In a message dated 1/29/98 5:46:18 PM Eastern Standard Time,
tjordan@pa.ausom.net.au writes:
> What exactly is the blue thing? I have always assumed that it is a
> capacitor for ignition noise suppression, but I cannot find it any handbook
> or parts manual to confirm that.
Trevor,
It is a capacitor (aka condensor). It is shown as item # 10 on page 339 of
the TRF catalog.
> I am trying to eliminate ignition noise by progressively replacing all the
> suspect parts and disconnecting the blue thing makes no difference at all.
> This either means that it has failed or that it is not the source of the
> problem. If I knew what it was, I could replace it.
If you have a modern radio, you probably don't need it. Most modern radios
come with a little plastic cube attached to the power lead, which contains
electronics to eliminate radio noise.
> Incidentally, can anyone explain why the blue thing is connected between
> the coil and the block, is there some advantage in connecting to ground via
> the block and the transmission ground strap?
Yes. There are two types of noises generated by most noise sources -
conducted and radiated. Conducted noise travels, as the name suggests, down
the wire from the source to the radio. An inductor in series with the power
lead is very effective in preventing conducted noise from reaching the radio.
The other type is radiated -- the noise radiates from the wire much like a
radio signal from an antenna. To prevent this type of noise from reaching the
radio, a capacitor is used in parallel with the coil. The capacitor works by
shorting radio frequencies (radio noise) to ground. The closer it is to the
source of noise, the more effective it is in getting radiated noise to ground
before it can cause interference. In other words, by placing it close to the
coil, there is very litttle length of power lead available to act as an
antenna.
Most of the noise generated by the primary side of the coil, evidently, is
conducted, rather than radiated.
As to replacing it, if you can't get an exact replacement, buy the largest
capacitor that will fit the space available, and that can withstand the
environment under the hood. I can't think of any reason why larger
capacitance values would be a problem, unless you got really HUGE values. If
the capacitor were big enough, it might take a few seconds for the engine to
shut off after you turn the key off, and it might take a few seconds for the
capacitor to charge up before the engine could be started. A capacitor of
that size would be so big, and so expensive, that you are not likely to buy
one by mistake.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
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