Need help with interference on CDM1550

[tbski]

Background: I have a CDM1550 VHF high band installed in a jotto desk console in my 03 Chevy Avalanche. I have a black Maxrad trunk mount base and antenna mounted on the drivers side front hood, exactly opposite side of the factory radio antenna.

Anyway my problem is intermittant interference. And it always is on certain freq's with one of them being one of my priority channels (154.0?? to 154.1??). When in scan it hits these "birdies" and i have to temp delete 'em from the scan list. Deleting doesn't always help as i can't delete my priority. Nothing worse than being engaged in an emergency type operation using the mobile and it gets all loud with white noise. It is so bad i need to turn the radio off and go to a portable.

I have tried redoing the power connection. I tried rerouting the running of the antenna wires through the dash. It just continues to break squelch. There is no whine per say so i don't think its the alternator. Only thing that makes any kind of a difference is - if it is doing this, if i go out and touch the top of the antenna it goes away, once i let go it returns. Is it a band connection? would a firmware upgrade help? Bad antenna location?

Any one have any experiance with what is going on, and could help me before i get rid of a perfectly good mobile unit along with losing my sanity.
Thanks.

[escomm]

Sounds like you need a real ground plane

[NCSHP311]

pop a hole in the roof!

[jackhackett]

We had a customer that used lowband CSQ and insisted on no holes drilled in the roofs, had a lot of trouble with the antennas mounted on the drivers side of the hood, picked up interference from the vehicle's electronics modules which are generally mounted right there.. I think the main culprit is the brake module. If you have to have a front fender mount try it on the passenger side.

[nmfire10]

This is most likely a very easy fix. Do your channels use PL or DPL? Most likely they do and if that is the case, I know what your problem is.

Under SCAN LISTS
PL SCAN TYPE: Set this for "Priority and Non Priority". The other settings make the radio go CSQ when it is put in scan. I have yet to determine the point of this.


Under RADIO CONFIGURATION - BASIC
HUB DEFEATS PL: Un-check this. With the option checked, the radio will go into CSQ when the mic is taken off the hook. If you are driving with the mic in your hand, on your lap, on the seat, etc, this will cause your problem as well.

[jackhackett]

Thinking about it a little more.... do you still get the interference if you have the vehicle off (key turned ALL the way off)? That should tell you if it's coming from the vehicle electronics.

As you say you have a portable, you could try using that as a "sniffer", put the portable in CSQ and see if you can pick up the interference with it, see if you can locate the source and move your mobile antenna away from it.

Making sure the radio always stays in PL will mask the problem, but if the interference is strong enough you could still hear it when receiving, and it could cause desense, covering up weaker signals. The best thing to do is eliminate the problem.

[tbski]

nmfire10; I thought some about a PL fix, but my county is still in the dark ages and out of all my some odd 60 channels programmed only about 5 have a PL or DPL that is in use, all other including all my problem freq's are CSQ.

jackhackett; Thanks for the suggestion on swtiching sides, tried, but reeked havoc with my stereo. Yes, with the key all the way off and the entire vehicle shut off if I override the ignition sense and turn the radio on it is still there if it is currently doing it. I should also say this is intermittant. Some days nothing, other days its unbearable. I 'd almost like to say it does it more when its clear and sunny but i would sound crazy. But it does seem to do it if the vehicle is on for quite awhile. It does it after i've been driving about 10 to 15 minutes.

While i know the hole in the roof is the best i am trying to use up all my options first. I went out today and sanded down the metal on the hood where the two screws for the trunk mount make contact with the hood.
"escomm" mentioned a ground plane issue. Will see how this works out.

[Jim202]

The digital clock in some of these newer cars seem to generate
a strong enough signal to reek havick amoung some radio channels.

Had one car parked at a gas station a number of years ago
and when I drove by on the street, it would lock up my radio
on a VHF channel. Stopped one day and convinced the owner
to lift the battery cable just to see if it would go away. It did.

Jim

[jackhackett]

The digital clock in some of these newer cars seem to generate
a strong enough signal to reek havick amoung some radio channels.

Also, I know that on some Fords you have to let the vehicle sit undisturbed for about 45 minutes after shutting the ignition off before all of the modules are fully powered down, even opening a door will reset this and you'll have to start over. I don't know if this is true for Chevys as well.

It does look like it could just be random interference, if so, without PL about the only thing you could do is tighten up the squelch.

[Garyf629]

I had a similar problem on a Chev. pickup. I ended up running a ground strap from the screws holding the antenna mount to a common ground point on the firewall. Talking this over with the service manager that works on our ambulances, he says that sometimes the parts are painted so well, that the ground is really poor on the fenders.

If you really want it to work well, nothing beats a hole in the roof.

[tbski]

I dunno. I got down to bare metal where the screws for the mount touch the metal of the hood. While some of the channels i had problems with in the past didn't act up, my main priority channel did come on after about 10 to 15 minutes of driving. I went and touched the antenna, moved the antenna wire slightly and nothing made it stop. So I turned it down and kept driving with it acting up. After about 10 minutes it proceeded to stop all on its own. Very strange.

I am starting to think it is some kinda electronic module somewhere that comes on and goes off after a set time. It sucks as i can't even nuisance channel delete it as its my priority.
jackhackett; you mention adjusting the squelch, what are any negatives of playing around with this?
Yes to all who have suggested it, i can cave and mount it to the roof..... but then again what if it still does it after i break out the hole saw.
Anyone know if he radio itself might need to be tuned up or something?
Thanks for everyones help so far.

[k2hz]

Before you drill any holes, try a magmount antenna and see if you find a location where the interference disappears. It also helps to "sniff" around with a portable.

You may find some vehicle interference is so strong you hear it everywhere around the vehicle and the only solution is tone squelch and living with interference on weak signals.

[mike m]

I know the 2003 Yukons have similar interference to VHF radios when the vehicle was shut off and this interference was traced to the ABS system.

You may want to do a google search of 2003 Yukon two way radio interference to see if you can find a fix for your vehicle.

Again this was with the vehicle turned off, I remember a thread some time ago on this subject but I cannot remember which internet users group it was on, I want to say that it was on one of the 4 X 4 users groups but as I said it has been some time.

MM

[mike m]

http://groups.google.com/group/rec.radi ... b3749ec5f2

I found one of the threads on this subject, go to the link above and read until you don't want to read anymore.


good luck

BTW, My 97 Toyota 4 runner is quiet from low HF all the way to 900 MHz.


Mike

[RADIOMAN2002]

Some of the GM products have been problematic, especially with low band equipment. Their computer or PCM would put out a signal on our mutual aid fire frequency, that could be heard for 100ft. When the scanner was on you could always tell when a GM product went by the shop.
I would try to see if the signal goes away when the vehicle is off, also try turning on/off different buttons in the car, am/fm radio on different stations, rear defroster, interior lights. etc. If you have access to a spectrum analyzer, try looking for those frequencies throughout the car.
Good luck

[Al]

The last resort is what a local fire chief did after noone could locate the source of his GM interference......he kept the radio and traded the truck!