VK Radio interference

[VK SL 3800]

Hi,
Have an annoying problem with my VK's radio getting a mild uneven crackling at idle and low revs, Doesn't matter if your on radio, CD or ipod it still does it but only with the engine running. Its not like your normal zzzzzzzz noise that changes with the revs but more of a clicking or chk sound. but doesn't seem to come through the sub, only speakers. Have had gearbox out recently and the Main engine harness connector apart, have not touched the coil or alternator, these may have been disturbed though.
Radio is an Alpine head unit about three years old.
Engine is a 202 still with EST.

 

[Luke0011]

Did this clicking or chk sound appear after the gearbox went back in and why did you have the main harness apart.. how loud is the crackling and does it disappear when driving completly and does it get louder when you turn the volume up as well as the sound distorting, and describe the system exactly. what amp and where are the speakers, in the doors or dash. Ive had this problem before and it can be a faulty unit or wiring but im leaning toward something caused by the main harness being apart. What exactly did you have apart.

 

[VK SL 3800]

Ok the clicking or chk sound started straight after i put the new gearbox in, it happens at idle, when you first start it up it takes a few seconds to start crackling sort of like static building up somewhere, it doesn't get louder as you turn up the volume, it get slightly worse and more frequent a little bit above idle then goes away when at normal operating revs. you can hold higher revs stationary and it goes away. I had the main harness apart because it was in the rode when i was taking out the speedo cable as i have put in a different gearbox, (tested with gearbox spinning in neutral or in gear with the clutch in, makes no difference). Speakers are JVC 6.5'' in the doors and 6x9s in the parcel shelf. Amp is a clarion APX 1301 420w, sub is a clarion sb12xb 600w. Head unit is a Alpine CDA 9856 I. It seems to be worse on the passenger side speakers to add more confusion. And i have had the main harness apart again to clean it up and has made no difference. System works fine on acc an with the ignition on and only starts to play up with the engine running.

 

[Luke0011]

ok i want to eliminate the amp as the cause of the interference. unplug the rca at the amp while off and turn unit on and test while engine running. if noise still there remove fuse of power wire at battery.
If noise still there unplug rca at the alpine unit as well as the trigger wire and the ipod lead. If still there then the amplifier can be eliminated as the cause of the noise. Which is what i was leaning towards anyway. Now i want you to unplug the antenna lead and test. Test again with engine running if unit doesnt power on then the antenna was earthing the unit and you need to run a solid earth wire and power wire to the battery directly. Re test again. if noise still present after test again disconnect all speaker wires and plug in a test speaker and have that sitting on your lap, now all you should have connected at the unit is two wires.... the yellow and red connected to the power wire you ran to the battery. And earth to the battery. As well as the speaker wire to the test speaker on your lap. Re test again on cd with engine running and check for static... if still there the fault is likely in the unit provided the alternator and battery get tested and working. Possibly a regulator or a noisy cell interference. Check power connections with a multimetre.
If i had it at the store i would have re tested with a brand new head unit from stock after doing the amplifier tests. That would tell me straight away if your unit is producing the crackling or the cars electrical system is. Check all speaker wires for shorts to earth or resistance to earth.
A parcial short could cause this problem.
sounds like a lot of work to test and find the fault but if you follow the steps carefully you will find what is causing the crackling.
Let me know how you go, cheers Luke

 

[VK SL 3800]

I unplugged the RCA cables, still noise, unplugged the amp at the fuse, still noise. unplugged the trigger lead & RCA cables at head unit end, no good. Wasn't being earthed through the antenna, unplug all speakers and test for any earth connections- all good. Couldnt find enough wire to run direct battery connections so i plugged in the original radio to the same wires and couldnt fault. no static at all. Must be the head unit...... but why would having the engine running or not effect it? i would have though that if it was the unit itself it wouldn't matter. The static was even changing speed with the engine revs.

Thanks for your help. Ben

 

[Luke0011]

Because the alternator only spins when the engine is running, the faulty noise filter inside the head unit which is designed to prevent cracks and whips and whine when the alternator is spinning. When the alternator is not spinning then the cracks and whips and nonsense cannot be caused as the power supply is pure DC. The alternator only produces rough DC voltage which the battery smoothes to an extent with the final filtering done by the head unit. If the filter is faulty then the unit is going to do itself a mishcief.