which 3 wire O2 sensor ?

[David A. Cooley]

Hi Ted,
Yep, the 4th wire is the - side of the sensor...
Most of the 4 wire sensors ground that lead to the shell internally as
well, but Bosch isolates it... found that out when replacing the factory
sensor in my 95 LeSabre... 80K miles and a bit lazy..., put the new bosch
sensor in, started it up and the car went full rich when it hit closed
loop... Set the MIL light etc.  Checked the O2 volts, 980 mV... thought the
sensor may be wired backwards internally... exchanged it and the 2nd one
did the same thing... Finally checked the old sensor and the - sensor lead
was grounded to the shell, the Bosch wasn't... it was floating about 2
volts above chassis ground.  Hooked a jumper to ground and the car ran
normal again.
Made a permanent ground connection in the harness at the connector.  Called
bosch and spoke with them and they are in the process of comparing their
sensor against the AC and Echlin sensors (both grounded to the shell).
The other 2 are heater leads. (may have one of those grounded as well...)

At 04:20 PM 9/21/98 -0700, you wrote:
>is the 4th wire a ground for the sensor ?
>and the other two are the heater ?
>
>it is possible to read the resistance value of the heater to determine the
>temperature of the sensor ? or I suppose the ECU does that and if it is out
>of range it goes open loop.

You probably could, but the ECU only looks at O2 sensor activity... if it
slows and the voltage stays steadier than it's preset limits, then it sets
a code.
OBD_II actually tests the heater... About 5-10 minutes after you shut the
car off, it starts measuring O2 sensor voltage... if it's .45 (bias applied
by ECU) then it kicks on the heater for a preset time.  It should see the
voltage drop to nearly 0 after the heater has cycled, then come back up as
it cools.  If not, it sets a code.


>
>what is the common scheme to check the temp and keep it within limits ?

Monitor activity...  The sensor should never hold a steady voltage at idle
or cruise... only WOT.  It should be bouncing from rich to lean and back
again if it's working ok several times a second.

Later,
DAve

===========================================================
           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!
===========================================================