1999 Chevy Cavalier no communication with scanner.

As mentioned in the title, I have no communication with my scanner.  The scanner works on another vehicle so It's not the scanner.  I have good power and grounds to the DLC.  I only have about 18 millivolts at the DLC on pin 2 which is the serial data line to the PCM for communication and 18 millivolts at the computer itself.  It should be around 7 volts.  Here is what I have done so far and if there is something that I missed or should do next.
1. Disconnected the connector at the PCM and placed a test light at the DLC with one pin at pin 4 and the other at pin 2.  I used my power probe on the disconnected connector at the PCM and put power to that circuit and the light lit up bright so no open or short in that communication line.
2. I checked all power and grounds to the PCM and they were all good.  
3. I also voltage drop tested the ground wires for the computer where they bolt to one of the transaxle bolts and got 2.2 mv and I also checked the case of the transaxle and it had the same 2.2 mv.
4. All tests were done with key on, engine off.

So, are there any other tests that I need to do to condemn the PCM?  Thanks.


 

Well, it turns out that my scanner is the problem on this car even though it works on other cars. It's an Otofix D1 that has worked on this car in the past and now for some reason it no longer does. I tried my Innova 5610 and Thinktool Pros on it and they both could communicate.
I did find in service data that there is not any voltage on the class 2 data line unless the scanner is trying to talk to the PCM so my voltage measurements with just key on, engine off with the scanner not hooked up were not valid. At least it seems like I don't need a computer for that problem anyway.

This car still has the intermittent long crank that I posted about a year ago. I've been hoping that the car would just break so that maybe we can figure it out.

I tried to talk to a 2000 ford ranger with an autel and whatever 2 other scan tool things my coworkers had the other week and I thought maybe something was wrong with the truck. A couple days later I had my snappy back and it communicated instantly and worked great. But sometimes it’s the other way around. 🤷‍♀️ also sometimes with imports I can view data pids on an autel that I can’t with my snap on. Software is weird. But! If you could communicate with it last year and now you can’t maybe you had some kind of update with your scanner that’s affecting comm with older cars? Or something new has happened to the car.

I don't know. The other vehicle I tried using it on to see if it was the scanner was my 1998 Chevy K1500 truck and it could communicate with it so I don't think it has anything to do with the scanner per say. It might have to do with the car itself but two other scanners were able to communicate with it so I don't know. I emailed Otofix but have not heard anything back from them.

I have seen exactly one vehicle with a played out obd port. It would work with just about any scanner, not with inspection station equipment. It's probably not your problem, but not completely unheard of

Well, here is an update of sorts. I had reached out to Otofix to ask them about this issue and they replied today asking for some information so I hooked the scanner up to give them a screen shot of the scanner saying no communication and what do you know, it communicated and I could get into the various modules and the blue light on the VCI was flashing like it normally should.

The only thing that I can think of was that when I was installing a new cooling fan assembly yesterday, I had unhooked the battery cables to move them out of the way and maybe doing that and reinstalling them must have reset something. Who knows how long the scanner will stay working.

Another update on this problem. Today the CEL came on and her son brought it over so that I could put it on the scanner. I used my Thinktool Pros this time and initially I had communication and was able to read the codes. I ended up clearing the codes in the PCM but when I went to clear them in the IPC, they would not clear and I had no communication. I tried going into other modules and doing a re-scan and had no communication.

I unhooked the battery and still did not have communication.

One of the codes that I had in the PCM was a P1629 which is an Anti-Theft device cranking signal code. I went through the flow chart for this code and the final recommendation was to replace the PCM. I'm wondering if the PCM is starting to malfunction and causing issues.

The other code in the PCM was a P0171 but the short and long term fuel trims looked perfect running only around 0-3% and never going over 10% when revving the engine up. After clearing both codes, neither came back immediately but I'm sure they may come back at some point.