Jon,
This is my first actual post to the digest, although Tim May and Gene Bea=
ird
have been indirectly posting some things for me in the past (I checked th=
e
archives <g>). I joined this list even though I don't have a Dakota, but =
I do
have a Magnum engined relative, a Jeep GC 5.9 Limited. On the GC list all=
they
talk about are lift kits and offroading at some place called Moab <g>, an=
d
personally I bought my GC as more of a high performance heavy duty statio=
n
wagon with the capability to handle Chicago winters rather than as a true=
=
off-road vehicle. The GC listers disconnect their swaybars, while I put =
BIGGER ones on. <g> I was drawn to this list because the emphasis here is=
on =
performance and handling, which is what my mods are all about. I have =
recently completed a host of mods to this vehicle, including many that wo=
uld =
pertain to Dak owners as well.
I have some experience with PCMs and OBDII in general, and O2 simulators
specifically, so I thought I should post some corrections and additions t=
o
the recent post(s) on the subject.
>The sole purpose of the O2 sensor after the cat is to determine the
efficiency of the cat.
This is true!
>(Basically, the computer compares the readings from the downstream senso=
r to
the upstream sensor, and it expects to see more oxygen by volume at the
downstream sensor than what the upstream sensor is reporting, since the c=
at
is supposed to remove the extra junk.)
Not exactly. The upstream sensor is defined as the one(s) BEFORE the cat,=
while the downstream sensor(s) is/are AFTER the cat. The cat functions to=
REMOVE oxygen from the exhaust, which is what the sensor is looking for. =
The
downstream sensor therefore expects to see LESS oxygen.
>(The upstream sensor is also used to determine the fuel/air ratio which =
the
PCM (computer) uses to adjust the mixture at power settings less than WOT=
=2E)
Exactly!
>Each sensor has 4 wires. Two are for the heating element (to make the s=
ensor
readings more accurate I'm guessing), one is a ground provided by the PCM=
,
and the other is a voltage to the PCM (0 to 1volts)
The heater is to heat up the sensor so it begins to function more quickly=
after a cold start. O2 sensors don't function properly below 600 degrees,=
and
the sooner they function the quicker the vehicle can go into closed loop
(sensor controlled) operation. Cold start emissions is one area the EPA h=
as
been attacking recently, so all O2 sensors now must have heaters to get t=
hem
functioning more quickly.
>Its been a while since I looked at this stuff but if I remember correctl=
y, 0
volts would mean 100% oxygen, and 1 volt would mean 0% oxygen.
Exactly the opposite is true. More voltage indicates MORE oxygen.
>its very easy to see what would happen if you removed it; just unplug it=
and
go driving. I did exactly that, and I drove for about 45 miles with no C=
EL
(check engine light). However, as I was going up a grade, I was behind a=
slow car which I was about to pass. As I was waiting for an oncoming car=
to
go by, I was in 3rd gear at about 3,000rpm for several seconds. That's w=
hen
the CEL came on.
The Catalyst Monitor (that's what the test function using the downstream =
O2
sensor is called) is a "once per trip" monitor, which means that sensor's=
function is tested once per vehicle trip, after certain enabling conditio=
ns
(such as reaching closed loop operation) are met. A trip is basically def=
ined
as a cycle where you: start the engine, drive (reach closed loop and meet=
some other criteria), and then stop the engine. The other enabling criter=
ia
make it a little more complicated than that, but for now this description=
should be adequate.
If the Catalyst Monitor (downstream O2 sensor test) fails, a PENDING code=
is
set, but the MIL (Multifunction Indicator Light, that's what the Check En=
gine
light is officially called) will NOT be lit. On the SECOND "trip", if the=
same test fails, then the MIL will be lit and the pending DTC (Diagnostic=
Trouble Code) or fault, becomes a full-blown stored DTC. In other words, =
you
have to have two failures in a row for OBDII to first find a problem (set=
a
pending code) and then turn that problem into a DTC complete with MIL or
"Check Engine" light.
Virtually all OBDII monitors work this way except the Misfire Monitor, wh=
ich
depending on the level of the misfire, can set the MIL immediately. It ca=
n
also cause a flashing MIL, which is an indication that the misfire is bad=
enough that the catalytic converter will be damaged.
Just as a side note, let's say you disconnected the downstream O2 sensor,=
or
your cat failed and the sensor was functioning. On the second trip where =
this
occurred, the MIL would be lit, and remain lit for every trip that follow=
s
where the test continued to fail. Once you repair the condition, the PCM =
will
begin reporting a passing grade for those tests. After three passes of th=
e
test(s) which initially caused the DTC and lit MIL, the MIL will be turne=
d
off. In other words, after a failure is no longer being reported, the PCM=
will count "good trips", and after three good trips, the MIL will be
extinguished and the DTC retired to history.
Of course you can manually remove DTCs (and extinguish the MIL) using a s=
can
tool, or in many cases by disconnecting the battery.
>There are a few Mustangs without cats that I have heard of who are NOT
getting CELs.)
