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Old 12-01-2019, 02:20 PM   #13
fdryer
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Join Date: Jan 2006
Location: NYC
Posts: 43,601
 

2003 L-Series 3.0L Sedan
Default Re: question about daytime running lights

Presuming the Park/Brake switch works (pull up and the red brake light turns on, release the p/b lever and the brake light turns off), this switch sends a signal to the IPC. In the wiring diagram, the p/b ground signal is sent to the DRL Logic Board where it determines when to turn on DRLs (high beams at half power from series wiring). I do not know from info provided where the problem lies yet. One way to see which way to go might be bypassing the DRL relay with a small wire jumper. The headlight switch should be in the OFF or AUTO position.

The standard 87A relay comes in two configurations, four or five pin types. It doesn't matter which one is used in DRL mode but important not to mix them. If you have the original one, examine it for labeling, often printed on one side of the cover or embossed on the case bottom. The four pins of importance on all 87A relays are 30+87 and 85+86. Technically, 85+86 are the command side where the pcm commands the relay to power up. 30+87 are the switch contacts, open when relay isn't energized, closed when relay is energized to send 12v to blower motor, fuel pump, rear defrost grid, drl circuit, etc. To test the DRL circuit and turn on high beams in drl mode, remove the relay and insert a small wire jumper across relay socket terminals 30 and 87 to send 12v to the high beams in series. Whether in drl mode or not, halogen lamps do not decrease in brightness due to halogen lamp design never dimming as the filament ages. The properties of halogen lamps replaced older filament lamps as the last improvement in headlights before HID and LED lighting became the next evolution in lighting. It's difficult if not impossible for anyone to tell the difference between any headlights using halogen bulbs running in series or parallel wired configurations. GM took advantage of this property to introduce and use an inexpensive method of DRLs with halogen lamps. Wiring circuitry is made a little more difficult but not for anyone familiar with basic electricity.

If you are able to insert a small wire jumper across the DRL relay socket terminals 30 and 87, this may manually turn on high beams in drl mode. If not then this may point to the instrument panel where the DRL circuitry governs when DRLs are needed.
Attached Images
File Type: jpg 87A-1 relay.jpg (53.6 KB, 4 views)
File Type: jpg 87A-2 (5pin) relay.jpg (36.2 KB, 3 views)

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