Join Date: Mar 2016
Location: Lake Mary, Florida
Re: Burnt tachometer
I would start with that loose and melted junction point, then troubleshoot further, if needed. You just may have found the issue. However, the tachometer circuit board will need to be replaced or repaired. Not sure if you could simply identify the burnt resistor ohms value and solder in a replacement. I see in the picture there is a chip, a diode, and other circuit components that may have cooked due to that resistor getting hot. Without a schematic of that circuit board, its a guess.
As to your question on circuit breakers and relays, relays work as a high current switch, and a circuit breaker works as a high current fuse. Because the head light doors require more current to operate, a relay and circuit breaker are used to protect this electrical circuit. Typically, anything requiring 20 amps or more can use a relay and a breaker. A relay uses a low current "trigger" (the head light switch in this case) to open/close the relay. The relay will have a large gauge wire connected carrying voltage (current) to the 'input' of the relay, and the circuit breaker is protecting this. A smaller, lower current gauge wire (from the head light switch) will operate the contact in the relay, and allow the high current 'input' to be switched to the high current 'output' of the relay, to the headlight motor.
A switch, such as your headlight switch would not be able to carry the higher current load of the headlight door motors without burning up its contacts quickly, so relays and circuit breakers are used.
Never argue with an idiot; they'll only drag you down to their level, then beat you with experience.