What’s wrong with my power seats?

Question:

I finally purchased my first collector car, a 1986 Mercedes 560SEC, for only $1300. Among its many problems are power seats that do not work. I checked the fuse/relay box and all there seemed well. I decided to start with the passenger seat. I tested the power feed to the controller box under the passenger seat and it read 12 volts. OK there. I then tested continuity of the wires from the door-mounted user switch to the controller box. No resistance on any of the wires. OK there. I then removed the user switch mounted on the door, took it apart, and cleaned and reassembled it with some dielectric grease. I then applied 12 volts, bypassing the controller box, directly to each of the three seat motors. They all worked. OK there.

My conclusion, duh, is that the controller box is defective, so I re-soldered the electrical connections on the board, thinking some of the joints may have cracked over time. No luck. After seeing the price on a new controller, ($500), I have decided to have the existing controller repaired.

So, my questions are, did I miss something in my diagnosis and can you recommend a diagnostic/repair service for electronic circuit boards? I very much appreciate your consideration of my project and thanks for a fabulous magazine!

Answer:

As a starting point, you should make sure the fault is with the board itself and not the board’s input or output. Corrosion or a physical break in a wire or connector going to or coming from the circuit board could be your problem.

If the input and output are good but the board isn’t then you’ve got to take a different approach and it’s not particularly easy.

If you’re manufacturing circuit boards then it’s entirely feasible to set up a dedicated test station to evaluate the function of every component and circuit on the board but as an individual there is no simple way to do this unless the problem with the board can be seen with your eyes (a cracked board, delamination, a physical break in one of the circuits, separation of one of the components, etc.).

But if the defect can’t be seen with your eyes, you have a much more difficult task. You can use a multi meter to test the continuity of every circuit on the board but your first hurdle is to come up with a detailed schematic for the board, specifying the correct voltage in and out of each transistor and other component. If you come up with this and have the patience, tools and eyesight, you can systematically go through everything on the board to figure out what’s not working.

(Editor’s note: For more on power seat repairs, see the series “Restoring Car Seats” in the October-December issues.)