Need help diagnosing OBD port comm failure
07-20-2019, 02:59 PM
#1
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Need help diagnosing OBD port comm failure 2001 GMC Sierra with a 4.8L 2wd. Just got the truck and everything in it works and feels like a few years old. Drove it around for over an hour and it was fine. On the way home, the SES light came on. By the time I got home, I tried plugging in three different scanners and none of them can communicate with the vehicle. Note that two of those were passive and getting power from the vehicle.
Did some research in this area of the forum of things to look at. Cigarette lighter fuse and associated fuses looked good. I tried draining the vehicle of power to reset everything. That got the SES light off, but still no communication. So, I got out the multimeter and looked up the pinout diagrams. So on the OBD side, I had the following results:
2 - J1850 - tested continuity OK to pin 58 on C1 ECU. voltage comes out in the >1 mv range, though i see other threads say 3-5V. don't know if my digital meter isn't responsive enough or not.
4 - ground OK
5 - ground OK
14 - ??? “HS-CAN” “E&C” “CAN J-2234 Low” ???
16 - voltage OK, does not deviate from battery
So my question I need help with, is what does pin 14 go to on the PCM side so that I can test that wire for continuity? I think if that wire tests continuous, then it's probably a PCM issue. I've also seen where some threads and internet sources say this is for the ABS module, and/or not needed for OBDII functionality. Can someone confirm that?
I think if the PCM is bad I might order a new PCM and see how it does. The parts store says it's unflashed though. Any suggestions?
Thanks...
Did some research in this area of the forum of things to look at. Cigarette lighter fuse and associated fuses looked good. I tried draining the vehicle of power to reset everything. That got the SES light off, but still no communication. So, I got out the multimeter and looked up the pinout diagrams. So on the OBD side, I had the following results:
2 - J1850 - tested continuity OK to pin 58 on C1 ECU. voltage comes out in the >1 mv range, though i see other threads say 3-5V. don't know if my digital meter isn't responsive enough or not.
4 - ground OK
5 - ground OK
14 - ??? “HS-CAN” “E&C” “CAN J-2234 Low” ???
16 - voltage OK, does not deviate from battery
So my question I need help with, is what does pin 14 go to on the PCM side so that I can test that wire for continuity? I think if that wire tests continuous, then it's probably a PCM issue. I've also seen where some threads and internet sources say this is for the ABS module, and/or not needed for OBDII functionality. Can someone confirm that?
I think if the PCM is bad I might order a new PCM and see how it does. The parts store says it's unflashed though. Any suggestions?
Thanks...
07-20-2019, 05:14 PM
#2
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I like your idea of trying a different PCM. I'd head to the junkyard and pick one up from as similar of a vehicle as possible if not exactly the same. Model year 2001 GMC Sierra and Chevy Silverado vehicles are common in the yards around here and their PCM's sell for about 60 bucks if I remember correctly. Ironically, I have one flashed (896 version) and one unflashed (411 version) from around that year sitting here on my shelf as spares that would work perfectly for you in this experiment. But it should be easy to find one of these in a yard close to you for similar short money.
Rick
Rick
07-20-2019, 05:57 PM
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I like your idea of trying a different PCM. I'd head to the junkyard and pick one up from as similar of a vehicle as possible if not exactly the same. Model year 2001 GMC Sierra and Chevy Silverado vehicles are common in the yards around here and their PCM's sell for about 60 bucks if I remember correctly. Ironically, I have one flashed (896 version) and one unflashed (411 version) from around that year sitting here on my shelf as spares that would work perfectly for you in this experiment. But it should be easy to find one of these in a yard close to you for similar short money.
Rick
Rick
Also, while looking at parts stores, I noticed I could order a PCM next-day with one of two descriptions for $90 (saves the time and hassle of going to a yard for $30 more):
- Application: Flashed
- Notes: Remanufactured
Flash technology. OEM #12200411. Requires factory reprogramming
- Application: Unflashed
- Notes: Remanufactured
Flash technology. OEM #12200411. Requires local reprogramming
What's involved with reprogramming and being flashed/unflashed? Do I have to take that somewhere before swapping mine out?
Thanks.....
07-20-2019, 07:04 PM
#4
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The issue with reprogramming and flashing is solved with somebody very skilled with a program called "HP Tuners" or "EFI Live". I'm not that person but I have access to somebody locally with HP Tuners. For HP Tuners, he has done this twice for me and each newly flashed PCM costs two "credits" at 50 bucks each to "license" the new operating system. Then he charges me another 50 bucks for his skill, time and effort and my reprogramming for him has been very simple low cost things. More complicated tuning would be more than 50 bucks each.
It helps to procure a replacement PCM from a derelict vehicle that is as close as possible to your target vehicle engine and transmission because less reprogramming will be required. Using an obscure 411 PCM from some odd, unlike vehicle would be doable but would require your tuner to perform additional "segment swaps" of what your target engine needs from stored program tunes in a library. All of this is achievable but the required skill level in your tuner person goes up. Cost goes up. Post tune, retune effort becomes more likely. Wideband O2 sensors for tuning help likely required and you REALLY need a local HP Tuners guy for this, not a mail order tuner.
