Originally Posted by was
Many thanks for all the feedback (no pun intended). This is one of the things I love about this forum. Now, being a completely inexperienced when it comes to electrical stuff, let me see if I understand some of what you all said:
1) Wiring the gps into an existing circuit probably causes the interference.
2) It is better to take power either directly from the battery (the supplied wire should be long enough) or from a relay (which continues to supply power when the engine is turned off).
3) If the power is taken from an existing circuit, a ground loop isolator has worked well to clear up the interference for some (jwd98056) but not for others (rixchard).
4) The ground loop isolator is wired into the audio line.
I hope I got that right. Now, two additional questions: if I take power directly from the battery, should I put a fuse in the power line? And what is a relay, and where do they live? Many thanks.
1) Wiring two devices from the same power circuit may or may not cause interference. In fact the grounding matters more than the power in most cases. Ground loops are caused when the voltage at the the grounds of two circuits are different for whatever reason. The cause of the voltage difference may or may not be related to the source of the power for the two circuits. When a ground loop condition exists in an audio circuit the shields of the audio cable provide a low resistance path for currents to flow between the two ground potentials. This current will include the noise that is always part of an automotive electrical system. This includes AC ripple from the alternator (alternator whine) and spark ignition noise. If the ground loop currents get large enough that noise will couple into your audio system and become a problem. There are two situations where the power source can contribute to the interference. One is if other devices are creating noise on the power circuit and the audio circuit is susceptible to that noise. The other situation is if a ground loop exists and the new device not only shares the power but the return ground wire as well. With additional current on the ground wire the voltage drop across the ground wire will increase and this could increase the effect of the ground loop. You can run a dedicated ground and power wire for the gps but that does not guarantee that a ground loop problem will be resolved. It is the sum of the whole and not the parts that matter.
2) This is a matter of personal preference so "better" is subjective. I personally have no problem with powering off the map light. If I am stopping to do map work and I want to stop the engine I just turn the key counter clockwise one step to the accessory position. The gps stays on and the motor is off. If I am refueling and need to remove the ignition key to unlock the gas cap then I have to turn the key all the way to the lock position and the gps is off. If I am refueling I don't have enough hands or eyes to be messing with the gps. Your mileage may vary. I also see no reason to use a relay if you want power available to the gps all the time. A direct hookup to a fused power source is all you need. If you want the power to the gps to be to be switched when the key is in the accessory position you will need to locate a circuit that operates the way you want your power to operate, such as the map light. If you want a dedicated power source you would tap into that circuit to control a relay you would purchase and install yourself. The control signal goes to one side of the control solenoid and the other side of the solenoid should be wired to ground. You would then run a fused lead from the battery to the common (C) or input contact on the relay. The output or normally open (NO) contact on the relay should then be wired to the gps or whatever it is you are powering.
3) As I have already stated the power source may not be the source of the interference. There are other sources of interference besides ground loops so ground loop isolators are not cure all solution.
4) Correct. Ground loop isolators are transformers and they block DC currents. To treat noise on the the power leads requires either a noise suppressor (commonly a shunt capacitor but a low pass filter is better) or ferrite beads or cores (to increase the the power lead inductance).
It is always best to have a fuse in the circuit. Unless you enjoy the smell of burning insulation
A relay is device that can open and close an electrical connection using a control signal. For mechanical relays the control signal is applied to a solenoid or coil inside the relay and the magnetic force opens and closes the primary circuit contacts. There are also solid state relays that use transistors and optoisolators to do the same thing. Relatively low power control signals can then control much higher power circuits through the switched contacts of the relay. There are a variety a relays on the LT and I believe most if not all are located under the gas tank. I'm sure there are LT gurus out here that would know better than I.
I hope this diatribe helps rather than hinders