Monday, December 10, 2007

Saving SEG meter

Mission: Saving SEG speedometer

Code name: Road runner

Symptom: As refer to my previous post about my ever make best car, my SEG is running at around 140km/h at 5th gear with 3000 RPM. I hope it’s true, but I know there is something wrong with my meter.

Action: I had been asking around the spare part shop to get a new meter but none of them selling and they told me the meter is very expensive if I want to get a new one. The cheaper way is get it from car scrap. Searching around internet and forum but get none positive result. Finally talk to a mechanic and he told me that there is a PCB board behind the meter, since I’m best electronic engineer in town  I plan to save my meter with all the knowledge I have.

Unscrew all the screw on my car dashboard (total got 6 screws) and unplug the cables (total three cables), the dashboard is detached from the car. Everything looks fine on the flexible plastic PCB behind the meter. So I plan to open the plastic cover of the dashboard. Two screws and compression fitting (not sure how to call that kind of connection :p). Once the cover is removed, the meter is now exposed. There are four screws at the back of the speedometer, so I unscrew it and the speedometer is detached. Looking behind the meter, there is a PCB board. No cool solder was found, no burned component, also no lose connection. Almost give up with the saving mission. Suddenly I notice a black material on the top of the meter. Using a multimeter to measure the connection between the two points. A very funny reading come out, 1.4kohms. Finally I got it.

The resistor behind the meter is 3.9kohms, and the black material is “eating” the PCB track and leaving 1.4kohms. So the additional resistance is causing my speedometer running at around 40% higher speed. 1.4/3.9 = 36%, so that the source of the problem.

Since the track can’t be repair, I just solder a small wire to replace the broken track and the mission is complete buy putting back everything to the car.

Result: No need to say too much, my SEG got it correct speed again.