1998 V70R: loud popping noise on radio on inrush loads (headlights/fan switching on). Persists after repair attempts

[atomicthumbs]

My car's got a weird electrical problem i'm having trouble tracking down. Whenever something that briefly draws a large amount of current switches on (radiator fan spinning up, headlights or high beams heating up, and at seemingly random other times), the radio emits a deafeningly loud staticky pop/crunch noise. I know it's electrical interference and not RFI because it's happening on FM.

This also was triggered by playing loud music on the radio and cranking the volume up.

I suspect this is caused by a brief voltage dip, and not a problem with the radio, because I've also noticed that when the radiator fan kicks on, the headlights dim briefly. Any more accurate diagnostics about that will probably need me to hook the car up to an oscilloscope.

In hopes of fixing this, I replaced the battery under warranty, and replaced the voltage regulator. Doing the latter seems to have helped partially, but it still pops when the fan comes on.

Is there some kind of filter somewhere that's gone bad, or another part of the electrical system? Is the alternator itself going? Could this be caused by bad grounds? I'm starting to lose my mind here.

[wizechatmgr]

I'd have someone do an alternator test just in case you have a bad alternator. You could install a small capacitor, maybe 500uF on the power side feeding the radio and see if it helps.

[BEJinFbk]

Stock stereo or aftermarket?

[atomicthumbs]

Stock stereo or aftermarket?

Stock. It's an SC-901.

[Clemens]

I had a similar problem with an aftermarket head unit when the wires of the subwoofer shorted to ground because of dailing insulation of the large power wires. I know this doesn't help alot, but better than nothing

[foggydogg]

Could be something as simple as a bad ground - especially since the headlights dim when the fan starts. Chase all of them on the engine,
especially the one down on the subframe.

[scot850]

+1 to what foggydog is saying. If you don't have test equipment or know how to use it, some of the larger parts shops have electrical testers that can check your charging system and battery. Typically that is a free test.

The fact that the lights dim says something is really causing a draw on your system or a bad battery/alternator or cable/ground is at work. Also check the B+ cable in the kidney shaped main fuse box in front of the brake booster. It is the cable that cones up into the front of it and has a brass end with a brass nut on it. If these are not shiny, then check the B+ cable by disconnecting the battery first! and then undoing the nut and carefully lift the connector so you can see down into the wires below the connector. In my case it was completely melted and shorting out.

As you have a SC-901, it could be the unit or the amplifier is failing. Being a 98 I don't know if the amplifier is behind the radio or under the passenger seat. I have seen the one under the seat get stuff spilt on it or get damp from blocked drains getting the carpet damp and moisture into the amp. If you have a center dashtop speaker then you will have the one under the seat.

Good Luck!

Neil.

[BEJinFbk]

You may just be experiencing a symptom of low voltage.
I’d suggest starting with something simple like a voltmeter
plugged into the cigarette lighter socket and see what
your voltage levels looks like while you’re driving.
You can find inexpensive ones online.
Added Bonus:
Many also include USB charging ports.