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MG MGF Technical - Interference on AM Band - wasn't there before replacement Alarm ECU

Hi Guys, I know I've touched on this before, but since having a replacement alarm ECU installed a few weeks ago, I've been experiencing an annoying buzz on the AM Band - it's like some sort of electrical noise or static type sound. The FM Band is fine, no problem there.

Does anyone know whether it is possible for the alarm ECU to cause electronic/electrical noise on the AM Band? Should the alarm ECU have an in-built 'noise suppressor' or is there supposed to be some shielding between the wiring of the ECU and the radio? Also, is the alarm ECU inter-changeable with other makes of cars like the ZT or the Freelander? I'm thinking that perhaps the alarm ECU as fitted to my car is of a slightly different design to what I should have? I dunno, just a thought. Is it possible perhaps, that the radio wiring is too close to the ECU wiring, which may be causing some EMI?

Any thoughts would be gratefully appreciated.

Thanks guys,

Phil
Phil

Hi Phil,
I have no idea what the answer is, but I have exactly the same problem. I changed radio sets and things improved a little. Then I upgraded to competition HT Leads from B&G, which state they remove radio static, and for a while things were marginally improved. I went on a 100 mile run Saturday afternoon and AM is now worse than ever. FM is fine. And it only happens when the engine is running too.
I'd love to know if anyone has any ideas.

Cheers

Kieren
Kieren Gibson

Phil,

Have the same symptom on mine even without the engine running. Strong AM stations are OK but weak ones are crowded out with interference from the Alarm ECU.

Bruce
Bruce Caldwell

Recent thread about voltage differential due to poor vehicle earthing may hold the key here.

The earth wire may be acting as a transmitter, which will have the greatest effect on weak AM stations.

Could also be a lack of radio frequency suppressors fitted. These are capacitors which are connected across componenets generating interference and "smooth" out the offending voltage.

Also check the instrument light dimmer switch by adjusting it when interference is apparent.

As a last resort, you can get an in-line signal booster which connects between the hean unit's antenna connection and the aerial, and gets its power from the +12 volts continuity connection at the head unit.

Good luck!
Sam Murray

Thanks everyone, I guess it's just one of those things.

Phil
Phil

Phil,

Just remind me. Does it occur when the engine is switched off?

Bruce
Bruce Caldwell

No I don't think it does but I'll have to re-check that.
Bruce

Please do not forget that there were two reasons for introducing FM VHF radio

1) Increased bandwidth to give a wider audio frequency range (this is the VHF bit)

2) Reduced susceptibility to interference (this is the FM bit)

Weak AM signals will always be subject to interference and one should not expect too much

Brian

I played around with the car radio on the weekend and managed to reduce the amount of electronic noise by playing around with the cabling. I did notice that the interference was more noticeable when the ignition was ON. When it was OFF, the amount of noise or interference reduced considerably.

Anyway, I managed to shift some wires around a bit which sems to have made a bit of difference, but not much. What I did notice however, is that there are two wires which were not connected.

A thin black wire, which is part of the wiring loom attached to two, rectangular black plugs which themselves, plug two other small plugs; an orange plug and a grey plug. This black wire was loose an not connected to anything, which seemed rather off to me.

The other wire, which has a slightly thicker diameter, was purple and was connected to the rectangular, white plug which connects into the backside of the radio. At the loose end, it had a clear plastic sheath, so I'm assuming that this is a protective coating as the wire doesn't plug into anything.

I might have squiz at Dieter's site later on but as we have a different ICE in our cars to those in Europe, I don't expect that there's much anyone can do - I'll probably end up going back to the dealer to get a wiring diagram.

Cheers,
Phil
Phil

Phil,

The wiring to the head unit should be ISO standard as follows;

Main Positive Power Red
Memory Positive Backup Yellow
Arial Switched Power Blue
Dial Illumination Orange
Ground (Negative) Black
Speaker Front Left + White
Front Left - White Black
Speaker Front Right + Grey
Front Right - Grey Black
Speaker Rear Left + Green
Rear Left - Green Black
Speaker Rear Right + Purple
Rear Right - Purple Black

Sound to me like the black earth wire is not connected to the vehicle, so the head unit is using the antenna earth instead - thus causing interference.

If you can identify the offending black wire as the earth connection, connect it to the an exposed part of the car's metalwork whilst interference is present.

If there is a big spark and a bang - it's not the earth wire, >;-X but if the interference goes away, make the connection permanent and pat yourself on the back for perseverence. {:-D

Good luck!

Sam
Sam Murray

Sam, thanks mate.

Phil
Phil

This thread was discussed between 02/09/2002 and 10/09/2002

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