RFI in 99 Dodge Grand Caravan

[WD4AOG]

Getting RFI in my 99 Dodge Grand Caravan.  Here're the details.

Rig: New IC-706MkIIG (also heard problem in previous rig, Kenwood TS-690S so it doesn't appear to be a rig problem.)
Antenna: 40 meter mono bander on tri-mag mount on roof

Symptoms:
1. Pitch of noise is much lower than alternator whine but does vary with speed.
2. Only hear noise when I am in gear (switch to neutral while moving, no noise)
3. Noise coming in through antenna (remove antenna, no noise)
4. Problem is only heard on weaker (@S5 or less) SSB signals and only on 40 meters.  Transmit audio is clean.

What I've done so far:
1. Wired both power leads directly to battery with heavy gauge wire fused at both ends.  
2. Grounded the tailpipe to the frame near the end of the pipe which seemed to help a bit.

What should I do now?

Thanks for any help.

Michael
WD4AOG

PS Thanks eHam.net.  Best ham site as far as I'm concerned.

[KA5S]

This is probably control computer sensor noise. And it's probably coming in on the antenna because you have not grounded the tri-magnet mount; those don't have enough capacitance to the car body to be a decent RF ground at 40 meters. Try running a strap from the mount to a nearby ground. If you don't have one, you can make one; I drilled a small hole for a self-tapping screw under door frame weather stripping (DRY side, thank you), grounded there, then weatherproofed it.

One side effect of adding a good ground is, your antenna will work better.


Cortland

[K5LXP]

You've done some good isolation efforts so far, I agree with the previous post that you're dealing with a 'logic box' somewhere.  Since this model vehicle is most certainly a computer controlled transmission, and you can change the characteristics of the noise by switching to neutral, you're on to figuring out where the noise is coming from.  I would start with basic bonding of the transmission housing itself, and perhaps some clamp on ferrites around any control cables exiting the transmission.  Since it's so quick to move the ferrites around, trial and error with different cable harnesses is quick and painless to try.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net

[K0BG]

The noise you hear is chopper noise from the speed detection system (used for a variety of electronic engine functions). Chrysler has had several TSB (technical Service bulletins) about this problem. Your dealer may or may not know about it, but can look them up. If the dealer is of no help (a lot aren't) call them on their 800 number and jump through the necessary hoops.

Alan, KØBG

[WD4AOG]

Thanks for the suggestions (and any that may be added later).  I'll give them a try.

73,
Michael
WD4AOG