Paul, I have always had problems with my tach overreading, but had always
assumed that it was due to a fault in the primary ignition circuit. When I
was wiring the XM receiver into the car I was picking up noise. Initially I
had hooked it up using the available, optional, FM modulator that supplies
the receiver's power and also takes it's line out and converts it to a
signal that it inserts into the antennae at 88.1 MHZ. I don't get noise
here in general but I do pick up a pulsing or static like sound under
acceleration. I tried wiring it using the supplied cigarette lighter
adapter and taking the line out into the aux input of my radio this is the
setup that I would prefer to use but when I do this I get a rhythmic pulsing
that sounds like ignition noise. I tried numerous different physical
routings of the wires to no avail. Lastly I have an AC to DC converter and
when I plug the cigarette lighter adapter into this device the noise
disappears. I tried and repeated this with two different power adapters. I
haven't had the opportunity to investigate any further, hopefully I will be
able to do it on Tuesday since that is my next day off from work. I have a
suspicion that whatever is causing the noise that the receiver is picking up
is also causing the signal in the tach to be corrupted.
James Nazarian
71 MGBGTV8
71 MGB Tourer
----- Original Message -----
From: "Paul Hunt (T)" <paul.hunt1@blueyonder.co.uk>
To: "James Nazarian" <jhn3@uakron.edu>; "mgs" <mgs@autox.team.net>
Sent: Saturday, December 27, 2003 10:20 AM
Subject: Re: Electrical Noise
> It is normal for the battery voltage to drop to about 10v during cranking
> even on a good battery, lower still with a weaker battery or bad
> connections. It is also normal for electronic devices to have a point
below
> which they 'switch off', instead of 'browning out' like simple electrical
> devices. 2ndly you may have bad connections in the low current wiring
which
> results in low voltage to the electronic devices during normal running,
> which gets 'catastrophically' low during cranking. 3rdly they may be
wired
> to the 'accessories' position of the ignition switch which loses its 12v
> supply during cranking anyway. Measure the voltage on the brown during
> cranking - you may need to use an analogue meter to be able to get a
> reading - and see how close yours is to 10v. If that is what you see then
> stop worrying and JFDI. If 9v or less take another reading direct on the
> battery posts and see what you get. If much the same then either the
> battery is weak or some other effect of the engine is putting too much
load
> on the starter. If 10v or higher then you have bad connections between
> battery and starter, or possibly your battery cable has been replaced with
> an inadequate item. To check the connections and cable put one probe of
the
> voltmeter on the hot post (not connector) of the battery and the other on
> the hot post at the solenoid and measure the lost voltage during cranking,
> and again between the ground post of the battery and the starter body. If
> you have twin 6v batteries also do it on the battery posts that carry the
> link cable. Ideally you are looking to lose only a couple of tenths of a
> volt lost in the connections and the cable(s). If you see getting close
to
> 1v or worse then it is worthwhile investigating the amount being lost in
> each connection and the cable and cleaning up bad ones.
>
> All of which is nothing to do with electrical noise, particularly when
> everything else is switched off. What makes you think you have that
> problem?
>
> PaulH.
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