Diagnosing flaw in daytime running lights

By: Jim Kerr Posted: 1:00 AM CDT Friday, Oct. 23, 2009

QUESTION: I am unable to get the DRLs (daytime running lights) on my 2000 Chevrolet Z71 pickup to work. I installed new bulbs, the old ones were burned-out, fuses are good and the DRL relay is fine. I have heard this is a common failure on this model. What else can I check?

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Hey there, time traveller!
This article was published 23/10/2009 (6150 days ago), so information in it may no longer be current.

ANSWER: The daytime running lights on your truck are two bulbs that operate separately from the other exterior lights. It was common for these bulbs to burn out, so GM has recommended a longer-life 4157k bulb number as a replacement that has a much longer life. Before looking at the control system, verify that the bulbs are getting a ground. Connect a test light between battery positive and the ground terminal (black wire) in the bulb socket. The test light should come on. If it doesn’t, you have a bad ground connection to the body in the engine compartment.

If the ground is good, the next place to look would be at the connections in the underhood fuse/relay centre. GM calls it a junction block and several wiring harnesses plug into the bottom of this plastic box. You are looking for the dark blue wire in connector A2, pin C4. A loose or corroded connection here could cause the problem.

The DRL relay is fed with two fuses. The DRL fuse provides power for the lights, while the IGNE fuse powers the relay control circuit. You can test for power at the relay terminals 30 and 86 to verify the relay is actually getting power from the fuses. Corrosion on the connections internally in the relay centre could be preventing power from getting to the relay or out of it to the lights. If this is the case, the relay centre needs replacing.

The rest of the DRL circuit is the control portion. The body computer provides a ground on the light green/black wire at terminal A11 to turn on the DRL relay. You can ground the terminal to verify the circuit is working correctly.

If the circuit is good, either the body computer is bad or the ambient light sensor on the dash is sending the wrong signal. Usually these light sensors don’t fail, but dirt or an oily film (from leather upholstery or cigarette smoke) can prevent it from working properly. Clean the sensor bulb and shine a bright incandescent light on it when testing DRL system operation.

QUESTION: I decided I’d opt for all-terrain tires as opposed to going back to all-weather. The all-terrain tires have more rubber on the road in my case and I should think the more surface contact with the road the better the control. If not, why not? Thank you.

ANSWER: All-terrain tires are designed for traction both on the highway and off-road trails. The tread designs are patterned to provide additional traction in any direction on dirt, sand, gravel and snow. Because of differences in tread design between tire manufacturers, the all-terrain tires may or may not have more rubber on the road than an all-weather (designed more for pavement use) tire. The brand you have chosen may have more rubber contacting the ground, and you are correct in assuming this is good. But there is more to traction than just the amount of rubber on the road.

Take racing slicks as an extreme example. On smooth, dry, hot pavement they stick extremely well. But add a little water and they quickly spin out of control. Part of good tire-tread design is to channel water away from between the rubber and the road. Channels between the tread blocks do this, so if there are less open areas, the tire may slide easier on wet roads.

Winter ice tires also remove the water from between the rubber tread and the ice surface to provide traction. There are many ways to do this, including micro-voids in the rubber and multiple sipes or small cuts in the tread blocks.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net