Where IS the Radio Interference capacitor ?

[JohnW63]

I've had bad AM radio interference for quite some time. FM is OK. It's definately something in engine bay, because it picks up the AC fan noise and it ALSO has a whine that matches the RPM, with some slight delay in time. By that, I mean that when you first step on the gas, there is a delay before the noise gets sent out the speakers.

I've verified the strap from the firewall to the passenger side rear support.

I just put in a "new" alternator, as my diode reading were off and the charge light stayed on.

I've tried different radios, from auction sites.

I put in a new anttena, since mine finally broke the gear strip and was stuck in postion.

So..lookin in the book, I found this cap. Where IS it ? The diagram isn't too clear.

95 , 3.2 SHO

 

[SHOZ123]

Should be next to the coil coming off the red/light green wire.

 

[JohnW63]

Yeah. I found it. One small 8mm bolt holding it to the coil pack, ( And I thought it was a distributor. ) with a lead going to a plug comming off a main bundle of wires, covered in wire wrap.

I used a multimeter and I think it tests bad. On the diode check function, it just shows infinity, both directions. Same with moving it to the 200 ohm scale. It seems wide open. If my memory is correct, it should show high readings in one direction and decreasinging readings in the other.

If I am right, and this is bad..is this a dealer only item ?

 

[Shoaz]

A cap will test open in steady state in either direction if it's good. Some failure modes will cause a short so that a cap reads low impedance. A diode tester is not effective in testing caps.

It is possible to test a cap, but without knowing the intended capacitance it's tough to know whether it's degraded or not if it hasn't failed completely.

It shouldn't be an expensive part. Just replacing it with a known good unit should be a reasonable strategy.

 

[hawkeye18]

I've been trying to figure out for quite some time what the capacitance of that cap is. It's not like I have any known good units to work with! Hey, who bought that brand new engine off of ebay? They should test that cap to see what the capacitance is!

A rise/fall time graph would be sweet too, but I thinks that's asking too much...

 

[projectSHO89]

The cap is going to be too small a value to test with a meter unless you have a meter that directlys measures the capacitance.

If I had to guess, I' suspect it's around .1 uf to .1 uf.

I'd also suspect it's not the problem. I'd suspect a faulty connection of the antenna's ground to the fender.

Steve

 

[shoclown]

RFI is also commonly caused by battery conections or low battery...i have chassed rfi for awhile before only to find a battery that had a cell that was right at specs for specific gravity...replaced batery and rfi was gone...

just a thought

 

[projectSHO89]

RFI is also commonly caused by battery conections or low battery...i have chassed rfi for awhile before only to find a battery that had a cell that was right at specs for specific gravity...replaced batery and rfi was gone...

just a thought

The battery serves as a capacitor in the circuit. I'd bet your issue was due more to the quality of the connections that a weak cell, however, it is possible.

RFI in an audio device can come from various sources and be introduced via various sources.

AM-only RFI is radiated energy and won't come in via the power source. If it were power source RFI, it would be noticeble regardless of the source.

Electrically noisy electric fuel pumps commonly radiate RFI. Another thing to check.

I'd still be focusing on the quality of the antenna's ground plane connection.

Steve