Center maplight with time delay

The E30 does not have a map light, but it is possible to construct a light from a side door lamp that replaces the SRS (Supplementary Restraint System) monitor above the rear view mirror.
A photo below shows the constructed map light next to the removed SRS monitor. A second photo shows the map light as installed above the rear view mirror.

Electrical connections to the map light were made by running four wires under the headliner from the driver side door lamp and a single wire from the passenger side lamp. This provides simultaneous dimming of all three lights as well as on/off control of all three lamps from the center map light switch.
The headliner should not be pulled down to install these wires as it is often difficult to re-tuck the headliner neatly back into place.

Rather, the wires should be attached to a piece of solid 14-gage copper house wire and pushed forward along the roof edge from the door light openings to the corners of the windshield (removing the sun visors helps) and then strung across the windshield, under the black headliner cover, to the vicinity of the SRS monitor opening. BMW uses 0.5 sq mm wire for these lamps; US gauge #20 AWG is a good equivalent. Stranded wire is recommended. The backing panel for the map light was constructed from the lid of an 8 x 3 x 1-inch plastic box from RadioShack (cat. no. 270-1808). As shown in the next photo, a rectangular hole with dimensions to accept a BMW side lamp was cut into the lid using a Dremel tool.

The box lid was trimmed to the dimensions of the SRS monitor panel and polished to a high gloss by buffing with emery cloth, starting with 320-grit and progressing, alternately dry and wet, to 1500-grit paper. Final buffing was done with liquid brass polish. Of course, the plastic may be left in its original matte finish if desired.

The SRS monitor is held in place by friction between its inside frame and the black plastic headliner cover. A similar frame was fashioned from the sides of the plastic box from which the panel was cut and the new frame mounted to the back of the map light panel by flexible plastic cement. See the next photo that shows the reverse side of the panel:

The map light was fitted with a time delay circuit that dims all three lights approximately twelve seconds after the doors are closed. The circuit was built on a 1-3/4 inch square circuit board that is secured behind the map light under the headliner cover. A picture of the circuit board is below, along with a schematic diagram of the circuit. The circuit can be easily modified to dim only the center light if desired:

All of the electronic components are available from RadioShack. A partial list of the required components and their RadioShack catalog numbers with year 2000 prices is given below.

Part Catalog No. Price

• MPS2907 276-2023 $0.59
• IRF510 276-2072 $1.99
• PC board 276-148 $1.49 (for 2)
• PCB terminals 276-1388 $2.29 (for 4)
• Heat sink 276-1368 $1.49

Also required are several resistors and a capacitor.
Any general-purpose pnp switching transistor can be substituted for the MPS2907 provided the appropriate base and collector resistors are chosen for saturated operation.

Almost any n-channel, enhancement mode power MOSFET can be substituted for the IRF510, but the drain-to-source resistance should be kept low. A better alternative to the IRF510 (whose drain-to-source resistance is approximately 0.5 ohm) is the NTE66 from NTE Electronics, Inc (drain-to-source resistance of about 0.1 ohm). The NTE66 costs approximately $10. See www.nteinc.com for a list of NTE distributors. Because each lamp draws about 0.75 amps, the IRF510 dissipates (as heat) roughly 2.5 and 0.3 watts for 3-lamp and 1-lamp operations respectively. The NTE dissipates only one-fifth as much. The recommended heat shield, while serving its principal purpose of dissipating heat from the MOSFET, also serves to insulate the metal tab of the MOSFET from accidental grounding. (The metal tab of the MOSFET is electrically common with the MOSFET drain terminal.)

The operation of the time delay circuit is as follows:

• Opening either door grounds the base of the pnp transistor through the door switch, thereby driving the transistor into saturation.
• This, in turn, applies (near) battery voltage to the gate of the MOSFET, causing it to conduct and turn on the lamps.
• When the doors are closed, the transistor returns to cut-off but the gate/capacitor voltage is held by the 1N4001 diode.
• The capacitor voltage decays through the parallel resistor (the input impedance of the MOSFET is essentially infinite) until it reaches about 5 volts, at which point the lamps begin to dim.
• Below 3 volts the lamps are effectively extinguished.
• The delay time in seconds is approximately equal to the product of the capacitance multiplied by the resistance of the parallel resistor.
• With a 2.2 microfarad capacitor and a 5.2 megaohm resistor, the delay time is about twelve seconds.
• Dimming requires about 1 second.

A wire wrapped around the ground side of the center light bulb and running through the 1N5400 diode to the two side lamps allows both side lamps to be switched on and off by the driver from the center map light switch. See the photo of the reverse side of the map light for details.
If desired, the lights could be extinguished during the delay interval by installing a momentary-on/normal-off switch (not shown) across the capacitor to ground. A 1000 ohm resistor should be placed in series with the switch to prevent shorting the pnp transistor to ground if the switch is accidentally pressed while a door is open.

The above circuit will not work with a factory-installed delay circuit. In such a case the original timer module, located behind the driver?s kick-panel, must be removed and appropriate jumpers placed into the socket terminals. From the wiring diagram in Chilton's ?BMW 318/325? 1989-?93 Repair Manual,? page 6-95, it appears that the terminals to be jumped are "A" to "TG" and "A" to "TK". The author has no experience in this regard.

Ray Pipkin