SOLVED! Strange OBD2 Port Issues

zerovandez

Well-Known Member
I'll keep this one short...

Cold start, while in Park, the OBD2 port responds as normal. As soon as the shift lever leaves Park and goes into Drive, the port still provides power and does not communicate with any scanner. I had an iDatalink bypass module connected to Pin 2 of the OBD2 connector and I have removed it thinking that it was the issue - it's not the issue.

Any ideas what might be causing this?
 

budwich

Well-Known Member
without some "fancy equipment" (scope or otherwise), you are left with basics. Get a meter, measure the pin (voltage) while in park and then measure it again while not in park.... any difference?

Further, have you checked all shift positions, including neutral and reverse?
 
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zerovandez

zerovandez

Well-Known Member
Hey budwich, i can confirm power - the obd device does maintain power going from park to other positions - there is power led indicator. Even out of park, the device can connect BUT fails with trying to communicate.

Specifically, im using OBD MX+ and have an actel handheld device - both express the same behavior. In ODB fusion - when in drive - the app connects to the adapter but fails at getting vehicle info. With the handheld - an error displays to power cycle the ignition.
 

TJBaker57

Well-Known Member
Have you tried engine running vs engine not running? I know...sounds loopy,,,I tend to try anything or everything I can looking for clues. Even with the engine not running you should get PID data like ECT and MAP and so on. I have been asking myself "what changes when the shifter moves from Park"? Time to refresh my memories of the wiring diagrams related to the shifter mechanism.
 

budwich

Well-Known Member
Hey budwich, i can confirm power - the obd device does maintain power going from park to other positions - there is power led indicator. Even out of park, the device can connect BUT fails with trying to communicate.

Specifically, im using OBD MX+ and have an actel handheld device - both express the same behavior. In ODB fusion - when in drive - the app connects to the adapter but fails at getting vehicle info. With the handheld - an error displays to power cycle the ignition.
That's not what I asked. You said you "mucked" with pin 2 ... that is a data bus +. check it to see whether there is any difference. Powering of the device (which is another pin) is another "story" which you already have said is there.

Further, you need to get another obd interface (preferrably hard wired) to check if it works... you might be able to do this by going to some parts stores and let them plug in and then do your same "test" (ie. park and then change of gears).
 

seanpooh

Well-Known Member
Hey budwich, how did you come across pin 2 being data bus +? Pin 2 is just serial data.
Pin 6 & 14 are you HighSpeed GMLAN, 16 is you B+, 4 & 5 is ground.
 

seanpooh

Well-Known Member
Working on diagrams for an 02 TB...
I can't get the whole diagram to put onto here
From looking at the diagram, there are two circuit options DLC Pin 2 goes through before getting to the modules for comm.

If you have an immobilizer (which we all usually do), it goes to the the theft deterrent control module, then to the PCM->BCM->Splice Pack which connects all the other modules together.

If you don't have an immobilizer, it goes to splice pack and then to all of the modules.

If there was any modifications made to the theft deterrent system, it would stop comm.

Grounds provided to your DLC is G102 & G201. G102 is left rear of engine compartment. G201 is right front of lower console.

Something is being terminated when shifting out of park.
 
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zerovandez

zerovandez

Well-Known Member
I do have another OBD interface which is wired and it has the exact same issues. I can plug in the OBD MX+ and the wired OBD device to another car and they operate without issues regardless of drive position.

The info in the following link seems to be consistent as far as OBD connector pin out goes


The manual for the iDatalink bypass module also states to tap into this pin as well. If the truck is in START (not RUN) I can get communications - can't remember the last time I test this but I'll verify soon. Yes, I've checked all shift positions - all fail communications except for Park.
 

seanpooh

Well-Known Member
Thank god these older vehicles don't have GMLAN.

Stupid question to the OP, can you pull the key out when the shifter is out of park?
 

budwich

Well-Known Member
I do have another OBD interface which is wired and it has the exact same issues. I can plug in the OBD MX+ and the wired OBD device to another car and they operate without issues regardless of drive position.

The info in the following link seems to be consistent as far as OBD connector pin out goes


The manual for the iDatalink bypass module also states to tap into this pin as well. If the truck is in START (not RUN) I can get communications - can't remember the last time I test this but I'll verify soon. Yes, I've checked all shift positions - all fail communications except for Park.
Do you know if either of these worked before the "wire tap"?
 

Mooseman

Moderator
Anything else happen when you move it from P? Gauges keep working? Does it do it with the engine not running? Wondering if something in the PCM (which incorporates the TCM in those years) kills the Canbus when the tranny position switch sends a signal to the PCM. Might have to isolate each module via the Canbus main connector by pulling the comb and jumping only the necessary modules and see what happens.
 
