I noticed something last night coming home from a car show.
with the fm radio on and tuned in on a moderate to weak fm station, every time I put the brakes on, the fm station would hiss. Then I noticed it all the time when I put the lights as it was getting dark.
I then tuned it to a different more local station and I didn’t seem to have the issue.
I can also blue tooth music to it with no issue, thus by- passing the windshield antenna.
I have LED lights in the car and on my brake lights, if this makes any difference.

Mia there something I can do to stop the hissing noise?
Jeff

 

You can get a mix 31 RF choke and install it on the antenna cable and 12VDC wire to your radio. This would help mitigate spurious RF noise.

https://www.dxengineering.com/search...eering-ferrite

Also, make sure your radio is properly installed and grounded per the manufacturers specifications.

 

If the noise happens when you increase the load on the alternator it is probably a "ground loop" of some sort. Even low positive voltage variations are called ground loops for some reason but the troubleshooting is still pretty straight forward. First thing to do is make sure your alternator has the same ground/voltage potential as the rest of the car, including the radio.

With the car running, use a volt meter and measure for any voltage difference between the engine block and the alternator. Set the meter to 20 volt range with the red lead to alt and black to engine, and then measure the reverse. If there is any voltage either way you need a ground lead from engine to alt body. Most people don't think about it but when an engine gets detailed with nice clean paint on all the brackets, it insulates things electrically. Also make sure the engine is grounded to the chassis and the body with clean, tight connections. The entire vehicle must have the same voltage potential for everything to work well.

Next thing to check is the antenna base. This needs to have a clean and tight connection to the fender. The fender creates a ground plane that isolates rf signals from the inside and outside of the vehicle. You cannot rely on the connector at the radio for this.

Last thing to do is make sure the radio is grounded to the vehicle properly.

These simple things will cure most radio noise problems .

 

Great ideas, I will check them out.
thanks

 

Does your ‘75 have a fender mount antenna or an in-glass windshield antenna?

 

Absolutely. Low quality LED lamps ESPECIALLY in combination with an after market stereo radio system with poorly designed audio (amplifier) driver(s). Try different LED lamps. There ARE significant differences in quality.

 

I found a loose ground on the back of the engine where the flexible braid from the fire wall attaches to the engine. It appears to be a bit better. I forgot I had run an alternator ground back the same location under the same bolt where it attaches to the head. Ordering some chokes as well.

 

Critical you have solid grounds from battery>engine block, battery to chassis/frame (next to radiator shroud upright) & as you noted rear of engine head to firewall. Ground contacts should be scraped/sanded so cable wire terminal ends and mounting points are direct metal-to-metal - no paint, no rust, no dirt, no grime. Ground contacts are the most important factor for any/all electrical on the car - this can't be overemphasized.

 

All good points about grounding and bonding the engine, antenna, etc. Use this on the ground points once you burnish them.

https://www.summitracing.com/parts/fil-as11

 

Here are ground points I know of and have found on my 75 Cutlass

1. Main negative battery lead to engine block
2. 2nd black lead from battery to drivers side fender
3. Rear of passenger side head (firewall braid to head)
4. Ground braid from head to firewall
5. Rear trunk, one black wire on either side latch ( probably for lights)
6. Radio ground
7. Gas tank sending unit ground to frame was cleaned when I replaced the tank and sending unit 3 months ago.

note: all lights etc are working with no issue

I didn’t find one to the frame, there are probably more that I haven't found, any suggestions?

 

Do you have an aftermarket radio? If so, I bet the install instructions note a grounding lug on the back of the radio that should be tied to the negative battery terminal.

 

Jesse - I thought same from initial post. It's most HIGHLY likely the OP has an aftermarket radio:

 

Original am/fm radio , antenna in windshield

 

Holy Guacamole. I would have never suspected an original OEM 1975 AM/FM radio was capable of blue tooth pairing.

 

Even for OE applications there is bonding that is done under the dash. I'm not sure if that is covered in the Chassis Service Manual, or...

 

Question: Is there an ancillary device you have to install/plug into the OEM radio which allows you to achieve bluetooth pairing?