Stick with an original GM PCM with your desired tune in it as close as possible for lowest risks. No matter what, it is absolutely required that with ANY new different PCM, some work with HP Tuners will be required to defeat the "Vehicle Anti Theft System" because that PCM will expect to see a different ignition key - no start - unless VATS is defeated.
Rick
It helps to procure a replacement PCM from a derelict vehicle that is as close as possible to your target vehicle engine and transmission because less reprogramming will be required. Using an obscure 411 PCM from some odd, unlike vehicle would be doable but would require your tuner to perform additional "segment swaps" of what your target engine needs from stored program tunes in a library. All of this is achievable but the required skill level in your tuner person goes up. Cost goes up. Post tune, retune effort becomes more likely. Wideband O2 sensors for tuning help likely required and you REALLY need a local HP Tuners guy for this, not a mail order tuner.
Stick with an original GM PCM with your desired tune in it as close as possible for lowest risks. No matter what, it is absolutely required that with ANY new different PCM, some work with HP Tuners will be required to defeat the "Vehicle Anti Theft System" because that PCM will expect to see a different ignition key - no start - unless VATS is defeated.
Rick
07-21-2019, 12:01 AM
#5
The obd2 port wiring is simple. 2 black grounds. 1 ign hot. And 1 serial data wire tht runs back to ecu.
If power and ground checks out ok. Jump a wire for the serial data to the correct pin at ecu. Thin see if a scanner will read out data. Ive had issues like this before with aftermarket radios etc. Couldnt get a read using hptuners. Esprcially with steering wheel and door chime adapters etc.
Ill report back with correct pinouts.
If u get a new ecu it will need to be a new flashed one to work correctly.
A used one will need the vats removed to be able to start (with vats it will crank and shut off) Any tuner can flash a base 4.8 tune into any ecu u get. Its pretty rare for a ecu to go bad.
If power and ground checks out ok. Jump a wire for the serial data to the correct pin at ecu. Thin see if a scanner will read out data. Ive had issues like this before with aftermarket radios etc. Couldnt get a read using hptuners. Esprcially with steering wheel and door chime adapters etc.
Ill report back with correct pinouts.
If u get a new ecu it will need to be a new flashed one to work correctly.
A used one will need the vats removed to be able to start (with vats it will crank and shut off) Any tuner can flash a base 4.8 tune into any ecu u get. Its pretty rare for a ecu to go bad.
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07-21-2019, 04:57 AM
#8
TECH Fanatic
If you don't have somebody local with HP Tuners, you might (might???) be able to take a same model number PCM to a local GM dealer and get it reflashed to what is needed. I have no idea what this would cost. Proof of ownership for the GMC Sierra might be required and the dealership will need your VIN out of the Sierra to know what tune to push into the replacement PCM.
But yes, I agree, its rare for a PCM to go bad. Probably a wiring problem. Brandon's advice above is good - run a supplemental, temporary wire to the PCM for serial data and see what happens.
Rick
But yes, I agree, its rare for a PCM to go bad. Probably a wiring problem. Brandon's advice above is good - run a supplemental, temporary wire to the PCM for serial data and see what happens.
Rick
07-21-2019, 06:16 PM
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Wiring issue? Hm, maybe. I did originally test for continuity between pin 2 at the obd and pin 58 on the PCM connector. Perhaps it's also grounding somewhere else. Maybe I can run down there tomorrow, disconnect the battery again, and see if there's continuity from pin 2 at the obd and the vehicle ground. Running the supplemental wire will be a bit of a pain picking apart the thick wire bundle (I'm also colorblind 
) but I guess I'll cross that bridge when I get there.
On the PCM failure being rare..... The history report for the vehicle shows it passed last time it went through emissions late last year, but yeah things can go bad anytime. I guess it's an excuse to get a mail order tune.
) but I guess I'll cross that bridge when I get there.On the PCM failure being rare..... The history report for the vehicle shows it passed last time it went through emissions late last year, but yeah things can go bad anytime. I guess it's an excuse to get a mail order tune.
07-22-2019, 10:50 AM
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I should be able to take another look this evening if the serial data wire might be grounding elsewhere . That wire tests as continuous, but things can happen.
Can anyone confirm what voltage signal should be traveling on that pin? One older thread said 3 to 5 volts. I get about 400 mV, but that might be due to my auto-ranging multimeter not adjusting frequently enough to read the data signals.
07-22-2019, 09:36 PM
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Well I rechecked the wiring, and the wiring seems fine on both ends and the multimeter shows continuous where it matters. But, I also noticed a new problem. The radio will play for about a minute, then reset itself and the clock goes back to 1:00 am. Something is amiss, don't know if related now. What a headache, lol.
07-23-2019, 01:57 AM
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Mass airflow sensor It's not a truck. It's a Firebird with a 3.8 v-6 and it's all factory. I'm getting a code saying voltage to the mass airflow sensor is low.
08-11-2019, 01:01 PM
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Well I rechecked the wiring, and the wiring seems fine on both ends and the multimeter shows continuous where it matters. But, I also noticed a new problem. The radio will play for about a minute, then reset itself and the clock goes back to 1:00 am. Something is amiss, don't know if related now. What a headache, lol.