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Daytime Running Lights; xB (Warning: MumboJumbo Ahead?)

Old 01-13-2005, 11:36 PM
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Join Date: Nov 2004
Location: Southeastern Virginia
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Default Daytime Running Lights; xB (Warning: MumboJumbo Ahead?)

Hi Guys,

As many of you probably are aware (esp. if you have tried to wire an HID ballast), the xB has a constant-hot (+) headlight system. It completes the circuit by supplying the (-) to the appropriate circuit rather than being continuously grounded and selectively supplying power (+) to the correct lamp circuit.

To wire a DRL-circuit that uses separate low-wattage bulbs in the lamp housings in a traditional constant-grounded system, you would run a relay-trigger (+) from an ignition-on circuit and connect the trigger ground (-) to the (+) supply going into the headlight. When the headlight is unpowered, the trigger (-) can find sufficient ground through the headlamp filament to the chassis to complete the circuit. When the headlights come on, there is sufficient (+) ahead of the "load" where the (-) is connected that it prevents the trigger from achieving a ground and the DRL lamps go out.

With the constant-hot system on the xB, there is no (-) available at the headlamp circuit until the lamps are lit. So having a DRL relay-trigger connected to an ignition-on circuit (+) and using the headlamp circuit to cancel out the ground won't work. Reversing (+) and (-) to convert this scheme would only work if the xB had a complete positive chassis ground system, which it does not; it merely uses a constant (+) or ground-seeking headlamp circuit rather than a permanent-grounded selectively-powered one.

The one solution I can come up with to have DRL that automatically come on when the ignition is turned on and go out as the headlights are lit, is to add a DPDT relay into the DRL power-supply circuit with the trigger connected across the headlamp circuit. In theory: When the ignition is turned to "ON", the ignition-on connection would supply (+) to an SPST trigger-coil with the other end connected to (-) at a grounded screw. This would close the N.O. contacts in the primary relay and make current available to the lamps through a fused direct-supply off the (+) battery. Next, I would connect this lamp-supply in series through a DPDT relay's N.C. contacts. This way, if the headlamps provide (+) and (-) for the trigger-circuit of the secondary-relay, the DRL would go out even though the ignition is on and power-flow is available through the first relay. Otherwise, the DR lamps would retain the power provided by the NO primary relay that closed when the ignition turned on since in an untriggered state it would pass through the NC contacts of the secondary relay(s) (1x for HI + 1x for LOW?)

This system works but requires the DR-lamp power to be connected through up to 3x relays in order to achieve the 1x N.O. + 2x N.C. switching required to set the correct switch logic.

Does anyone have a simpler more direct way of setting up DRL on these xB?

BTW: I have my DRL as small low-wattage bulbs in chromed 4" diameter driving-lights that are set into my center-grill, so I can wire the lamps themselves in normal fashion with a standard (-) ground. I just need to work out the triggering-circuit so that they go on/off correctly.

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