I'll answer my own question.

Yes. There has to be an ancillary bluetooth adapter plugged into a 1975 OEM am/fm radio to achieve bluetooth pairing.

Aside from ground(s) connection(s) as well as specific LED lamp quality (they vary significantly), providing a dedicated & independent external power source (not tied to your battery as primary power source) they sell small external power sources you can employ which removes the car's battery power as the source of possible interference and instead use a separate dedicated external power source. Tough calls from a desktop armchair when providing best workable solutions.

 

Spot-on regarding the under dash bonding.

 

If you are still using the windshield antenna, the screws that hold the base to the cowl will oxydize reducing the electrical ground connection. That connection is what grounds the coax shielding of the antenna connector that plugs into the radio. You can loosen and re-tighten those screws gently and that can resolve a lot of reception issues. If you are using an antenna injection module for the bluetooth connection that antenna ground will be particularly important.

 

Great point(s).

 

What may or may not be notable is this hissing develops only w/ the headlights turned ON. I haven't put my head completely around this other than to suggest this is apparently isolated to the headlamp circuitry. You have LED headlamps? You stated:

You don't say if your headlamps are LED.

 

Reading your scenario (again) you have this hiss when applying brake LED brake lights & w/ headlamps turned ON. If you have LED headlamps both scenarios speak to LED lamps. Yet, it certainly speaks to one specific circuit and that's your lighting circuit. I'm leaning towards quality LED lamps.

 

I have device that plugs into the cigarette lighter, tunes to 107.7 I think. Blue tooth through it. No issues.
I will check the antenna screws.
Mom leaning towards LEDs

happens on braking or when I pull the running lights on.

I bought LEDs through Super Bright LED.
All good points everyone.

 

As did the OP, I purchased a gizmo some years ago that plugs into the cigar lighter socket and transmits audio to your car's FM radio (setting both the gizmo and radio to the same frequency) and will pair with your phone via bluetooth. Not just audio, but hands free phone calls too (using its built-in mic). Cost me ~$13 at Wally World. I've used it in multiple cars that don't have BT built-in. Best gizmo ever.

 

Yes, understand; yet, it doesn't address the OP's issue regarding static/hissing only when brake lights &/or headlamp switch is turned to ON.

To the OP. There's certainly a chance the LED conversion wasn't wired exactly as described. Just saying it might not hurt to examine conversion wiring. Pretty sure you've narrowed it down to the lighting circuit - so, what's changed (maybe)?

 

Head lights and front blinkers are incandescent bulbs.
interior lights , rear brake and running lights and side marker lights are LED

no wiring changes, just removed the old bulb and installed LED bulbs.

I will check the windshield ground today
I hope the chokes will work so I don’t get into replacing the bulbs again.

I may put the 1157 bulbs in the rear brake and tail lights and try the brakes to see what happens.

 

Update today
i cleaned the windshield antenna screws and mount surface. Appears to bring in more clearer stations sitting my garage. This is good. Screws and surface were corroded.

Still hissing on weak stations when brakes applied though.
New installed chokes installed on coax and 12v feed at the radio ,no change
I changed my super bright red LED bulbs back to incandescent bulbs on the back. NO hissing when brake applied on weak stations !!!
Also tested this with the lights on with the same weak station even though my side marker lights are LED, no noise. (Note: front blinker bulbs are incandescent as well as the head lights.
So my noise is coming from the 4 1157 super bright LEDs at rear.
Now to try figure out good Red LED 1157 bulbs that don’t cause this problem.
Interestingly enough when I had my trunk open with the same radio station on, my radio hissed with the trunk lid LED. Not worried about it, but interesting.
Thanks everyone for your suggestions. My radio is working better and I have found the issue and also fixed a loose ground at the back of the engine, All this has to help.
Jeff

 

Good that you got the problem fixed. Sorry the RF chokes didn't help.

 

A few years ago I read about folks installing LED bulbs in their garage door openers and the remote controls quit working. Apparently some LEDs generate so much EMI it interfere with the wireless communication.