PCM can't communicate with scanner
 

Hi all. I'm having a bit of an issue reading codes on my PCM. The motor seems to start and run okay, but the CEL is on all the time. I've tried 3 different code readers, tried with the key on, engine off & key on, engine on. Batt voltage was a bit low at 11.3-ish, so I ran jumper cables to my truck and that didn't work either (I read on here that a guy could only read his codes when hooked up to a battery charger).

The OBD2 port is hooked up right next to the PCM...there is battery voltage and 2 grounds present, and the data line ohms out to the PCM connector, and the "floating" portion of the dataline has been cut. I ran one GND to the engine and the other to chassis, like the diagrams say. I have also tried tying both of these together with no luck. All power and ground pins at the DME look good to me. All connector contacts look good to me...no visible corrosion.

Full disclosure: this is an L33 swap into an e36 BMW using a 99 F-body PCM with the later OS, a 2000 F-body wiring harness, and I re-pinned the PCM connectors to use the rear O2 wiring for the front sensors (pretty straightforward). I got a mail order tune and had all the typical stuff deleted: rear O2s, CAGS, VATS, Evap, EGR, AIR.

I'm not sure what else to try, other than seeing if there is a local (Colorado) tuner who could hook my PCM up to a benchtop harness and read the pending code(s). That doesn't quite fix the problem, but would be a good interim solution so that I can get back on track. Does anyone have an idea? I have read that a bad PCM typically won't run the motor, so I've discounted that, but I'm running out of ideas here.

My money is on wired up wrong. Do you know anybody with a '99/'02 F body, '99/'04 Vette, or '99/'06 truck you could plug your PCM into, just to turn the key on, to see if it communicates?

Just to make sure the connector is wired correctly, the two grounds are on pins 4 and 5, 12v on pin 16, and the data is on pin 2. Is that the way yours is wired? Many scan tools require both grounds (i.e. 4 and 5 together).

Yes, as far as I know. I'll triple check the wiring tonite...mainly the data line, since I know that connector has power and ground. I went off of the 99-02 F-body PCM pinout table on lt1swap.com (http://www.lt1swap.com/2001_PCM_CONN_PINOUT.htm). I physically traced the wires from the PCM connector and soldered the OBD connector in about a foot away.

My OBD port only has those 4 positions populated, so no confusion there. All pins look to be damage and corrosion free. Not sure what else could be going wrong, assuming my data line is spliced into the right wire.

Does the scan tool light up it's display when you plug it into the DLC? Make sure you know exactly the orientation of the connector pins of the DLC. Sometimes the diagrams show the cable, not the connector, which would be a mirror image. If the tool lights up, you probably have that correct. The data should be at PCM pin C1.58; C1 is the blue connector. It should be a dark-green wire there. The DLC data pin has a purple wire, which then changes to dark-green from the star connector to the PCM.

The tool (not self-powered) does power up and tries to connect, so that kinda rules out the power and grounds.

I did tap into the DK GRN wire at p58, right next to the ORG Batt V wire used to power the DLC.

Regarding connector orientation WRT pins, I went by position number, which is marked on the connector housing. All of the populated pins look to mate up to its respective mate on the scan tool. I'll verify this tonite as well.

As far as I see it need to have good grounds as stated before on both pins 4 and 5 of the DLC. I would use a volt meter and read the battery volts if not like 12.5 or up I would use a trickle charge. Now check volt at DLC pins 4 and 5 (grounds) to pin16 (+) I would put a load on these pins like a halogen bulb to check the integrety of the circuit and check the available volts. About .3 volt below battery ok.with that good I would want to see 7 volts approximately on pin 2 (data line) key on and it should vary the volt a bit when run ( data exchange). Whatever is missing I would back track to fault. All should be present for proper operation and nothing more.

It does seem like these motors/PCMs are very sensitive to grounding in general. I'll maybe try to ensure that the grounds are as good as they can be. I like the light bulb idea...I'll see what I can scrounge up.


Also, I did some reading on the data protocol that the PCM uses on that serial data line...are you sure that you get +7V with key on? It looks like that line idles low. Also, with the engine running, do you actually get a ~7V reading on a multimeter? Given the data rate on the line and the RMS nature of multimeters, I'd think the voltage would read as much lower. Not doubting you...just wanting to be sure of what to expect when I measure.

Might have to borrow an oscilloscope from work...

Wow...sorry to waste your time, guys. As it turns out, my connector wasn't pinned the way I expected it to be. The first row of contacts on the OBD port is symmetrical...the data line was in pin 7, which looks exactly like pin 2 (with respect to the end of the row and pins 4&5).

I relocated the wire and the scanner worked! Thanks so much for all your help. As it turns out, reading pin numbers is helpful...

Nice! All good when it works!

Not a waste of time at all. I have made that exact same mistake myself.