o2 price?

[Robertson, Nigel]

David

I know absolutely nothing about the ECM you are referring to, but can I
offer this:

It electronics it is quite common practice to keep a common earth point for
various parts of the system and I would suggest that in auto electronics
with quite a few sensors that could be prone to noise the same is true.  The
ECM on my car (ROVER MEMS) certainly has this set-up.  The idea is to avoid
creating ground loops so running a ground wire, which is isolated from the
chassis ground, to the sensors as well as a signal wire prevents a ground
loop being set up around the ground wire and back through the chassis back
to the ECM common ground.

If the joints between various bits of metal aren't perfect, i.e. rusty body
panels, then every bad joint forms a higher resistance than normal.  Any
ground current flowing through this higher resistance will cause a potential
voltage to be developed across it and could therefore alter the ground
voltage at the sensor. 

If your ECM also has a common ground for its sensors then it would be
sensible to use the 4 wire O2 sensor.  The fact that the 4-wire sensor you
tried did not work could suggest that either the ground reference in the ECM
is not at ground ( check voltage at ECM pin to battery negative terminal) or
the ground wire usually a shield in the cable to the O2 sensor is broken.
Your non-isolated sensor worked because the ground was made through the
chassis and in your case this was a good electrical connection so luck came
into a bit.

You may have noticed the idea of running ground wires has increased in auto
electrics even for standard accessories, gone are the days when you just ran
a wire from the fuse box to the radio and bolted the outer casing to the
nearest bit of bare metal.  Those of you who may have built HIFI amps will
know the effect of ground loops.  They manifest themselves as an annoying
audible hum which stops you using the amp until you fixed it.  Your O2
sensor may be "humming" if you could here it you would certainly do
something about it.

Well that's my 2 pence, if anyone feels I need educating please let me know.

Nigel R

_________________________________________________________________________
Nigel Robertson, BEng. CEng. MIEE.
Senior Engineer
Roke Manor Research Ltd
Old Salisbury Lane
Romsey
SO51 0ZN
Tel 01794 833524
Fax 01794 526943
email  nigel.robertson at roke.co.uk


> -----Original Message-----
> From:	David Cooley [SMTP:n5xmt at bellsouth.net]
> Sent:	09 June 1999 16:49
> To:	diy_efi at efi332.eng.ohio-state.edu
> Subject:	RE: o2 price?
> 
> At 04:17 PM 6/9/99 +0100, you wrote:
> >Can you elaborate a bit more on what the extreme problems with the ECM
> are.
> 
> 
> The sensor, not being grounded to the outer shell, causes the inputs to
> the 
> ECM to "float"... mine went to -2.5 volts and made the ECM thing the
> engine 
> was starving for fuel...  The ECM compensated by going full rich and the 
> engine started chugging, missing and pouring black smoke out the 
> exhaust.  At first I thought the new O2 sensor was bad and made autozone 
> give me a new one... same thing... After a couple hours of playing, I 
> hooked a wire from ground to the wire that should have been grounded to
> the 
> shell of the O2 sensor and it ran right.  Pulled it out and checked with
> an 
> ohm meter, and it was totally isolated from the shell... Checked the old 
> AC, and the low side of the sensor was connected directly to the 
> shell.  Bought a new AC and all was well again.
> 
> 
> >When you say they are isolated from ground do you mean the ground wire is
> >not physically connected to the outer metal casing?
> 
> Exactly!
> 
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> David Cooley N5XMT Internet: N5XMT at bellsouth.net
> Packet: N5XMT at KQ4LO.#INT.NC.USA.NA T.A.P.R. Member #7068
> We are Borg... Prepare to be assimilated!
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