screen alternator whine caused by pc fan!?

m60power · 04-29-2008, 12:46 AM

I have been trying to get rid of the alternator noise on my xenarc 700tsv forever, it's really starting to **** me off. It is in the form of horizontal lines across the screen that change frequency with engine rpm, and a faint whine through the screen itself. (there is no interference with the engine off). The odd thing is what causes the interference: I recently added a molex powered fan to my computer because it runs too hot and the fan is what seems to cause the noise on the screen. Without the fan plugged in the alternator noise goes away. I am trying to get rid of the screen noise without removing the fan. This doesn't make any sense to me how a fan causes interference to the screen. What is going on? I imagine there is a ground loop somewhere?

Here is the basic setup:
The screen is powered by a 12v cigarette lighter source and connected to the pc via vga and usb.The pc is powered by an inverter that is hooked directly to the battery. Originally I had alternator whine through the speakers but I solved that by grounding the pc case to the car chassis. I have tried grounding the ground of the screen power at the battery, and at different points on the chassis, and it still doesn't solve the problem. Also the rca audio cables have nothing to do with it, if they are unplugged, there is still noise. getting rid of the molex powered fan is the only thing that gets rid of the alternator interfence on the screen.

Any ideas on how to get rid of the interference without removing the fan, because it gets really hot in my trunk.

dMand · 04-29-2008, 01:45 AM

i'd say your best bet is to get rid of the inverter, and shop for a dc-dc power supply. it will save you from those noisy headaches in the long run.

m60power · 04-29-2008, 12:04 PM

I know I should be using a dc-dc power supply, and I will get one eventually, but right now I'm am sticking with the inverter. Also, I don't see what it has to do with using an inverter. The inverter is properly grounded and the case of the inverter is grounded. The problem is with the pc itself and when I plug in that fan. Without the fan plugged in there is no noise/interference/alternator whine whatsoever through the screen.

I am trying to figure out how to eliminate the noise without removing the fan. Since the interference is alternator/engine rpm noise, I imagine it is a ground loop, but what does a fan have to do with it?

JimmyFitz · 04-29-2008, 01:08 PM

Try connecting the fan to the 12V from your car's battery switched lead instead of the PC PSU to see if that helps. Fans won't care about the voltage being a little high or low anyway.

m60power · 04-30-2008, 12:17 AM

Hooking the fan up to the car is a good idea...but I resolved the issue another way. Instead of powering the screen through 12v dc, I used the screen's ac adapter and plugged it into my power inverter and now the screen noise is completely gone. I really didn't want to use that method becuase it required ripping the car apart again in order to run the power cable from the screen to the inverter.

lukec · 04-30-2008, 10:32 AM

I had the same issue, and the same resolution is working for me in test scenarios.

It's strange to me that the inverter would fix it because video equipment is supposed to want a true-sine wave which the 12v supposedly puts out while an inverters usually put out a modified sine wave.

I'd say that based on that, the cigarette adapter isn't putting out the true-sine.

Just a guess though.