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2003 Cadillac CTS 3.2 V6 automatic with wiring problems at the MAF and the ECM grounds- P0103 P0118
What is known at this point:
MAF: Pin4 is a 5 v ref
Pin 3 is a low ref (sensor ground line) that goes to SP111 and two other sensor grounds, terminating at ECM C1 Pin 26 (the line I cut near the C1 and grounded to the block). Should I reroute this sensor ground line to another ECM ground line??? Which one?
I don't know why I'm not getting email notifications for updates to this topic...
You said earlier on that you measured ground on pin 3 of the MAF. If there was elevated voltage I would suspect a bad low ref circuit. In any case, just connecting it to battery ground or block ground temporarily to see if the code clears or if the MAF starts reporting should be good enough if indeed the low ref circuit is at fault.
I took a video showing a drive and the scanner PIDs, including MAF sensor values in gr/s, and also V. The communication to the OBD port would cut out sometimes, and then work other times.
I don't know how to attach a 29 MB quick time video so I just posted the picture below. From left to right it shows RPM, grams/second and then the bottom is MAF volts, and MPH.
The best way to post a video is to upload to an external server like you tube or something and then post the link here.
Is the loss of comm a symptom or do you think it is a scan tool issue?
It's not the scanner, I've seen the comms drop from the OBD port from different scanners. The drop comes and goes, sometimes on the instrument panel of car, the MPH is off, or twice the normal speed values. Sometimes the tackometer and speedometer stay at zero, while other gages like fuel level work. Mostly it works and I can get PID values from a scanner connected to the OBD port. Should I put an oscilloscope on the OBD ports 2 and 10, the SAE 1850 ports?
Hmmm, that is unexpected. I wonder if these problems are connected or unrelated.
This car sounds like a real headache. I take it the car never stalls when comm is lost between the scanner and the ECM?
Does it set network or loss of comm codes when this happens?
It runs fine all the time. When I loose comms with the ECM ( no mph, no rpm, the ECT looks off) I can access the transmission PIDs with the scanner. And by the way, you were correct that the MAF pin 4 is a 5 volt reference. And the pin 5 MAF sensor line (ye) apparently is not sending a 5 volt reference on this line, but the IAT shows 5 volts unplugged. At this point, I want to confirm that a rebuilt ECU is necessary (to restore the fuel economy) so can you guide me on this?
Forgive me, I don't follow the logic. Why do you suspect a replacement PCM will improve fuel economy?
I just read through the post you linked in from before, it sounds like there was extensive harness damage in the past. That was quite a saga
You ask a good question. By the display on the radio, it gets about 28 mpg highway so that may not improve with a more functional PCM. I'm all for fixing wiring problems and if a sensor/computer ground needs to be rerouted from the PCM that is ok. I'm just questioning whether the non communication issues, or a not functional MAF can be fixed. It helps to know the correct circuit design for the MAF (e.g. pin 4 is supposed to be 5 v). I'm all ears as to what to do next instead of looking for reasons to replace the PCM... also, I got some advise from Kevin at Just Answers. and some OEM wiring diagrams
It turns out the Splice Connector located in the fuse box under the left side back seat had water damage and corrosion due to a recent back glass breakage. I think it caused communication problems in this GM network using a single line, star-like configuration. It's a serial network with a signal of 0 to about 7 volts digital signal. I cleaned up this splice connector and everything talks again. I will replace the MAF with a new Delco MAF to see if I can get a good working MAF. The current one makes no difference whether plugged in or unplugged. The car runs the same.
Replaced the existing MAF with a new Delco one. It seems to run fine. I still can't see a digital frequency based signal to indicate mass air flow on the sensor line (Ye) on pin 5. Everything I've read online says this car uses a digital MAF so I am still not sure I have a real reading of g/s or if the ECM is using substituted values. Perhaps I can use the generic OBDII mode on the scanner to show the actual signal from the MAF? What do u think?
It seems the next test is to go to the ECM and measure volts on the MAF sensor line. I think it should be 5 volts if the MAF connector is unplugged. What do you think?
Replaced the existing MAF with a new Delco one. It seems to run fine. I still can't see a digital frequency based signal to indicate mass air flow on the sensor line (Ye) on pin 5. Everything I've read online says this car uses a digital MAF so I am still not sure I have a real reading of g/s or if the ECM is using substituted values. Perhaps I can use the generic OBDII mode on the scanner to show the actual signal from the MAF? What do u think?
Is it setting a MAF or fuel trim related trouble code with the new AC Delco MAF installed?
No DTC for the new Delco MAF. There is only a DTC for an O2 sensor at this point. I'm still struggling with the design of this GM sensor which I thought is a digital type where airflow is reported by a frequency change on the signal wire for the MAF. My oscilloscope does not show this frequency, it appears that its an analog MAF...
Nice, you can't do better than Pico. There's a learning curve, like anything else naturally, but there's a lot of support and content online centered around the Pico software.
I will be surprised however if the Pico reveals a different frequency or waveform compared to what you have all ready measured with your other tools.
Please keep us updated as you move forward.
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