I hope I'm in the right thread but if not forgive please. I installed a Pioneer stereo last week and notice on weaker stations that when the key is in ACC the reception is perfect, however when the key is turned to on (motor not running) the reception cuts out almost completely? Turn the key back to ACC the reception is perfect. I checked all wiring, grounds, antenna etc and all is in order. Took it to a pro installer over the weekend and they were completely lost too? I figure it has to be something in the ignition wiring to cause this but really at a loss. Anybody ever come across this problem with their stereo? Thanks maxx

 

I would guess that when you turn your key to the ON position that the PCM and other computer type systems are firing up and emitting some level of RFI. I'd be tempted to run a power line direct from the battery to the radio, at least as a test. You may have tapped into a "noisy" 12V source.

 

thought about so I did run a line direct from the battery to the head unit. Same affect, as soon as I turned key to "on" the interference kicked in. I agree about the PCM because there really is nothing else on? No accessories etc are on. Had 2 thoughts, one was to start pulling fuses to see if I can get the interference to stop while having the key on and the secons was to put a antenna filter in line between HU and antenna. Never had any experience with antenna filters and dont know if they work or not. thanks for the reply...Maxx

 

I used to deploy low pass filters often back when the CB radio was pretty much standard issue. Maybe someone smarter than me might have some insight.

 

Try this -- leave the key set to "on" for like a minute and see if the interference stops at around 20 seconds or so. The fuel pump can cause noise in some cases.

 

Tried that too. I thought it might be the glow plugs also, but this happens all day long, well after the pump and plugs turn off. May be a bad ignition switch or poor switch. Got me baffled..Maxx

 

Have You tried Direct grounding the radio to a good ground source? Idk, just a thought.

 

I have the radio grounded directly to the battery post. Same affect. Think I will try putting a noise filter on the antenna line and then a booster. Got nothing to lose....Maxx

 

Try isolating the radio power source completely from the truck. IOW, run it off a power supply from the wall or a separate battery to see if the noise is coming from the truck power supply or being picked up by the antenna. If it's being picked up by the antenna, then the noise filter *might* help...

 

That is a good idea. I will give that a shot.......Thanks Maxx

 

Have you ever pulled PCM? Installed a chip? How close is pyro probe wire from radio wires/components?

 

One additional test. Take a small hand held radio and hold it near the heater control area. I found that in my 2000 F-250, something in that unit is emitting RFI (radio frequency interference). My problem was a pulsating noise at the low end of the AM band. It was being picked up from the truck radio's antenna. (By pulling the antenna cable out - the noise stopped. Yes, I made sure that the antenna cable was in tip-top shape.) I wasn't able to solve the problem. Good Luck.
BTW 30 years as a broadcast consulting engineer did help me!

 

The only thing I know of there is the GEM/VSM (depending on model). You have a GEM, I have a VSM. Not sure what the OP's truck model year is (02s & 03s have a VSM), but I haven't had noise issues on AM (I listen to 570 here in Dallas often). I also am an Amateur Radio Operator, and have other noise issues that I've minimized on other bands, but still battling, but nothing on the BC bands.

 

I looked in the O/P's profile and found that this problem may be in a 1996 truck, so all bets may be off in what I write below!

There are a number of square-wave signals running around our trucks.

Square waves are defined as being made up of "an infinite number of odd harmonics", meaning that some of the radio frequencies that may be "broadcast" from these signals could fall into the broadcast radio spectrum.

The higher-voltage pulses from the IDM to the injectors, while in a shielded cable, could be a source, but would be variable in frequency depending on engine RPM and completely absent if the engine is not running.

The CPS is another example of a square-wave signal, but is very low-level and again absent if not running.

The most likely source is the Data Link Bus, which is the square-wave digital communications link between all the on-board computers, as well as running to the OBD-II connector. This is why bobmiers (above) found the noise at his heater control unit. The OBD-II connector is directly below this controller. This bus runs anytime the ignition switch is on, engine running or not.

Ideally, that bus should be a twisted, shielded pair, but I don't know that it actually is. The wiring diagrams don't show it as such, and if it's not, will be a real source of radio interference.

It is shown on Page 1 of the Engine Controls wiring diagram, and runs from the PCM to the APCM (Auxillary PCM), to the PATS (if equipped) system, to the OBD-II connector.

Probably the easiest place to determine if it's a twisted pair would be at the OBD-II connector, and it will be an TAN/ORG on Pin 2, and a PNK/LT BLU on Pin 10.

Good luck, as running down RFI problems can be a real pain, especially if you aren't well-versed in radio theory.

Pop

 

Good point, Marv. Forgot about the OBD-II being so close. I also have a ScanGauge running off mine, and don't notice any abnormal interference on the AM band. Or FM for that matter.

This may help, so I'm throwing it out there... I do notice some on VHF (around 146 MHz), and some on HF (around 7MHz). The 7 MHz stuff I'm sure is injector (IDM) hash. I believe the noise on VHF is from the data bus, since it's there with the key on, engine off. 7 MHz noise is only there with the engine running.