RFI driving me nuts

DoTheDew · 07-22-2012, 08:17:11 PM

I've got a problem with RFI. I've always had it, and it wasn't really an issue until now. I have had it since the car was bought. After start up, once reaching 2000 rpm or so (this is when the alternator comes to life), I can hear the rfi through the radio. Didn't really bother me.

I then installed new Equus gauges. This did not agree with my tachometer. It would flip out at random, work for a while, and usually under hard acceleration, it would flip out, dancing all over the scale. I tried changing coils (since I had a high output hei), to a stock coil, and this seamed to fix the tach problem for the most part, but the radio interference was still there.

Now, I've moved into the world of nitrous. For this, I installed a digital window switch (summit brand). It has a digital readout display for running rpm, and upon reaching a certain rpm, completes a curcuit for the nitrous to work. However, after installing this, (without nitrous installed), it will work normally for the most part, but will randomly 'flip out' like the tach would, stable at a 6-700rpm, then all of a sudden 5600rpm, then back to normal. This causes a problem, because it could accidentally trip the nitrous before it should.

I've tried moving the tach wire away from the distrubutor as much as able, and this seems to help, but I can't seem to rid of the problem. I've disconnected the alternator, still get it (tach flipping out, rfi noise through radio goes away though). I've changed coils, wires, modules, caps, whole distrubutors. Everything has a good ground (including tach and window switch) but I can't seem to rid of the issue.

Help?

Mr.Bill · 07-22-2012, 08:28:15 PM

First off you need to find out if it is alternator whine or ignition noise, Two different things. Alt whine is a steady whine that increses with rpm, Ignition noice sounds more like a plug firing(tick tick tick). In most cases you can get a ground loop isolator from a good car audio shop that can fix it.

Gary S · 07-22-2012, 08:58:44 PM

What kind of plug wires are you using? What kind of plugs are you using? Do you have the filter capacitors in place that the factory used?
I don't know where the filter caps were on HEI, but coil ignitions have one on the coil.
These little things all help.
Many aftermarket radios are not filtered adequately, so you might need additional filtering at the radio itself.

DoTheDew · 07-22-2012, 09:55:51 PM

The Rfi through the radio is the alternator. It changes with rpm. Disconnecting it rid's the radio problem, but no the tach problem. There is no arching/snapping noise evident at all.

Plug wires are summit replacement 8mm spiral wound wires. I swapped them with an oem 7mm set, issue still present.
Plugs are R45TS

Filter Capacitors? Where? Assumed HEI doesn't have one. There is a capicator next to the module in the HEI, but that was swapped out with no difference.

Radio doesn't bother me so much as it effecting my tach and window switch. I've tried everything I can think of, shy of wrapping the tach wire in tin foil :P

**77RS** · 07-22-2012, 11:57:01 PM

The tach filter is located on the firewall. It is an RFI filter and has no function in the operation of the tach itself. If your car wasn't originally equipped with a radio (or a tach), it probably doesn't have a tach filter.

Sounds like you have a 78-81 tach, correct? If so, the tachometer circuit board is most likely damaged. Also, if it jumps very quickly from 700-5600 the bi-coil meter needs to be redamped. I can make the repairs in my shop. Just let me know if you would like to send it in.
Thanks!
Daniel

DoTheDew · 07-23-2012, 09:25:43 AM

Yes, the car is an 80. I'm not sure it was originally equipped with radio or not, assuming it was, since the windshield (which is held in with original butyl) has an antenna built in. It was not equipped with a tach, hence why I added the Equus tach. It is not just the tach that jumps, its also the digital readout on my window switch that does the same thing. I suppose both could be damaged, but if the tach and window switch both 'spaze out' like that, it tells me there is some sort of interference issue, no?

usmcz28 · 07-23-2012, 10:20:15 AM

How are your engine/body/frame grounds looking? You may want to see if any are corroding or missing.

If your missing any of these you should replace them and this could help with most of your issues.

DoTheDew · 07-23-2012, 11:21:07 AM

Tried that too. Body to engine, frame to engine, and engine to battery. Still nothing changed.

usmcz28 · 07-23-2012, 12:50:12 PM

You may want to either upgrade your ground wires to a larger gauge wire, move the ground you have for your radio. There are several kits available on audio websites to upgrade the grounds, you can't go wrong with a larger ground.

(Is it aftermarket?) If it's and aftermarket unit wire a ground to the chasis of the radio to ground, some have crappy internal grounds. Also if you're using RCA cables you could get an RFI filter for them. Make sure your power and audio cables are not near each other, that will induce noise into the audio cables.

As far as the gauges I say check your connection or like said above regarding the gauge set itself. Rebuild or replace