RFing the black box 2003 Avalanche on 75mtrs

[AA9YP]

I recently attempted to install HF in my 2003 Chevy Avalanche. The rig is a ft897 and the antenna is a Hustler mo4 mount with resonators for 40 and 80 meters.
 
 The problem I am having is on 75 mtrs.  I am RFing the black box (brain) of the truck. Every time I use a power level greater than 20 watts on 75 mtrs the trucks gauges, seat belt alarm, door ajar alarm and check 4wd lights and bells come on.   When using it on 40 mtrs, it does great.  

  I am assuming at this point that for some reason the frequencies from 3.890 to 3.930 are such that they swamp the black box with rf.  This is the only frequency range I tested on 75 mtrs since the hustler was resonant at the center and rose to 2 to 1 at the edges.

   Again, 40 works great and I recieved great reports on it.  I have no other resonators to try so I cant say whether it has problems on any other band,  

   I have tried changing the antenna cable routing and also tried a chunk of rg-8 and still had the same results.  
 
    The power cables and antenna cables exit the cab of the truck through the rubber gromet that also has the fuse/relay bundle running through it.  

    I tried a isolated source for power i.e a marine battery in the cab... but still same results.

    I put a telewave dummy load on the rig and could key down 100 watts with no problems, I even tried the dummy load on a long piece of rg8x and keyed down and ran the dummy load all over the cab and engine compartment hoping to spot the affected area.  

    Nothing happened, the rig perked along at 100 watts and the vehicle had no problem.  So I need the help of some more experianced mobileers.

    Any and all suggestions cheerfully appreciated.

     Tnx in advance  de AA9YP

[K0BG]

Some of the information you didn't give was where and how you mounted the antenna. Further, you didn't state whether you have bonded the various pieces of the truck to the frame, such as the doors, hood, and cargo box. All of these are important parts of preventing ingress of RFI to the EEC. Visit my web site for more information.

Another problem ifs the size of wire you used (?) and how you ran it to the battery. Running the power cable next to existing wire, and impressing RFI on to the coax due to a poor antenna installation is a sure way to get frustrated.

Lastly, GM products are no worse for RFI ingress than any of the others out there. Nonetheless, they (GM) publish a rather good treatise on RFI prevention. The URL has been published here on this forum several times. Unfortunately, I don't remember what the URL is.

Alan, KØBG
www.k0bg.com

[AA4PB]

My guess would be that it is an insufficient RF ground at the antenna (coax shield). The mount where the coax shield is connected needs to be bonded to the frame with copper straps. Once you have done all the bonding you can do, if you still have the problem try a string of ferrite beads (i.e. a current balun) on the coax back near the antenna. This will help to minimize the amount of RF flowing back down the outside of the coax.

[N9QKG]

You mentioned you ran the wireing and coax through the firewall along with a bundle of wireing.  You bypassed that power wire by setting a battery next to the radio.  You mentioned you put a different piece of coax on with a dummy load and the problem was gone.  You did not say if it was the same coax running through the firewall.  I am assumeing it was a different coax run out the door.  I first thought the power wires was bringing something in, but I wonder if the coax is doing it since it runs around and with a bundle of wireing.  Now, you would think the coax shielding would protect from that, but RF is a funny animal.........