Posted a short while back with an issue over radio interference in my '97 300tdi from window motors/wipers etc. Got some great feedback and advice but the problem will not go away.
I have run a clean wire direct from the battery to the radio power lead, and another to the memory back up permanent live. I ran two new ground wires to clean new earthing points ( one from the ground terminal on the plug and another for good measure from the radio chassis).
It has made no difference; perfect reception until I start the engine and get a constant background crackle, and the windscreen wipers and washer motor and all the electric windows give interference when they're operated.
Is there a suppressor somewhere in the electrical system that may be missing or broken? I've looked through all the fuse and relay boxes and cant see anything obvious.
Any more help and advice please, I just dont know what to do with it next.

Seeing as you've got interference from a number of sources, it's unlikely to be a single suppressor somewhere.

So far you've done the start of tracing the noise, now check the condition of the aerial cable(s), paying particular attention to the screen in the cable and the earthing of the screen at the aerial end.

Sounds like a coax problem as above. I have a cheap aerial I plug in to test if it fixes it then I check the coax aerial and amp.

Thanks for those pointers but already tried. The radio that was in the disco when I bought it had the same issue and, after changing the radio with no improvement, I fitted a completely new antenna with new coax and connections. Made no difference .

Is the interference on fm or mw/lw
You may need to fit supressors to the items causing the problems
Will try and dig out my intrerference hand book from my ham radio days later today

owl said:

Sounds like a coax problem as above. I have a cheap aerial I plug in to test if it fixes it then I check the coax aerial and amp.

Click to expand...

Having just re-read your reply Owl, I need to ask 'what amp?'
I did read a thread on this relating to a jap spec disco which had a new head unit fitted but ran through an O/E fitted amp for the original LR head unit which needed bypassing, but this seemed only to apply to jap disco's, not UK models. Have I read that wrong?
The original LR head unit was long gone when I got the disco, and the head unit in its place was plugged straight into the standard multi-connector behind it in the dash. The head unit I then fitted was just swapped in and connected in the same way. Is there an O/E amplifier unit in the vehicle somewhere that needs removing or bypassing?

just some pointers for you
Identifying and suppressing radio interference | How a Car Works

Noise Suppression Guide - Noise Suppression Guide


A whining that rise and lowers in pitch in time with engine speed is generally caused by the alternator.
The problem could be that the alternator is not grounded properly. The cure is to get a good sized grounding strap from an accessory store or motorfactor and connect the alternator casing straight to the vehicle body. Some alternators have a grounding post on the rear case for this. Make sure that the body end is connected to a good clean ground and that the strap is well away from moving parts.

The problem could also be caused by a faulty diode inside the alternator, the alternator will appear to work fine with a faulty diode but the problem is there. A possible cure is a large capacitor across the output from the alternator (mounted as close to the alternator as possible, on the rear casing is best).

Any electric motor can cause a crackling or receive or transmit or both. Most small electric motors in vehicles are permanent magnet motors. Some possible cures are:
Heater, Radiator cooler. Make sure the metal casing (if it has one!) is well grounded. If the motor is all plastic and gives interference you could try covering the motor housing with glued down foil and grounding the foil (see Plastic Bonnet above). Add C1 & C2 or L1 & L2 (all if all band suppression is required).

HF: Use C1 & C2 1.0µF
VHF: Use L1 & L2 30µH, 7Amp rated

Diagram showing how a permanent magnet motor is suppressed

For wiper motors it is a little more complex as they have the self park function:

HF: Use C1 & C2 1.0µF
VHF: Use L1 - L5 30µH, 7Amp rated
If your wiper motor is only a single speed unit you omit C2 & L2.

Diagram showing how a permanent magnet windscreen wiper motor is suppressed

One Final, but important, Tip:

The only amplifiers are depending on model 1. the aerial amp fitted above the rear window aerial, 2. the bass loudspeakers in back door 3. and as u mentioned Japanese spec vehicle loudspeakers.

All decent head units are fully suppressed for interference such as wiper motors, indicators, alternators, heater motors, petrol ignition systems etc.
As stated try a portable radio in the vehicle
Take the head unit back to where u purchased it as it looks as if it's faulty.

Is the interference as bad on FM as it is MW? The aerial on the inside isn't going to help so maybe try the original wing mounted one again but check for continuity on both centre aerial connector pin to aerial and the outer of the connector to a ground near the head unit. This will confirm the shield is connected at both ends, the aerial mount is grounding on the wing and that there is continuity from the wing back to the chassis. If someone has changed the cable on the original aerial they may have mistakingly used coax like RG58 or 59, car stuff needs low capacitance cable like RG62, it can seriously effect MW/LW performance if you don't due to the electrically short aerial.

If you want to continue with the internal aerial it sounds like it will use the coax screen for the supply ground so you need to check continuity otherwise the amp wont be powered.

Pull the head unit out, connect only a speaker and run in a seperate power supply, try it with and without the aerial connected, hows the interference?

Thanks for all the pointers, the suppression wiring diagrams are all a bit too technical for me, but I should have thought that LandRover would have fitted some kind of motor suppression at manufacture. The fact that every motor on the car ( wipers/washer/windows/fan) causes interference makes me think it unlikely that every motor suppressor has failed.
MW has worse reception than FM.The head unit isn't at fault, I tried it in the wife's car and it works perfectly, but her radio has the same problem when I plug it into the disco.
I have checked the aerial and co-ax, all seems fine, and bear in mind I fitted a new additional aerial to try and eliminate the problem. A portable radio in the car has fine reception, but I can only run it from its own batteries as I don't have a ciggy socket adapter to run it from the Disco 12v system. I have seen a couple of audio interference suppressors on Fleabay for fiver or so, they are supposedly intended to wire into to head unit supply lead and eliminate interference in the line. Does any one know how effective, if at all, these are?