PWM signal causing RF interference.

[torch]

I have an irritating buzz when tuned to an AM station, particularly a weak station. It came with the car and survived replacing both the radio itself, the antenna and the antenna lead. A PO already installed a capacitor and inductor on the power supply wire. The buzz affects AM only, does not vary with engine speed and disappears if the antenna is unplugged. It is not apparent with the key set to ACC, but is definitely there Key On Engine Off.

Pulling fuses, one by one revealed the buzz is related to the Variable Effort Steering. Looking at the Factory Service Manual, I think it's probably generated by the PWM output controlling the steering pressure. The FSM indicates the PWM frequency is 60Hz.

I'm thinking a ferrite bead will not be effective at that frequency, so perhaps a small capacitor between the VES output and ground might attenuate the RF signal. Of course, if it's too big, it might damp the PWM signal too much and affect the steering. So my question is: how would you approach this problem?

[AG6QR]

The PWM is 60 Hz?  or 60 kHz?

Regardless, what you need to get rid is the portion of the signal that interferes with broadcast radio, which will be in the range of roughly 540-1700 kHz.  A ferrite bead may not do much at the fundamental frequency (especially if it's 60Hz), but it still could help attenuate those troublesome harmonics.  I think that might be the easiest and least disruptive thing to try as a start.

[torch]

The PWM is 60 Hz?  or 60 kHz?

No, it's definitely in the FSM as "Hz" not "kHz". I thought it might be a misprint, but elsewhere in the fault diagnosis it talks about injecting a 30 Hz signal with a signal generator and slowly increasing to 60 Hz. So the manual is self-consistent. That said, it's raining outside so I haven't dragged out a scope to check it myself.

[Mosaic]

There's a solenoid valve run by that 60Hz signal that is causing the harmonic into the AM band. You need to snub the coil on that solenoid as close to it as u can as  the interference is propagating into the wiring harness.

[torch]

Reading the FSM again, I realized that the 60 Hz is not the output frequency, just a test frequency. Since there was a break in the rain, I crawled up under the dash, moved the fuse box out of the way and back-probed the output with a VOM frequency counter. The output is actually 251 Hz.

Oddly enough, when I was done, the buzz was gone. There must be a loose connection somewhere but  no amount of wiggling wires will bring it back.

I hate intermittent faults.