You seem to have an intermittant problem and each sensor can be checked with a scope or you can ohm meter each. If you are having a start/running problem, it would more likely be the Crank sensor . Look at your WIS under CPS tetsing.
Does the tach work?
You may also just have to run the car a few days b/c after a memory clear , the car management has to relearn . That takes some driving.

Do you see a CPS code anywhere on the 8 module?

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A Dalton

Car management has to re-learn? I didn't know they were that sophisticated in 1990. I will check my WIS under CPS testing. The tach works perfectly. With an ohmmeter, am I just checking for contuinity on each wire looking for breaks?

I am not sure I understand your question about a CPS code on the 8 module. If you are referring to pin 8, I only have readouts on pins 3,6,7 & 14. Except for the push button and LED, all other pins are blank.

Ok .. Pin 8 would be Ign module on later models w/16 pin DM
So, yours will be Pin 3.
Like I said , I just can't find any info on that Cal version , mostly b/c it was early Cal w/16 pin...so I do not know what they have . I did find codes for that chassis, but Not Cal. Emmssions.
But I do have TN codes on Fed versions and you can ohm the sensors from the end of the cable and see if your WIS gives you those values..you can also flex the cable/harness and watch the meter for intermittant readings, indicating poorconnection/wiring.
The AllData just has "Check speed sensor wiring" as test for TN code. They also have a scope pattern test at ECU, but that does not help you. [ unless you have a DSO?]
I will do some more searching, but that is an early Cal car and info is hard to find...Your WIS would prob have more info than my AllData does.

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A Dalton

According to the documentation that I have, Pin (socket) 3 is for the Continuous Fuel Injection System (CFI) and as you indicated earlier addresses the same DM as the built-in LED for Cal versions. I suspected this earlier since the flashing light on the home-made reader flashed in sync with the built-in LED while my yellow wire was in this socket.

I now have 2 codes present on that socket: 17 "Oxygen sensor is shorted to positive or ground" , and as of yesterday 18 "Current to idle control valve is illogical".

The code 17 is the same one that I had about 2 years ago. Replaced Oxy sensor, tested all wiring, then someone on this group suggested checking the Duty Cycle. Made adjustments to that and the code has been gone since then. About 1 month ago my CE light came on and Code 17 returned. That is what got me motivated to build the code reader and get everything tested and fixed.

Code 18 is new since I erased the memory yesterday and started having the idle trouble.

That could be 02 sensor sig, duty cycle, or even OVP relay . Tap that relay and see if any change and check the fuse on top..I would even pull it and see what the socket and pins look like. That is power feed for ICV

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A Dalton

Thanks for the input. OVP was replaced within the last 18 months. Big weekend planned with USC-Ohio State game in town. I will test on Monday and report findings.

Well, you may just want to pull the rug and check those 02 sensor wire connections under the rug there ....the code is telling you an open or shorted 02 sensor, so it could be right at the plug from footwear..I have seen that happen in cars with a lot of passenger traffic.
You should also have 12v there for 02 heater.

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A Dalton

I will certainly check the O2 wires again. When I did so 2 years ago, they were perfect and the sensor was performing within spec. Still had the Code 17 though. The code finally went away when I adjusted the duty cycle.

Can the idle control valve be damaged by clearing the memory on the diagnostic module?

I only ask because the idle trouble and Code 18 started after doing so. Will the idle issue be corrected and the code disappear after the car rmanagement re-learns?

No. or at least I doubt it. I just can't find any info on that chassis b/c it was in between on the diagnostics and emmssions mandates, being early cal.

The reason I had you try the relearn is that being an early 16 pin CAL system , it may have had adaptations, but if it did not, then trying the Memory clear procedure would just do nothing b/c there would be nothng in there to address. If there was no memory, you would just be redoing the code clear procedure. I think , as you do, that memory came a couple of yeares later w/HFM and LH 12 pin.
That car has MAS Controller according to my info , so I am unsure on this point.
I would not rule out the 02 sensor as that is where things are pointing. Duty could also be the problem, being out of 02 range. And OVP is feed for duty, so is there a feed problem there ..???
I would look at 02 first...

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A Dalton

Recap: 1990 300te California car, 148k mi.

I pulled the OVP and checked its fuses and connections. All good. Was replaced July 2006 at 120,000 mi.

Since clearing, the code 18 on socket 3 has not come back.

Pulled the rug and all wires to O2 sensor test good.

After all testing outlined below, Code 3 on socket 14 returns (TN/TD signal (RPM) interrupted). I have ordered both speed sensors and will install later this week.

Car stalls upon hard braking. Stalled on a freeway offramp this weekend. I put it in nuetral, and restarted while coasting. This problem has now happened about 10 times in the last 4 months.

I share your concern of the O2 sensor/duty cycle. After I clear code 17 on socket 3, the CE light goes out but returns immediately after re-start along with code 17. With the Sears multimeter I get a duty cycle reading of 81.36%in KOEOff mode. I know from past experience that I should get the 85% CA reading.

When O2 sensor warms up, the duty cycle starts and ranges from 37-46% at idle. O2 sensor stabilizes at a reading of .17 volts. If I increase the RPM to 1500, the duty cycle defaults back to a fixed 81.36, but then kicks back in after about 30 sec, and displays in the range of 73% (O2 signal then jumps to .8v). Increase to 2500 RPM and the duty cycle defaults back to fixed 81.36 for 30 seconds and then displays in the range of 120% (O2 signal still at .8v)

The O2 sensor appears to be functioning correctly according to the Landis article (.2 to .8v). My understanding is that if the mixture is rich it will read above .7v, lean below below .3v).

My duty cycle is totally wacked. I hesitate to start adjusting when I can't even get the 85% baseline CA reading. What do you make of my getting readings of 73-120% at 1500-2500 RPM?

Man, do I feel stupid.
I accidently had hit the HOLD button on my Sears Duty Cycle meter and that's why it was displaying 81.36% in KOEOff mode. Also, output signal type must be set to %. Now it reads 50% in KOEOff mode (perfect).

When Arthur had me clear the memory to reset Code 17 in socket 3, it must have reset my duty cycle as well since after I corrected the buttons on the meter, my duty cycle was about 80% (way to lean). Turned the set screw CW and now it runs between 45-50% on idle (perfect). Only problem is that at 2000 rpm, duty cycle is 65-71%. Spec calls for it not to vary by more than 10%. Is this cause for concern, and how to correct?

Issue # 2: (Original problem)
I also just replaced my front speed sensor and my rear crank sensor. Still getting the code 3 TN/TD signal (RPM) interrupted on socket 14. It comes back immediately even after clearing the code and clearing the memory of the module. Any ideas on how to correct this code?