There are five main issues that cause this problem:
- Poor ground or shared ground with other electronics. Move the ground to a clean metal point without other parts, or ground the radio directly to the negative battery terminal. Check the ground from engine to firewall.
- Radio voltage is not stable. Voltage too high or too low can cause issues. Test to make certain both power wires are getting at least 12 to 14.9 volts. Use a digital multi-meter, not a test light. Make sure both the RED ignition and the YELLOW constant do not have spikes or drops in voltage at anytime. Test when the engine is running and not running.
- USB cable routed near or over ignition, tach or high current wiring. Make sure the USB cable(s) are not wrapped up with the ignition harness and are routed away from the vehicle’s ignition switch. Do not coil extra USB cabling.
- Old vehicle wiring. Remember that in most cases, we are dealing with vehicles that are over forty years old, with forty year-old electrical systems. Older ignitions — such as Lucas wiring — and many other older vehicles simply need to have updated plugs, wires and caps installed. Solid core spark plug wires offer almost no resistance to RF interference; braided plug wires are best.
- RF (radio frequency) interference caused by HEI or MSD after-market ignition systems.
- MSD: Make sure main control box is isolated from the firewall (MSD ignitions come with rubber boots for the module. Make sure to use them!). Also make sure it is not sharing the same ground point as the radio. An MSD 8830 ignition coil filter is often effective at isolating the RF interference.
- HEI: If your ignition is too advanced it will cause premature spark, creating RF interference. Adjust the ignition so that it does not fire prematurely. Some vehicles with HEI ignition need RF shielding on the ignition part. Another alternative is to put shielding — such as ferrite coils — on the radio’s USB cables.
- Inspect HEI: Check for cracked or loose distributor cap. Check for carbon build up. Check rotor for burned black spot on wiper, or for pits in the surface. Test the coil to make sure it is functioning properly.
Because our radios feature an external USB cable on a three-foot umbilical, the USB cable itself can act as an antenna for RF interference. Some vehicles simply either have too much RF interference or other wiring issues to be able to use a unit with external USB inputs.