Ignition RFI

[W3MPS]

Ok guys, here's a question for all of you. I've had a CB in my '92 Jeep Cherokee since 2009. No funny stuff, just a plain ol' CB. And I had no tx induced problems problems at all.

I put a new engine in in the summer of 2010. Now I am having all sorts of problems with the radio making the ignition go nuts when I try transmitting. And by nuts, I mean miss fires, popping, backfiring, and general bad juju for the motor.

So far, I have cleaned up all the chassis ground points, made sure nothing had been touching that shouldn't be, and bonded everything. The only thing that really changed was I had to splice on a new connector for the pick up coil for the distributor, and that part of the harness ended up being about 6 or so inches longer.

Where should I start troubleshooting at? Cleaning up the grounds decreased the severity of the problem, but did not eliminate them.

[N9FE]

For grins try putting some tin foil around the new splice by the distributor. Wierd a low wattage CB would do that though.

[W3MPS]

Who knows, maybe the splice made it near resonant on or around some harmonic or whatnot. That's my current theory, it's on tomorrow's to do list.

[N8YX]

Undo the splice and put a few slip-on ferrite beads along the conductors, then redo the connection. Use beads with an Ft of 20MHz or lower as a start. Then add a couple clamp-on (split) ferrite cores of the same mix to the main ECU harness.

[PA5COR]

Large enough clip on ferrites will clip over the cable without having to redo the splice.
Put a few next to eachother to amplify the rf choking.
Get your + and - Cables directly from the battery, ( fuses in BOTH) and also get some ferrite over those.

That should sort it, put some ferrite next to the transciever too, some rf might leak into the wiring in the car and reach the computer.
Find computer, check ground connections, and add ferrite there too.

Glad my van has only a stupid driver as computer ;)

[W3MPS]

Thanks guys. It looks like I have a nice project on my hands for next week. (Ahh, the fun of having to work weekends)

I had already done a direct run to the positive post with an auxiliary power bus, guess now I'll finally get around to running the ground bus to match.

And of course, I should know better. Adding or subtracting wire length is a great way to make things RF sensitive that you don't want to be.

[N5RLR]

Try a "twisted pair" of wires from the rig to the battery; offers some self-shielding properties.

[W1GUH]

'FE said...


Wierd a low wattage CB would do that though.

He's not talkin' about how many pills he's usin'. Just sayin' ;):evil:

[W3MPS]

He's not talkin' about how many pills he's usin'. Just sayin' ;):evil:

Nope, no Illudium Q-36 Explosive Space Modulator is in use.

[W1GUH]

Forty-roger on that, good buddy. Keep it between the ditches, y'hear?

[Jerry]

The ignition in the Jeep is pulse width modulated.
The fuel pump in the Jeep is Pulse Width Modulated.
The shift for the transmission is pulse width modulated.
The alternator is pulse width modulated.
The more computers you add to the vehicle, the more problems you will have.

The problem with Jeeps is - you don't have a steel roof to mount the antenna to because most people wants to be able to drive it into a parking garage or home garage and they usually don't fit if there is a 4'+ antenna on top of the roof.

When you mount the antenna to the quarter panel - half the signal wanders under the vehicle and is not transmitted. Plus you get nulls in one or two directions.

I highly doubt if adding 8 inches of wire to the harness would make it resonate due to the size of the wavelength 27 mhz - 11 meters


Usually the problem is the third harmonic because it is the one that is the most closely related to the original frequency in question.

[Jerry]

One sure way to check it would be to disconnect the radio from the vehicles charging system and hook it up to a independent power supply and key the mic and see if the ignition still cuts out.

The problem with trying to diagnose a problem over the internet is that you are there and I am here. I can't see what you might have done wrong or diagnose the problem over the internet.
Your best bet is to find a CB shop that has knowledgeable people who can help you to diagnose the problem. Most if not all of the truck stops around here has abandoned the CB radio and most new radios are disposable - hence the need for trained, qualified radio repairmen has gone the way of the Dodo bird.