That simply cannot be true on stock '96 and up vehicles. If it were true,=
Ford would be in SERIOUS trouble and fined by the EPA, BIG TIME. Honda wa=
s
recently fined many millions of dollars (I seem to remember hearing a fig=
ure
over $200M) for inappropriately turning off the misfire monitor under cer=
tain
conditions in some of their vehicles, so they take this very seriously!
>So, there is at least one documented case of a CEL as a result of no O2
sensor.
I can assure you that in ANY '96 and newer (OBDII) vehicle, simply removi=
ng
or disconnecting the downstream O2 sensor will result in at least one, if=
not
several, DTCs. It would notice the open heater circuit, as well as the
failure of the sensor itself.
>(I have no idea what would happen if you just removed the cat and left t=
he
downstream sensor in place; that might work as well.)
The PCM would definitely sense too much oxygen and fail, there would be a=
pending DTC on the first trip, and a full-on MIL with stored DTC on the
second.
>If a CEL doesn't bother you, you can just reset the computer whenever yo=
u get
one, but I'd rather not see it at all since then I've got to pull the cod=
es
and make sure the CEL is because of the O2 sensor and not a more serious
problem.
Sure, you could disconnect the battery, or use a Scan Tool to delete the
pending or stored DTC (and thus shut off the MIL) after each first or sec=
ond
trip. However there are several reasons why IMHO this is NOT a good
technique. <g>
>So, I'm working on a module to simulate a downstream O2 sensor behind a
functioning cat.
No need to re-invent the wheel. I've been testing one for some time now, =
and
so far it works perfectly. It was developed by a friend of mine several y=
ears
ago initially for GM products, but a simple wiring and connector change h=
as
it working beautifully for Chrysler as well.
BTW, the circuit either may not be as simple as you might imagine, or in =
some
ways may be simpler than you think. One thing to know is that most are
looking for true O2 transitions, not just a steady voltage.
>This module will plug into the wiring harness using the same connector a=
s an
O2 sensor, and will also have to read the voltages being sent from the
upstream sensor to the PCM.
I can assure you at least for now, that type of connection is not necessa=
ry.
The properly functioning circuit I am testing plugs directly into the
downstream O2 sensor connector, and is powered by the heater circuit on t=
hat
connector so no extra connections are necessary. It is a completely seale=
d
(in fact potted) unit about the size of your index finger which is totall=
y
impervious to dirt and moisture intrusion. It has a hole drilled through =
the
potting material for tiewrapping securely to the undercarriage.
>If anyone knows of a place to get the type of snap-together connectors u=
sed
in the wiring harness and O2 sensors, that would be helpful, since this w=
ould
allow someone to simply plug it in without having to do any cutting or wi=
ring.
In lieu of purchasing the connector, terminals and seals separately, I us=
ed
the original stock downstream sensor, cut off the wires, attached a Packa=
rd
Electric 4-pin weatherpack (sealed watertight) connector, which mates to =
the
same type of connector on the finger sized simulator unit. It then become=
s a
direct plug-in to the Chrysler wiring harness.
>As far as killing low end, I don't have enough info to be sure.
The cat delete is normally worth 10 HP or more on most vehicles. I agree =
that
with headers AND a low restriction DUAL exhaust, subsequently deleting th=
e
cat may produce so little restriction as to somewhat compromise low end
torque. However with the stock manifolds, and either a stock or modified
SINGLE exhaust, the cat delete really wakes things up. Not only power is
improved greatly but the sound as well!
In SOME rare cases with backpressure sensitive EGR valves, reducing
backpressure drastically can result in problems with EGR operation, on so=
me
vehicles setting a code. The '96 and newer Magnum engines don't use an EG=
R
valve, so that definitely would not be a problem there, and I honestly do=
n't
think it would be a problem on the '95 and earlier ones either (although =
I
haven't tested that).
>The sound definitely was different. It was louder, and had more of a
"tuned" or "race" note.
Exactly! If you haven't heard it, the difference is more than you would
expect.
>Once I get my truck back, I'll install my module to do some additional
testing. Once I'm satisfied with it, I'll post the parts and info requir=
ed
to make one yourself on the DML page.
I'll be interested to hear IF your module works, and how it was construct=
ed.
Remember that you'll need to successfully navigate at least two complete =
trips
to verify no MIL, and you'll really need access to a scan tool to see pen=
ding
codes, as well as to view the PCM's interpretation of what your module is=
doing. Without a scan tool you'll be testing blind.
>My eventual plan is to have a "quick connect" type of arrangement which =
I can
use to swap the cat in and out with a minimum of hassle. (I'm planning t=
o
use stainless band clamps.)
I found that band clamps unfortunately wouldn't give enough clamping forc=
e to
prevent the muffler and tailpipe tip from rotating. Part of the problem i=
s
the anti-seize which I used to coat the pipes where they join, but that i=
s
important to prevent them from rusting together, thus facilitating an eas=
y
removal later. I ended up using some nice stainless steel muffler clamps,=
which have the advantage of never rusting, which looks better and allows =
them
to be easily removed in the future.
Another thing I did as much for function as appearance was to have the pi=
pe
Jet Hot coated inside and out to prevent any corrosion from occurring. Wi=
th
this coating, the anti-seize, and the stainless clamps, it should be no
problem to separate these pipes even years in the future. Scott.
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