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zerovandez

zerovandez

Well-Known Member
Do you know if either of these worked before the "wire tap"?
Never had to use the OBD prior to the tap. Actually, I soldered the connection from module to #2.

I haven't tried to move the shifter position when engine not running. I might try this tonight. Gauges still operate as usual without issues. SES light comes on when triggered. Everything works as normal.

Stupid question to the OP, can you pull the key out when the shifter is out of park?
Can't remove key's when out of Park - I found out the hard way when I replaced my battery lol.
 

MRRSM

Lifetime VIP Supporter
Double Check your Solder Work against this GM Specific OBD2 Female Port Wiring Diagram;

16PINOBD2DIAGRAM.jpg
 
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zerovandez

zerovandez

Well-Known Member
Yeah it's the right pin. I actually removed the remote start entirely - cool idea but not great if I can only remote start the thing at 2ft away lol.
 
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zerovandez

zerovandez

Well-Known Member
I don't think anyone would have ever guessed....

So I've also been having strange headlamp issues - they would stay on at night and never shut off. During the day, they seem to be working normal, at least I thought. I let it go for a few days because it doesn't drain the battery - I'm using LED's for the headlamps.

Last night was different - I got tired of it. I pulled the #46 relay and will replace it soon. This morning, I connect my OBD2 scanner and it FINALLY WORKS while in any other position than Park!

This truck is so strange. Consider this resolved.
 

Blckshdw

Moderator
:confused: Pulling the headlight relay fixed the OBDII issue? That's a new one on me! Glad you were able to get it solved.
 

TJBaker57

Well-Known Member
I am one of those guys that has a need to know just how things fail. I tear apart old replaced parts to see how they work. I have no experience with LED stuff. Is this a case of electronic 'noise' from the LEDs drivers somehow finding itheir way onto the class 2 data bus? Where? How? And why does it not interfere with normal vehicle comms other than the OBD scantools? :smile: Enquiring minds you know.
 

Mooseman

Moderator
Quite possible the relay module just doesn't like the LED headlights. You might have to do a DRL delete like the HID guys do. But that affecting the OBD port is whacked!
 
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zerovandez

zerovandez

Well-Known Member
I've had the LED's on for a while without any issues of sorts. The light issue just started to happen last week. I swapped with the fan relay and didn't have any issues with light or OBD2 port. Would be interesting to take a deeper dive and find out why this relay would affect it. Maybe when I have more time to look at wiring diagrams.
 

Mooseman

Moderator
It's actually an elecgtronic PWM module. It modulates the headlights to 70% in DRL mode. Kight not be good for your LEDs neither. I do know it kills HIDs.
 

MRRSM

Lifetime VIP Supporter
I agree with @TJBaker57 completely.... FWIW... I think I have an idea on how this problem developed:

(1) The Steady Headlight Drain on the Battery can weaken it and ruin the Battery Cells if Drained and Re-Charged many times before this problem could be discovered.

(2) Even after the Engine Starts and the Battery ostensibly takes on a new charge... The system runs off of BATTERY POWER.. .NOT ALTERNATOR POWER. So if the Battery has suffered enough internal damage, the cascade of this problem would follow in (3)...

(3) The PCM is VERY Sensitive to Insufficient Voltage and according the "Pinnacle of GM OBD2 Scanners"... The OEM GM Tech 2 Scanner uses an OEM DC to DC Voltage Conversion mechanism is to ensure that the Tech 2 can operate within a Min-Max range of 8 Volts DC to 20 Volts DC. However ALL of the After Market Voltage Regulator "Chips" in the "GYMKO" flavor Tech 2s only function between 9 Volts DC to 18 Volts DC.

(4) The GM, Single Wire Class 2 Network operates within a range of 0 Volts DC to 7 Volts DC oscillating generally in the 0-5 volts DC Range.

(5) If the HIB Module(s) were being fed Volt DC via a problematic Voltage Relay in the Power Distribution Center...AND there was a Short to Positive in the Electro-Viscous Fan Clutch... Even with the Engine running and the Alternator throwing 13.5 + Volts DC into Bad Battery... Once the Vehicle is Shifted into Drive ... the Engine and Transmission System load vs. Voltage demand might DROP below the 8-9 Volts DC Lower Threshold powering the Class 2 Network.. and this would involve the Power Feed to the OBD2 Dash Panel Female Connector in particular.

(6) The conclusion might be that the Problem with the OBD2 Scanner is NOT a Cause... but rather is a Symptom of these Recombinant DC Electrical issues. There is probably nothing wrong either with the Port itself ... or with the Wiring in between all of these Class 2 items in communication with one another...and the PCM-BCM.
 

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