Saturday, February 8, 2014

Porsche Signals

The other weekend, we started working through some issues we were having with the signal lights on the Porsche. We were having issues where the individual signals would only turn on when the stalk was moved in a direction, and the hazards would flash once or twice and then stop responding.

Low voltage electrical relays. Picture taken from driver's seat.

As we delved deeper into the vehicle, we remembered that the vehicle is 38 years old and even though it came to us in good condition, it is hard to stop the aging of the electrical terminals.

Note the coloring of the contacts.

The signal/hazard relay is the square box located on the bottom of the picture. We thought our signal issue was an issue with the signal relay, and we found a replacement online. It should be a plug and play type of situation. Right out of the box, we noticed that the original was significantly heavier in weight, and about 80% of the size of the original.

Original on left, replacement on right.

The replacement relay physically fit quite well, and we powered up the low voltage to test out the system. Interestingly, the signals would flash very slowly (~2 seconds on, ~2 seconds off). The hazards also had a different frequency. While this might satisfy the Massachusetts state inspection, we were concerned that it wasn't fast enough to catch another vehicle operator's eye.

After scratching our heads for quite a while, we remembered that the signal lights were replaced with LED lights a few years back. LEDs (Light Emitting Diodes) consume much less power, and are great for a variety of applications. About the only environment that has had some difficulty adapting to LEDs is the traffic light and runway light environments. Since LEDs produce so much less heat than conventional lighting sources, ice can build up on these surfaces and either block the light or change the light refracting properties of the lens such that the effectiveness of the lights are diminshed.

For us however, if we applied the RC Circuit concept to the system, the change in the headlight source may have been changing the dynamics for which the relay was designed to switch with. Thus, we switched back to the original relay and used the opportunity to take a look inside.

The relay is simple in concept, relying on the magnetic field created by an applied voltage to close another set of contacts and close the circuit. A solid state relay is traditionally viewed as being more energy efficient since it does not require the currents needed to hold the contact closed. For reference, efficiency is the output per unit of input. Since the mechanical relay (as shown above) used some energy and arguable produces no work (output), it is seen as a negative efficiency influence. Solid state relays have some pitfalls when used for certain applications though. Feel free to contact us and I should be able to elaborate on this in some detail.

Using a lab bench, the relay worked perfectly fine. However, we noticed it was hard to get a good connection through the 38 years of age. We never considered this as a potential problem being that the contacts should not have moved within the housing, therefore the connection should never have been broken. We were wrong.

Using microgrit sandpaper (CAMI-600 in this case), we sanded down the male ends of the connectors and reinserted the original relay. Everything worked fine, the timing of the hazards/signals were within the expected range (~0.75 sec on/~0.75 sec off), and both the hazards and signals had the same frequency.

For us, this was surely a learning experience in that we shouldn't make assumptions without some verification, and that we should always check our contacts before changing parts.

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