'98 XK8 sound system interference

[Macavity]

Hi all, first post, so apologies if I miss out something obvious.

I've a '98 XK8 which I've had from new. In keeping with its age, it's starting to have the odd eccentricity.

The current one is an intermittent electrical interference heard over the sound system. This has only happened this year, and it's not to do with the famously poor radio reception. The details are as follows:

- affects both radio reception, and also cassette playback (haven't listened to a CD with the boot changer, so can't comment on that)
- sounds like a whir/whine, that starts and stops suddenly, lasting 2-3 seconds
- happens on average every hour or so, but I can't detect a pattern

It sounds to me like some sort of motor or pump which only activates sporadically, and is generating interference which is being picked up by the wiring and/or amp of the sound system. I've no idea why it's only started recently. The car has had various repairs done over the years, including recently, but I can't be sure that this started after a specific job.

I'm totally stumped by this, and would appreciate any pointers or fault-finding suggestions.

Many thanks and Merry Christmas!

[Astromorg]

I would start by checking the suppression module that is fitted to the alternator as it may have started to breakdown internally due to age. You should find it rearward of the righthand headlight with two wires from it to the altrnator, one brown and one black. Trouble is, I don't know if it's possible to test such an item other than by replacing it. I'd be looking for a used one from one of the specialist Jaguar breakers such as www.eurojag.com but phone them for an instant response, don't bother emailing!
If that angle doesn't work, it might be necessary to consider any recent repair work. For instance, in the original design of the electrics, great care was taken to forestall interference problems; this quote from a Jaguar technical paper

More generally, the wiring was designed to give maximum separation between noisy circuits ‘sources’, and potential sensitive circuits ‘receptors’. This was achieved by locating sensitive signals and low current signals in a separate harness. The noise signals were located in another harness and routed to provide separation of the harnesses in the vehicle. The wiring has also been routed to avoid in potential sensitive areas e.g. near the In-Car Entertainment components. This has improved the immunity of the vehicle to electromagnetic coupling.

[Macavity]

Thanks so much for your suggestions. I'll certainly look in to the alternator. Just one question on that - wouldn't the alternator always be running? If so, why would the interference I'm hearing only happen intermittently? When I hear it, it really does sound like a motor or pump which is whirring in to life, and then stopping again...

[davidr]

The intermittent nature of the interference does point to something that only comes on now and again but it could be that the suppression of your alternator breaks down occasionally?

[Astromorg]

As David says, my feeling is that if it's the suppressor it's because it's in the process of breaking down internally or perhaps there's a poor or corroded connection.

Also bear in mind that although the alternator is always running, the load on it varies greatly. If there is a pump or motor occasionally coming on, that will be reflected in the alternator load and that additional load may be too much for the suppressor. I think it's unlikely to be that, but you could check by winding the blower motors up and down.

Things that cut in and out; radiator fans, A/C clutch, VVT solenoids, fuel tank vapour purge system (in front near side wheel arch). Can you tie anything like them to the problem?