CAT – canadian automotive technician

Intermittent electrical faults can be the most frustrating of all service procedures. Lighting circuits, fortunately, haven’t yet evolved into the hyper-complexity of engine controls, so most problems can be tracked down with little more than a test light and common sense. Here’s a classic newer model lighting problem that can fool even experienced techs.

The issue was intermittent operation of the left turn signal in a mid-size GM N-body. The right signal worked reliably, so the flasher unit wasn’t a factor. The bulbs, however, all glowed steadily with the signal stalk showing left turn. This suggests the bulbs are O. K. but most experienced techs would go into the bulbs and sockets anyway. The first step is obvious but often overlooked: accident damage. Broken lenses, for example, let moisture into sockets not designed for it, and combined with road salt, can corrode a socket in days. There’s also the other extreme: if the car looks too good, has it had recent bodywork? Bodymen aren’t mechanics, and it’s not uncommon to find some creative wiring under a new front clip or fascia. In this case, neither applied so the tracing started in the trunk. Why the rear? Because in a 50/50 coin toss scenario, start with the easiest repair first … if you’re right, you’ve saved time and if you guess wrong, you’ve wasted little. In this case, the rear bulb retainer looked clean, and the flat power connector was sound (see image 1).

Removing the retainer frames and examining the 3057 lamps didn’t show anything unusual with two good filaments each (see image 2). Pulling the trunk lining aside down the harness run showed that the rear harness connector was sound, although there was evidence of water leakage and rust formation in the trunk area (see image 3).

Could some other rear harness lighting issue be loading down the stop/tail/turn circuit? Continuing the visual inspection revealed two other issues. The first was a missing license plate bulb while the second was the commonly-found broken trunk dome light lens and missing bulb (see images 4 and 5).

Filling these holes however, didn’t improve the situation, suggesting that the rear harness was intact, with no short circuits or weird grounding issues. Pulling the 3057 bulbs from their sockets, however, revealed the problem once the dielectric grease was wiped away from the base (see image 6). The wire base is burned and blackened, loading the circuit just enough to hobble the flasher unit, but not enough to keep the filaments from glowing. Both the socket and bulb were replaced to solve the problem.

How did this one evade the tech early? One reason is the way modern 3057-type bulbs fit into their sockets. 3057’s are electrically very similar to the traditional 1157 (see chart), but use the filament support wires as the electrical contacts. The old 1157’s used brass bases with soft solder point contacts. Corrosion was a known problem, and it often showed externally in the metal socket. The 3057, bathed in contact grease and in a clean plastic housing, doesn’t telegraph overheating and/or corrosion as clearly. To find issues like this, it’s necessary to pull all the circuit bulbs (one at a time!) and inspect carefully. Reinstall good ones with more dielectric grease, but don’t forget the possibility of a long-life product up sell…if the bulb has been there for seven or eight years and its buddy has burned out, why not change the set as a form of preventative maintenance? For high-end consumers, there are also LED replacements for some applications that offer lots of light with lower current drain. And the interior trunk lamp? A tie-wrap retained the lens with its broken mounting tabs, an appropriate repair for this ten-year old vehicle. Remember to add this simple service to the work order … it’s only value-added service for the consumer if they’re aware that you’ve performed the work.