Alternator Whine in Crown Vic

[AciDFluX]

Hello,

I have just installed a new larsen trunk lip motorola type mount on my 06 Crown Vic. I'm using it for my Harris P7100 EVC on the San Antonio 800mhz EDACS system. I have noticed the alternator whine while sitting at idle and acceleration/deacceleration. I have ran the RG58 from the trunk through the fender down to under the rear seat then going under the carpet to the transmission hump then up to the console. If I disconnect the coax from the EVC then noise goes away. Any ideas what to try next as i've purchased some ferrite chokes to see if that would help but it didn't.

Thanks

 

[rescue161]

It could be a loose ground from the alternator to the frame of the vehicle. I had a car that did this exact same thing. I was never able to stop the whine on my scanner, so I made a short jumper, which omitted the shield of the coax. This was YEARS ago and I'd never do this today, but back then I didn't know any better.

More recently, I changed my setup in my truck and it started whining, then my alternator bit the dust. Upon replacing it, the whine went completely away.

 

[prcguy]

More than likely its a ground loop problem and not noise being picked up by the antenna, especially at 800MHz.

Try removing the antenna mount from the trunk lid (but with whip connected to mount) and isolating the mount electrically from any metal and test again, I'll bet it will work fine.

If so there is a difference in DC potential between the trunk and whatever you have the radio grounded to and there is current flowing on the coax shield trying to equalize potentials.

Usually if you have the ground lead for the radio running directly to the battery this problem is not as common. A large capacitor between the hot and ground leads of the radio can usually smooth out the problem but its better to eliminate the ground loop. 20,000uF of capacitance will probably remove the noise on receive but if its there on transmit you might have to use a much larger cap.
prcguy

 

[rescue161]

I tried filter capacitors, tightening the grounds, moving the antenna, but nothing worked. The only thing that did work was doing what the OP has done, except I used the center conductor only.

Looking back now, it was probably a bad diode in the alternator causing the DC produced to not be as smooth as it should have been.

If there is a difference of potential, you should be able to see it with a meter. Do you have access to a meter or an o-scope? If you have an o-scope, I'd see if the DC being produced is clean or dirty (lots of ripples or more AC in nature).

 

[Skypilot007]

Those crown vic alternators loose rectifier diodes commonly along with lots of other cars with similar alternators. Another thing to look for is if using trunk lip or even if you drilled a hole in the trunk you need to bond the trunk to a good chassis ground. The trunk hinge mounts are all painted before assemble and some cars don't get a good ground on the trunk lid. If you have an intercepter model most have it from the bonding done at the factory.

 

[AciDFluX]

The trunk lid has grounding straps on the trunk lid from the factory. The power & ground cables are coming directly from the battery. I also have a VHF mobile installed and it doesn't have this issue at all and it's wired identical.

 

[jim202]

Before you all go to all sorts of efforts to eliminate the noise, it might be worth the effort to take a digital volt meter and do a few simple checks. Put the meter in the DC meter scale and measure the voltage with the engine off and not having run the engine for a while. Your looking for the at rest battery voltage. I would expect it to be about 12.5 volts or so.

Next, start the engine and measure the battery voltage again. You should see something like 13.8 to 14.3 volts or so.

Next, turn on the headlights and again measure the battery voltage. It should be near the same voltage as when you first started the engine. Now put on the high beam headlights. Measure the battery voltage. You may see a slight voltage drop from just the low beam headlights.

The next test is to now turn on the heater or air conditioner with the high beam headlights still on. here is where the good alternator and a bad alternator will part the voltages. If the battery terminal voltage has dropped down much, I would expect that you have a bad alternator.

While we are at max load, change the digital volt meter to the AC range and go into the milivolt range. Again place you meter across the battery terminals. You should not see very much in the order of an AC voltage with a good alternator. If you have a shorted diode in the alternator, I would expect to see a reading over about 30 to 50 milivolts.

As others have mentioned, you need to make sure there is a good ground wire going from the negative post of the wiring to the vehicle frame. You should also have a heavy wire going from the negative side of the battery to the starter and or the engine itself.

You can sort of keep an eye on the status of your alternator by watching your headlights at night. If you pull up to a stop sign or traffic light and the lights dim way down, your looking at a problem. The get brighter as you start up from the dead stop.

Another issue is if you battery is about 3 years old or more, Your living on borrowed time. As soon as it gets cold, you won't start the engine. Batteries have a useful life of about 3, maybe 4 years. The internal resistance starts going up and their capacity to provide a bunch of amps goes down.

I buy the largest battery that will fit into the battery holder when I change out my vehicle battery. Being in the radio field for some 45 plus years, I always have radios in my vehicles. Generally it's on the first cold morning that I find out just how good my battery is to me. Have had to change out a few alternators in the various vehicles I have had over the years. The first sign is the alternator whine. Another sign is the clock looses time and resets to midnight when you start the engine.

Hope this provides some clues on your search for the problem.