Testing O2 Sensor Installed
The engine must first be fully warm. If you have a defective
thermostat, this test may not be possible due to a minimum
temperature required for closed loop operation. Attach the
positive lead of a high impedence DC voltmeter to the oxygen
sensor output wire. This wire should remain attached to the
computer. You will have to back probe the connection or use
a jumper wire to get access. The negative lead should be
attached to a good clean ground on the engine block or
accessory bracket. Cheap voltmeters will not give accurate
results because they load down the circuit and absorb the
voltage that they are attempting to measure. A acceptable
value is 1,000,000 ohms/volt or more on the DC voltage.
Most (if not all) digital voltmeters meet this need. Few
(if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter
to look for 1 volt DC. Many late model cars use a heated
O2 sensor. These have either two or three wires instead of
one. Heated sensors will have 12 volts on one lead, ground
on the other, and the sensor signal on the third. If you have
two or three wires, use a 15 or higher volt scale on the meter
until you know which is the sensor output wire.
When you turn the key on, do not start the engine. You should
see a change in voltage on the meter in most late model cars. If
not, check your connections. Next, check your leads to make sure
you won't wrap up any wires in the belts, etc. then start the
engine. You should run the engine above 2000 rpm for two
minutes to warm the O2 sensor and try to get into closed loop.
Closed loop operation is indicated by the sensor showing several
cross counts per second. It may help to rev the engine between
idle and about 3000 rpm several times. The computer recognizes
the sensor as hot and active once there are several cross counts.
You are looking for voltage to go above and below 0.45 volts.
If you see less than 0.2 and more than 0.7 volts and the value
changes rapidly, you are through, your sensor is good. If not,
is it steady high (> 0.45), near 0.45, or steady low (< 0.45).
If the voltage is near the middle, you may not be hot yet. Run
the engine above 2000 rpm again. If the reading is steady low,
add richness by partially closing the choke or adding some propane
through the air intake. Be very careful if you work with any
extra gasoline, you can easily be burned or have an explosion.
If the voltage now rises above 0.7 to 0.9, and you can change it
at will by changing the extra fuel, the O2 sensor is usually good.
If the voltage is steady high, create a vacuum leak. Try pulling
the PCV valve out of its hose and letting air enter. You can
also use the power brake vacuum supply hose. If this drives the
voltage to 0.2 to 0.3 or less and you can control it at will by
opening and closing the vacuum leak, the sensor is usually good.
If you are not able to make a change either way, stop the engine,
unhook the sensor wire from the computer harness, and reattach
your voltmeter to the sensor output wire. Repeat the rich and
lean steps. If you can't get the sensor voltage to change, and
you have a good sensor and ground connection, try heating it once
more. Repeat the rich and lean steps. If still no voltage or
fixed voltage, you have a bad sensor.
If you are not getting a voltage and the car has been running
rich lately, the sensor may be carbon fouled. It is sometimes
possible to clean a sensor in the car. Do this by unplugging
the sensor harness, warming up the engine, and creating a lean
condition at about 2000 rpm for 1 or 2 minutes. Create a big
enough vacuum leak so that the engine begins to slow down.
The extra heat will clean it off if possible. If not, it
was dead anyway, no loss. In either case, fix the cause of the
rich mixture and retest. If you don't, the new sensor will
fail.
Testing O2 Sensors on the Workbench
Use a high impedence DC voltmeter as above. Clamp the sensor in
a vise, or use a plier or vise-grip to hold it. Clamp your
negative voltmeter lead to the case, and the positive to the
output wire. Use a propane torch set to high and the inner blue
flame tip to heat the fluted or perforated area of the sensor.
You should see a DC voltage of at least 0.6 within 20 seconds.
If not, most likely cause is open circuit internally or lead
fouling. If OK so far, remove from flame. You should see a
drop to under 0.1 volt within 4 seconds. If not, then the sensor
is likely silicone
fouled. If still OK, heat for two full minutes and watch for
drops in voltage. Sometimes, the internal connections will open
up under heat. This is the same as a loose wire and is a failure.
If the sensor is OK at this point, and will switch from high to
low quickly as you move the flame, the sensor is good. Bear in
mind that good or bad is relative, with port fuel injection
needing faster information than carbureted systems.
ANY O2 sensor that will generate 0.9 volts or more when heated,
show 0.1 volts or less within one second of flame removal, AND
pass the two minute heat test is good regardless of age. When
replacing a sensor, don't miss the opportunity to use the test
above on the replacement. This will calibrate your evaluation
skills and save you money in the future. There is almost always
no benefit in replacing an oxygen sensor that will pass the
test in the first line of this paragraph.
Resetting the O2 Warning Light
Randy Wilson
First off, this reset has nothing to do with the O2 sensor itself. It
is merely a maintenance "reminder" light, required by our dear government,
telling you it's time to do the x0,000 mile service. It's driven by the
speedo, and turns on a light on the dash after a cetain mileage has passed.
In the case of Jag, the light says Ox sensor. Other brands had the light
labeled "EGR" or "Service" or ...
The early cars with mechanical speedo had a Smiths unit in the middle of
the speedo cable. It's a white box mounted in the engine bay at the
bulkhead. This unit has an odometer looking counter that shows the
percentage of mileage that has passed. It is reset by turning the recessed
knob on the side until the counter shows 0000. The knob has two holes in
it to turn it. Smiths makes (made?) a little key that fit. I have sucessfully
reset them with a set of small circlip pliers, though this is a pain.
The later cars with electronic speedos use a different system. This is one
of my favorite examples of British parts bin engineering. The box, as you
mentioned is mounted in the boot on the forward bulkhead. This box is black,
and has a push button to reset it. That's it. Push the white button (it is
recessed behind a guard), and kerchunk, it's reset for another 30K miles.
What makes this such a favorite of mine is the box. It's a VDO piece that,
like the Smiths, is designed to go inline with the speedo cable. Jaguar,
unlike most everyone else, did not go the all electronic route when they
converted to the electronic speedo. They made a interface that takes the
signal from the pulse generator, and drives a stepper motor attached to
this mechanical counter.
Hey, it works... :>
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