[RFI] Ferrite Beads
Ford Peterson
ford at cmgate.com
Thu Oct 21 02:00:07 EDT 2004
I previously wrote:
> otherwise perfectly good toroids if it is not going to work.
> How much RF resistance would be required to make a noticable
> difference? I'm not smart enough to know. Perhaps others
> on this list could provide some guidance for me. Is 20 ohms
> on each COP enough? Is 50 ohms enough? How about 300 ohms?
> I have no experience with this type of problem and therefore
> I'm truly clueless.>>
To which Tom-W8JI responded:
>
> No one could possibly answer that unless they worked on the
> same vehicle.
>
> The amount of impedance you need depends on the common mode
> impedance of the system you are working on and it could be
> anything. The lower the common mode impedance, the larger
> effect a low impedance bead will have.
>
> This is why a combination of bypassing, grounding, and beads
> is far more effective than beads alone.
>
> 73 Tom
Lucky me... I get to be the first to blaze the trail...
I have the schematic for the truck. There are 4 wires to each COP.
All the brown wires are tied together and grounded at the rear of the engine compartment. The Red/Black (13.8vdc wired back to the ignition switch) wires are tied together on each side of the V8. So the 4 COPS on the left bank are tied together, as are the 4 on the right bank. Two separate 0.047uF capacitors are located at the rear of the engine. One for the left bank and one for the right. Two caps (shown as 'noise filters' on the schematic) are tied between the + and - lines at the rear of the engine--one for each bank of the V8. Both of these have been replaced with no change in noise. In fact, removing them from the circuit results in no change in HF noise. A large gauge braid is located between the engine and firewall at the same location. I have jumpered this braid as a test at various spots along the fire wall and engine and no observable change was noted.
The Black/White wires are connected in an odd combination. Cylinders 2, 3, 5, and 8 are connected in parallel and run back to the engine controller to a pin labeled IGF2. Cylinders 1, 4, 6, and 7 are also connected together in parallel and run back to the engine controller to a pin labeled IGF1.
The remaining Black/Blue wire going to each COP goes back to the engine controller individually. These are labeled on the schematic as IGT1, IGT2,...IGT8.
I have placed a 0.047uF capacitor between the +/- pins right at the COP, only to find that the noise becomes worse. I am unwilling to attempt any sort of capacitive loading on the other two lines running to the COP for fear that the engine controller may puke, costing me $1200 and lots of heart-burn for my trouble.
Ferrites encasing the entire bank of 4 wires going to each COP is my next thought. But perhaps there is a way to isolate which of the 4 wires is causing the trouble. I don't think it's the +/- lines since the noise filters don't seem to matter whether they are in the circuit or not.
Any and all assistance is greatly appreciated.
Ford-N0FP
ford at cmgate.com
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