Classic cars forum & vehicle restoration.

[FAB Cruiser]

Guys,

Sadly I spoke too soon in saying my interference problems have gone.

Thanks to the great help on this forum, they are much reduced, but still causing problems with my alarm.

So far I have suppressed the alternator with a capacitor (which reduced the whine on AM radio) and fitted resistive spark plugs which much reduced the interference to my alarm system. Also checked all earths are good.

However, what I'm still getting is random 'pops'. They increase in regularity with engine revs and can be heard on AM radio simultaneously with making my alarm siren squawk.

I've deduced it must be radiated interference as snubber circuits in the supply don't help, and in the case of the alarm, it gets worse when the ultrasonic sensors are plugged into the unit (they are obviously behaving like antenna).

I found this great article from the '70's which suggests a resistive lead from the coil to distributor cap (where can I get one of these? - screw fitting at distributor end and push fit at coil.)

http://www.quest4.org/radio/reducing_noise_interference.htm

I don't think I can add resistive leads to the plugs as I already have resistive plugs and the article implies that both would be too much resistance.

I also want to try a capacitor on the supply to the coil. Again, the article says I should use a 0.1uF capacitor. Does anyone know where I could get one? The capacitors for alternators are much higher value and I can't find anything for a coil in modern car shops.

I could also try a 0.5uF on the voltage regulator although I thinks that's a less likely cause.

My main sources for bits would be Maplin, RS, Halfords or eBay.

Cheers
David

[peter scott]

Hi David,

The capacitor value is not terribly critical so just use what you can find.

It might be worth puting a capacitor across the charge warning light connection at the alternator if you haven't already done so.

If your (now metalboxed) alarm is mounted under the bonnet would it be possible to relocate it under the dash somewhere?

Again try removing the fan belt to see if the occational pop is from the alernator.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[FAB Cruiser]

Good tips Peter. That'll be next weekends homework!!

I miss the long evenings already!

Thanks
David

[ukdave2002]

[FAB Cruiser]

Dave,

No electric fuel pump. The cap, points and condensor is a good idea.

Thanks
David

[FAB Cruiser]

Guys,

Before I takle this at the weekend, can I just a detail about the capacitor on a coil.

I was planning to use one sold for an alternator i.e. metal case with lug and a single flying lead. I was going to attach the case to earth and the lead to the battery supply.

However, I've just found another version of the article mentioned above but with diagrams. It shows a capacitor with two leads which goes in-line on the battery feed with the case to earth. I can't find a source for one of these though.

Is what I'm planning OK or do I have to find a source for a coaxial capacitor?

Thanks
David

[peter scott]

Hi David,

The "co-axial capacitor" is most likely a filter consisting of a capacitor to chassis followed by a series inductor (a coil) followed by a further capacitor to chassis. This configuration would give more effective suppression of noise passing along the connection in which the filter has been placed.

The tagged capacitor that you have should give some improvement if the connection you are applying it to happens to be the source of the noise.

I would try puting on the alternator connection that has the thinner wire connected to it.

As to connecting to the coil, if you can't identify which connection is which this doesn't matter. Try it on one of the Low Tension connections and if that does nothing then try the other. You can't use it on the High Tension connection.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[FAB Cruiser]

Guys,

I think I've cracked it.

I put the capacitor from the battery terminal on the coil to the coil's mounting bracket (earth) and....no chirping from the alarm and good old white noise on AM.

For good measure I added clip on ferrites to the ultrasonic sensors as they were acting as antenna and amplifying the interference.

So in total I used: Resistive spark plugs, capacitor on alternator, capacitor on coil, put alarm in earthed metal box and put ferrites on alarm sensors, alarm +12V wire and siren +12V wire. Also tightened spark plug cap connections, cleaned rotor arm contact, points and distributor cap contacts.

phew!!!

Thanks for everyone's help. I hope that's the last of it!!

Cheers
David