s.o.l. meter
Car Modification

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the s.o.l. meter

 - overview
 - car modification
 - meter design
 - meter input

I. Overview of Objectives and Process

    The goals of the car modification are straightforward and simple: insert a wire to transmit the voltage across the speedometer itself to the s.o.l. meter. The voltage serves as the input to the s.o.l. meter.

    The car which will be modified is a 1996 BMW 318ti, seen in Figure 1. To add the s.o.l. meter, all that is needed is the voltage across the speedometer. This involves three basic steps:

    1. Remove and disassemble the instrument cluster.
    2. Attach a wire to the positive and ground inputs of the speedometer.
    3. Reassemble the instrument cluster and reinsert into the vehicle.

    Once these steps are complete, the s.o.l. meter only needs to be attached to the input leads.

Figure 1. 1996 BMW 318ti.

II. Removal and Disassembly of Instrument Cluster

Removing the instrument panel from the car was done in eight simple steps:

    1. Turn off car.
    2. Unscrew instrument cluster's two Torx mounting screws.
    3. Pull instrument panel onto steering wheel.
    4. Turn car on.
    5. Rotate steering wheel one radian clockwise.
    6. Disconnect car battery.
    7. Disconnect three connectors from back of instrument cluster.
    8. Wedge instrument cluster through gap between wheel and center dashboard.

    Unfortunately, during the actual procedure, the order of the procedure was not followed correctly, resulting in the "actual procedure" taking approximately 74 steps. The removed instrument cluster can be seen in Figures 2 and 3. In Figure 2, it should be noted that the position of the speedometer is not in any way indicative of the normal position of the speedometer. Under regular operation, the speedometer should be deflecting at least 180 degrees further clockwise. Once the instrument cluster was obtained it was brought to a well-lit area.

Figure 2. Front view of instrument cluster.

Figure 3. Rear view of instrument cluster.

    On the back of the instrument cluster, a row of white holes with black lights in them can be seen. These are the LEDs that light the warning lights on the front (air bag light, seat belt light, check engine light). These can be removed and replaced easily by twisting the LEDs and pulling them out gently. The larger green LEDs are the light sources behind the speedometer, tachometer, gas meter, and engine temperature meters.

    To disassemble the instrument cluster, five Torx screws had to be removed from the perimeter of the back of the cluster. After this, the front and back of the instrument cluster was pulled gently apart into two pieces. At this point, any form of warranty covering the panel was verifiably voided. In Figure 4, the rear side of the front end of the cluster can be seen. The leads of all the analog meters can be seen, and the positive and ground leads of the speedometer are indicated.

    The speedometer can be assumed to function in the same way as a d'Arsonval meter movement or a variant thereof. A quick explanation of the functioning behind the d'Arsonval meter movement can be found at 'All About Circuits' [1]. When a positive DC current is applied to these two terminals, the meter reading deflects. As the current increases, the meter deflects further. It was determined by observation that the speedometer (and all the meters on the instrument cluster being studied) were overdamped; however, this knowledge is irrelevant to the project at hand.

Figure 4. Speedometer leads.

III. Modification of Instrument Cluster

    Once the positive and ground leads of the meter were discovered, the next step was to create a connection to them. To accomplish this, two wires from a CAT5e cable were used. CAT5e was chosen because of its resistance to wear and tear, its shielding, and its flexibility. The cable was cut open at one end and the green and white-green twisted pair were used. The wires can be seen in Figure 5.

Figure 5. CAT5e cable.

    Simple soldering was used to connect the two leads of the CAT5e cable to the speedometer. Since the leads of the speedometer are inserted deep into the back panel of the instrument cluster, there was very little room for the CAT5e cable and care had to be taken to solder it as close to the base of the lead as possible. Theoretically, it would have been very simple to attach leads to all of the other meters for other possible projects; however, that could be done at a later time just as easily so there was no pressing need to do it during this project. With the leads connected, the next step was to create an escape path for the CAT5e cable.
    Due to the assembly of the instrument cluster, there is no "hole" in the back which a cable could be threaded through easily. However, as can be seen in Figure 3, there are holes along the bottom where LEDs are inserted for control lighting. Upon some inspection, it was found that there are several LED holes whose lights correspond to no control (which means they will never be used). One of these holes was chosen to thread the CAT5e cable through. However, the casing around the LEDs are flush with the front of the instrument panel, meaning there is no way to thread the cable through and close the cluster completely.

    Fortunately, by using state-of-the-art technology such as diagonal cutters and pliers, an area of the LED casing was removed, creating a viable pathway for the CAT5e cable to run through. This professional, sophisticated, and sleek solution can be seen in Figure 7.

Figure 6. Soldered CAT5e cable.

Figure 7. CAT5e pathway through LED casing.

IV. Reassembly and Reinsertion of Instrument Cluster

    Once the CAT5e pathway was created, the CAT5e cable was threaded through it, both pieces of the instrument cluster were put together and secured again. The back side of the reassembled modified instrument cluster can be seen in Figure 8. With this complete, the instrument cluster was reinstalled into the car, following the procedure outlined earlier for removal in backwards order. However, the addition of a CAT5e cable presented a challenge. The cable was threaded through the unused foglight control on the right side of the dashboard and left to dangle for accessibility in making measurements to establish a speed-voltage relationship. The modified dashboard can be seen in Figure 9.

Figure 8. Back side of modified instrument cluster.

Figure 9. Modified dashboard.

V. Inadequacy of Original Modification and Improved Modification

    After the car modification was complete and the meter assembled, it was discovered that the voltage-to-speed relationship was nonlinear. This presented a large problem, and more on that problem can be found at [1]. Eventually, it was decided that the leftmost lead on the speedometer may contain useful information, so the instrument cluster was removed and disassembled once again. Another wire was soldered to that lead, which can be seen in Figure 10. Then, the instrument cluster was reassembled and reinstalled into the car.

Figure 10. Soldered CAT5e cable with extra lead.

VI. References

[1] http://www.igglybob.com/projects/sol_meter/speedometer_input.php