Date: Wed, 30 Jul 2014 14:07:17 -0700
Reply-To: mark drillock <mdrillock@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: mark drillock <mdrillock@COX.NET>
Subject: Re: Another question about O2 sensors..
In-Reply-To: <Ycdn1o01108X5Fr01cdqyQ>
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Unless you know the exact year and model car that your inline 4 wiring
harness came from it may be difficult to know exactly how it should be
connected. Because VW used different variations of that 4 pin O2 sensor
wiring there is no guarantee that what you had before was right either.
The old O2 sensor may not have been an exact match to the one that
harness was fitted with from the factory.
In any case, the shield for the coax should be grounded but only at 1
end. So it needs a ground at one end and no connection at the other end.
Never should both ends be connected to ground. From your description my
guess is you have a harness where the shield is connected only to pin 3
of the 4 pin. If so that would mean it should have a wire running from
there to the engine, grounded to bare metal.
But even if that capped wire is supposed to be grounded it is possible
that you would notice no big difference by doing so. That is because the
grounding of the shield is only to let the shield absorb stray
electronic noise that otherwise could be picked up by the inner signal
wire. Since the normal O2 sensor signal is so small, the circuit is more
susceptible to interference and that is why the signal wire from the
sensor to the ECU gets the coax shielding. So grounding one end of the
shield lets it protect the signal wire from electronic noise but it
isn't used by the ECU for processing the O2 sensor signal. It is not the
reference ground, that is elsewhere.
Mark
Don Hanson wrote:
> That "reference ground' is one of the things (there are others, of course)
> that I am unclear on.
> My original heated plug, the one I just replaced with the single wire
> Bosch splice-in sensor, that original one had 4 wires from the ECU to a
> flat inline plug. Two were the heating wires, one was a black ground which
> I 'capped off' and one was the signal wire, which I crimped with the
> provided connector to the output of the single wire Bosch O2 sensor.
> The original heated sensor, it had a matching flat plug to connect to the
> one from the ECU, but one of the plug connection slots was blank,, and
> instead, the fourth wire from the O2 sensor went around the plug with a
> loop ring connector and was grounded at the engine on a valve cover bolt.
> I am unclear about what happened to the ground wire from the ECU at that
> plug, originally..
> The plug from the O2 sensor was arranged with a gap between the signal
> wire (purple from the ecu) and the other 3 wires...like this _ _ _ _ .
>
>
> I was told the single wire O2 sensor gets it's ground from the exhaust
> system. So as of now, I've simply left the ground wire, the one
> returning to the ECU inside the shielded sheath,, the one that was formerly
> connected (?) to the O2 sensor but is not required there any longer, I left
> that wire un attached to anything.
>
> So now I am wondering if that one (the black wire inside the shielded
> sheath that returns into the ECU) is required to go to ground somewhere in
> order to actually supply the signal from the new single wire sensor? I
> guess I could connect thtat ground wire somewhere and TRY it, but I am
> leery of messing up the ECU or the O2 sender with stray electrons in "bad"
> paths...
>
> Can anyone advise here?
>
>
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