Ok folks...need some help here. I've searched the message board and did not find references to my problem...if this is a repeated question, please point me in the right direction....

I have an 89 RS convertable, 305 TBI automatic. The car runs (ok) but most of the time runs in open loop. At times the car tries to go into closed loop yet throws an error denoting "open or shorted O2 sensor"....sounds pretty straight forward. This is what I have done to (try) to correct the problem.

1. Replaced the O2 sensor (many times...)
2. Checked voltage out of the new O2 sensor +.2 to .6 volts depending on throttle position
3. Traced wiring back to ECM and checked to see if the wire is bad, open or has a high resistance...wiring is fine.
4. Cut O2 signal wire (Purple D7) on ECM and measured voltage...same voltage reading as directly out of the O2 sensor +.2 to .6 volts
5. Repaired cut wire....
6. Replaced ECM...(using the same PROM that came with the car)...same problem.
7. Measured voltage, while the engine is running from pin D7 Purple O2 sensor wire to D6...supposedly O2 ground reference....this shows a negative (-.2 to -.6) volts....hmmm....

Car runs ok...yet idles rich, (burns your eyes)...yet always shows running in a lean position.

Actron Autoscanner readings follow;

Codes Present - No (sometimes shows bad O2 sensor)
4th gear switch - On
AC Request - Off
Air Divrt Sol - DIV
Air Sw Sol - CONV
Battery - 13.9
BCM - 128
Coolant - Varies based on engine temp
Engine RPM - Varies based on throttle position
Exhaust O2 - Lean
High Battery - No
IAC Position - Varies 0-26
Integrator - 128
Loop Status - Open
Map V - Varies .78
Mat F - Varies
O2S mv - 457 (fixed...not changing)
P/N Switch - P/N
Prom ID - 7501
TCC Lockup - No
TCC Solenoid - Off
TPS Sensor - .73 varies based on throttle position
VEHSpeed - Varies

Any help would be appreciated...I'm not sure about the accuracy of measuring between ECM O2 sensor wires D7 and D6...(positive lead of the multimeter) on purple wire D7 O2 sensor feed....while the car is running...yet may be an indicator that (perhaps) the ground/reference is screwed up....

Thanks in advance....

 

Folks, solved my own problem...thought that I would share with the group. The problem was a bad O2 sensor ground (pin D6 - tan wire). Talk about a head scratcher! Cut the wire, ran a separate ground to the chassis and everything works wonderfully...the car goes into closed loop within 3-4 minutes and stays in closed loop!

Again, just to re-iterate...the majority of electrical problems are due to faulty grounds!...this just happened to be an O2 signal (reference/ground)....

 

where is this ground you speak of for the oxygen sensor and ecm?

 

On the ECM 32 pin large connector...looking at the back right side find the tan wire, it should be in the 6th position refered to as D6 pin. This is O2 signal ground, D7 or the 7th position again on the right side, purple wire is the O2 signal.

 

so the o2 is grounded through the ecm?

 

For a single wire O2 sensor the physical connection to the exhaust is ground for the O2 sensor....for a multiwire/heated O2 sensor a separate ground wire is used.

In my case, I have a single wire O2 sensor, I ensured that the sensor, and then the exhaust/engine was grounded to frame ground...yet the ECM was not reading the sensor correctly. D6 according to Chiltons should be connected to a braided wire which the connects to frame ground (somewhere in the wiring harness). When I measured the resistance of D6 to ground, it showed and open condition. I then measured the voltage between D7 and D6 (O2 signal to ground) and was recieving a negative voltage....I ran a separate ground to D6 (cut the wire, and connected the (cut) wire to frame ground)...and everything works fine now.

 

now what happened to tell you it did not see the o2 sensor? did you get a code 13? for no or bad o2 sensor?

 

No, it would always run in Open Loop, therefore the ecm would never want to read the O2 sensor. Open loop is a default setting that when you first start your car...

 

how can you tell is it stays in open loop or how can you tell when it goes into closed loop?

any reason why idle would be horrible in open loop and fine in closed?

 

I have a OBD I scanner that plugs into the ecm/car diagnostics port, tells me when the car is in open/closed loop mode. If you do not have a scanner/tool you can also get the check engine light to blink, by jumpering certain pins on the diagnostic port...check/search other entries on this discussion board...search for blink and open loop and you should find it.

First things first check to see if your in open/closed loop when idling poorly then take it from there....different components come into play based on if the car is running in closed loop versus open loop.

 

how much did your scanner cost and where would u go to get one just curious?

 

if you have an old pc-compatible laptop you can make a DIY scanner for ODB-I compatible cars, using the printer port. I've made one too, costs a few euros or dollars (about 10 or maybe 20) and is fun to do...

mine looks like this

 

Mine's an Actron Autoscanner that works with both OBD I and OBD II autos...does real time display of (99) components...ie O2 sensor(s), MAF, temperature...majority of devices that feed the ECM. I find it handy when troubleshooting....

You can get them it at many auto supply chains...I purchased mine from Sears...it set me back about $350 US dollars. I maintain all of my autos...it was money will spent...