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The Hummer Knowledge Base

Accessories/CB/Noise (Eliminating):

>Date: Fri, 27 Feb 1998 19:37:54 -0600
>From: "Michael Pavolka"
>Subject: HML: CB Radio Noise
>I have had a new CB installed 40 channel with the sidebands.
>I have lots of engine noise.  High pitched whistle the increases as I rev
>the motor.
>If the engine is not running there is no noise or whistle.  Now if I turn
>the key to the ON position and the blower fan is running I also get noise. 
>When I turn the key to the on and wait until it warms up enough for the
>Diesel to start there is feedback then also.
>I have added a heavy duty noise filter.  This took out little noise,  if
>any at all.  This whining whistle does not happen with the PA Speaker on
>only when the CB is being used or one of the sidebands.  
>The question is any suggestions how to get rid of this noise?
>What may be causing it?
>Its too annoying to ignore.!!
>Any help would be appreciated.

I have a 1997.5 4DR HT 6.5L TD ( that has an aftermarket (e.g. Premiere) 200A alternator. I have been trying to clean up the radio noise from that. It is just like your noise.. S5-6 on the CB s-meter.

Unplugging the antenna from the CB, made the noise go away, so this CB has good power line filtering, and the noise is getting picked up by the antenna. Unplugging the alternator field winding, disables the alternator and also gets rid of all noise.

Apparently the alternator and under hood wiring are radiating all the noise, and the fiber glass hood is transparent to RF and so the noise makes it to the antenna.

Add-on "heavy duty" noise type filters, were designed to cover a single device, like a stereo or CB, that is having noise coming in the power line. So unplugging the CB ant, and noise goes away, this type of filter won't help.

I have been looking for a commercial filter that can take 200 Amps to install right at the source, the alternator main output lead. So far, no luck, I may just have to build one from parts.

I have an external voltage regulator. It appears to "pulse" the field winding, rather than steady DC current. Sharp edged pulses from the regulator may be "amplified" by the alternator into producing several watts of "hash/whine" at 0-50Mhz ???

The regulators probably "pulse" the field so they can use smaller transistors, with the field driver transistor being always "ON" or "OFF", with the "width" of the on pulse being varied to control the amount of charge. This is a sure fire way to generate RF noise. An "analog" regulator that supplied steady DC current to the field might eliminate much of this noise.

One thought may be to get a radio-shaft "noise filter", but instead of hooking it to the CB radio, connect it to the "field" or "F" terminal of the alternator and the regulator.

I think the stock AMG (GM) alternator has an internal regulator, which makes this difficult.

I will let U know if I can find a filter capable of 200A.



What you are hearing is alternator whine. This may be caused by several scenarios.

I assume that you are using one of the "switched" spares located near the fuse panel. There is a ground bar just under the fuse panel. You may need to drill a small hole in order to use a screw holding the negative lead. It is important this lead have a low resistance to ground. If the coax shield to the antenna is grounded to the body at the termination point, a ground loop may exist between the two grounds, fuse panel area and antenna location. The coax shield ground should be grounded at the antenna location. The ground loop condition may exist if the ground connection at the fuse block area is resistive.

With the radio on, disconnect the antenna from the radio. Does the alternator whine stop? If it does NOT, try installing a choke type noise eliminator on your positive D.C. lead near the radio. These have been available at Radio Shack over the years. It will look like a small transformer with some condensers etc.

If the alternator noise STOPS when you disconnect the antenna lead, remove the ground connection at the antenna end if it is connected to the body. DO NOT TRANSMIT. If the noise stops, you have a poor connection between THAT part of the vehicle body to frame ground. Most alternator's diodes do not radiate much R.F. up this high in frequency so it is probably a ground loop caused by a poor body ground.

There is a fix but Radio Shack won't have what you need.

Most alternators used on Hummers are of the "self excited" type. Without going into detail, it makes noise elimination fairly easy since there is only one heavy wire leaving the unit. Since we are dealing with high current output at this point, the standard coaxial condensers from Radio Shack will be useless. They will not handle the current. Go by a Motorola communications shop and tell them about the alternator whine. More than likely, they will have a 100 plus amp. coaxial condenser that you will need. This thing ain't cheap so try everything else beforehand. This device goes in series with the main D.C. lead from the alternator. SOLDER the lugs that are used to connect this device. VERY IMPORTANT.

The noise heard when running the heater is caused by the brushes in the motor. Unfortunately, this can be normal in most installations but there is a remedy.

Install a .5 MFD 100 to 250 volt tubular capacitor to the main D.C. lead at the motor location as follows:

You will notice this capacitor has a black ring at one end. This end goes to the ground nearby or the negative lead of the motor. The other goes to the main +12V. lead going in the motor assembly. This capacitor is not polarized in the usual sense, but the "outside foil" is connected to the end with the ring. Maximum noise reduction will be had with the ring side connected to ground.

Some motors' outside cases are not at ground potential and this can also be a source of noise radiation. Check for continuity to ground using an ohm meter by placing one lead to the motor case, the other to ground. Use the voltage scale (12V) first before the Rx1 scale to make sure there is no voltage present as this could "pop" your meter movement. If there is no continuity to ground and no voltage potential, try connecting a wire from the motor case to the nearest "body" ground. Most of these motors have screws holding their cases together. Be careful when loosening these screws since the end case bearing assemblies are sometimes centered by the tightening of these bolts.

As with the the capacitor above, the maximum noise elimination is achieved by connecting the motor case, if needed, to a "body" ground rather than the negative lead which wanders around aimlessly going to ground radiating R.F.

Good Luck, I've been there, done that on my '96.

John W. watkins Jr.

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