Most people buys a cheap Cobra 29 LTD type radio - because of the name and reputation and the idiots that needs more power buys the " Import" type radios.
The funny thing is 3 watts will do most of what you want it to do if you use a 103" style whip. To double the power - you have to put out 50 watts. That would put a S-6 radio signal into the S-7 range. Not a lot of gain for all the bother.
To go from a 10/S-9 to 20/S-9 you need to increase the power 100 times.
Hence a 100 watt rig would need 10,000 watts of power.

It's pretty hard for me to believe that a legal 3 watt rig would cause so much grief in a modern vehicle. Most times it is the exact opposite - as I said before, with everything in the vehicle making the noise and the radio not being able to hear and not the radio making noise and the vehicle not being able to function properly.

Just thinking out loud here.
What about some type of condenser - back in the day, when vehicles had points or HEI, there was a condenser that would filter the power going into the ignition.
I wonder if your Jeep has something like that on it and if maybe it has gone bad.

[W3MPS]

Why the hell do some people assume that I have a non type accepted radio or illegal linear amplifier. A 4 watt AM carrier or 12 watts PEP on sideband it all that is allowed..... Yes I DO understand that basic underlying principal of the Citizen's Band.

I had a couple of ideas as to what might be causing problems with my Jeep and thought I might ask and see if anybody else had some ideas. Some kind souls did chime in with a few pointers that I am very grateful for. Right now I have been very busy at work and have not had time to pursue troubleshooting things, but do intend to work on it. And once again to those of you who were helpful and offered sound advice I thank you and I will use your input to deduce what is going on.

Lastly, I am offended by the insinuations that I am engaging in some form of illegal radio operating practice. Citizen's Band radio does have a legitimate use, which is why I own a CB. And I abide by the regulations that govern the equipment allowed to be used in that radio spectrum allocation.

"I'm just saying....." or "I'm just thinking out loud......" Bullshit! If you have an accusation to make, be a real man and come out and say it. Let your yes be yes, and your no be no.

[KC2UGV]

Why the hell do some people assume that I have a non type accepted radio or illegal linear amplifier. A 4 watt AM carrier or 12 watts PEP on sideband it all that is allowed..... Yes I DO understand that basic underlying principal of the Citizen's Band.

I had a couple of ideas as to what might be causing problems with my Jeep and thought I might ask and see if anybody else had some ideas. Some kind souls did chime in with a few pointers that I am very grateful for. Right now I have been very busy at work and have not had time to pursue troubleshooting things, but do intend to work on it. And once again to those of you who were helpful and offered sound advice I thank you and I will use your input to deduce what is going on.

Lastly, I am offended by the insinuations that I am engaging in some form of illegal radio operating practice. Citizen's Band radio does have a legitimate use, which is why I own a CB. And I abide by the regulations that govern the equipment allowed to be used in that radio spectrum allocation.

"I'm just saying....." or "I'm just thinking out loud......" Bullshit! If you have an accusation to make, be a real man and come out and say it. Let your yes be yes, and your no be no.

Well, you do have to admit, it's odd. I'm starting to think about this: What kind of CB is it? An old school one? If so, it might be a combination of drawing a lot of power, and RF getting right back into the power distribution.

[kb2vxa]

Before this gets out of hands here's my 2c worth. Vehicle manufacturers bank on the odds against transmitting equipment being installed after market and provide little to no shielding and filtering the computerized electrical systems. This makes it easy for even low power RF to get in and screw up the works so it's a rather common problem; a trucker friend had to remove his ham and CB equipment because it drove the dashboard nuts, lucky it wasn't a "safety surprise". How the police cope with it I don't know but maybe the ubiquitous Crown Vic has something to do with it.

[K7SGJ]

The Crown Vics have to have all kinds of shielding what with the voice com, computer com, the light shows, and whatever else they have in them. A ham I used to work with was also a sheriff deputy. He bought a Crown Vic from a PD in the midwest and had it shipped out. He had ham, PD, Fire, and stuff from dc to light in the thing. He never had to do anything but add more radio equip.

Maybe that's the answer, get a CV to put the CB into.