red, green and black.

And one piece of interesting new information. The passenger side turn signal (right-hand turn) is starting to act strange intermittently. The relay cycles faster, maybe double the normal speed. The front turn signal does not light at all, while the rear on acts normally. This is the behaviour I've seen before when the front indicator bulb has burned out.

I'm starting to think the problem is not the DRL relay, but rather an electrical fault somewhere between the relay and the bulb. Since the turn signal and the DRL use the same filament but different relays, the fact that both are failing makes me think there's a problem in the wiring downstream of the relays.

Answer 1

With the additional information you have posted, I am in agreement with your initial assessment that a relay or relays are to blame. It sounds like you can get both high and low intensity out of the affected bulb under certain circumstances (high works OK when using the turn signal, and if I understand you correctly, low works OK as a running light when the headlights are on), therefore the wiring to the bulb must be OK.

Answer 2

Before you order a new relay you should test the old one. It's not hard and the internet is helpful with info https://www.google.com/search?q=how+to+test+a+relay

By far the most common problem with electronics is bad grounding. First thing to check is the resistance between the black wire and different grounds on the car, looking for high resistance. If you get high resistance anywhere (higher than a few ohms) you'll want to trace the wire back until you find a good ground and check the connections between there and the bulb.

If the ground tests out fine turn the key until the other running light comes on and check the voltage at the red wire, it should be enough to turn the bulb on (12V? you can check the other bulb for the exact value, it might be helpful). I'm guessing it will some other, low value, since the light doesn't turn on. What you want to do next is to trace that wire back toward the battery. For this it might help to turn the key off and check for continuity from the bulb to upstream connections.

Tracing those wires without a diagram can be a real pain, but luckily the circuit for running lights doesn't need to do much, so it's likely very simple with only a few connections.

Answer 3

I had a similar problem with my Mitsubishi ASX. It turned out to be a glove box bulb issue. Hope this saves others and service centre staff some pain... My drivers side running lights would flick on when I unlocked the car with remote entry then turn off. Mitsubishi Service centre suspected an ETACS fault. They disconnect the tail light and tried/tested but the problem still persisted. They then checked the wiring diagram to trace the power flow and it all came back to ETACS. Service guys found that the glove box bulb was on the same circuit. They removed the blown glove box bulb, fitted a new bulb, locked car and waited. They unlocked the vehicle and the issue was resolved! Cost me $4.16 for a new bulb!