Can you have a sel code 12 on obd-1 that isn’t actually the initial diagnostic startup code

[PlayingWithTBI]

I can not find your senairo that in any of the diagnostic trouble shooting trees. No SES light while running and a code 12 indicates normal operation. If you have some thing to support your explanation please share it with the rest of us.

yes, you'll never get a Code 12 while the engine is running. Here's a useful link, it's a little difficult to navigate from the list to the individual code without hitting the "Back Arrow" so, I posted 2 links - the 1st one shows all GM OBDI codes. If you click on one, it'll redirect you to an explanation and possible causes. The 2nd drills down to Code 12.

GM OBD I Trouble Code Chart

Code Description Code 12 No distributor reference pulse (diagnostic test active) Code 13 Oxygen sensor circuit open or no

repairpal.com

GM OBD Trouble Code 12

No Distributor Reference Pulse (Diagnostic Test Active) The Engine Control Module uses the distributor reference pulse to

repairpal.com

 

[evilunclegrimace]

yes, you'll never get a Code 12 while the engine is running. Here's a useful link, it's a little difficult to navigate from the list to the individual code without hitting the "Back Arrow" so, I posted 2 links - the 1st one shows all GM OBDI codes. If you click on one, it'll redirect you to an explanation and possible causes. The 2nd drills down to Code 12.

GM OBD I Trouble Code Chart

Code Description Code 12 No distributor reference pulse (diagnostic test active) Code 13 Oxygen sensor circuit open or no

repairpal.com

GM OBD Trouble Code 12

No Distributor Reference Pulse (Diagnostic Test Active) The Engine Control Module uses the distributor reference pulse to

repairpal.com

That is what a FSM for driveability and emmisions indictes. A 12 code does not indicate a problem with the system on KOEO. The absence of a code 12 would indicate a problem with the system with KOEO.

 

[A97obs]

Im pretty sure it’s just a wonky worn distributor or icm .. couldn’t hurt to swap it out … 91’ s-series trucks never had a crank position sensor or reluctor wheel .
The whole code 12 was at this point a misinterpretation on my part. With KOEO it’ll blink code 12 three times and keep repeating code 12 until you turn the key off .
I thought it only should blink code 12 three times and then STOP If the vehicle has No Real Actual codes stored. In my case it kept blinking 12 over and over and over and over until I shut key off .
Everyone understand and interpret what I’m saying ? It’s not a big situation just trying to understand why obd1 code 12 blinks past the 3 times it’s only supposed too. The next set of 12 made me think ok this is the real code issue now but apparently it isn’t it’s just the same diagnostic startup code 12

 

[termite]

Im pretty sure it’s just a wonky worn distributor or icm .. couldn’t hurt to swap it out … 91’ s-series trucks never had a crank position sensor or reluctor wheel .
The whole code 12 was at this point a misinterpretation on my part. With KOEO it’ll blink code 12 three times and keep repeating code 12 until you turn the key off .
I thought it only should blink code 12 three times and then STOP If the vehicle has No Real Actual codes stored. In my case it kept blinking 12 over and over and over and over until I shut key off .
Everyone understand and interpret what I’m saying ? It’s not a big situation just trying to understand why obd1 code 12 blinks past the 3 times it’s only supposed too. The next set of 12 made me think ok this is the real code issue now but apparently it isn’t it’s just the same diagnostic startup code 12

When/if you have active codes stored, it’ll flash the series of 12 followed by the stored codes then repeat code 12 three times before starting over.

 

[thinger2]

Im pretty sure it’s just a wonky worn distributor or icm .. couldn’t hurt to swap it out … 91’ s-series trucks never had a crank position sensor or reluctor wheel .
The whole code 12 was at this point a misinterpretation on my part. With KOEO it’ll blink code 12 three times and keep repeating code 12 until you turn the key off .
I thought it only should blink code 12 three times and then STOP If the vehicle has No Real Actual codes stored. In my case it kept blinking 12 over and over and over and over until I shut key off .
Everyone understand and interpret what I’m saying ? It’s not a big situation just trying to understand why obd1 code 12 blinks past the 3 times it’s only supposed too. The next set of 12 made me think ok this is the real code issue now but apparently it isn’t it’s just the same diagnostic startup code 12

Nope. it will just keep bliinking
12 just means that the ecm is working and has no codes.
It will 12 till the battery dies.
A repeating 12 just means that there are no codes stored.
It will start with a twelve with a twelve and end with a twevle always.
And it will repeat as,long as you have it in diagnostic mode.

 

[Road Trip]

A fella on the s10forum who’s fairly knowledgeable on early era Gm trucks
I frequent there often seeing as I own a 91 s-series pickup

Greetings A97obs,

That repeating Code 12 is to the TBI OBD1 system as the dial tone was to landlines
and the Indian Head Test Pattern was to NTSC broadcast television.

In all 3 cases it is a distinct method to let the human know that the system is
up & running and ready for use. Curious about how the gentleman on the s10forum
got the impression that he did, I went straight to the horse's mouth and read up on
what the GM technical writers had to say about Code 12:

You must be registered for see images attach

"A flashing Code 12 does not mean an engine problem; it simply means that the diagnostic mode is working."

So getting into the Diagnostic Mode requires grounding pin B to ground pin A in KOEO, a very specific sequence. (Key On Engine Off)

****

Now here is an interesting blurb about Code 12. Code 12 is
actually triggered when the ECM is powered up (Key On)
but there are no ignition pulses being received by
the ECM from the EST system. (O rpm = Engine Off)

You must be registered for see images attach

One thing you learn pretty quick is that computer system designers play
fairly elaborate "What If" games during the design phase. For example,
you can tell from the design that they absolutely did NOT want to spray
fuel at all unless the engine was spinning in order to consume it.
(Imagine if it were possible for the throttle body injectors just sitting
there spraying WOT fuel into a non-running engine that's parked in your garage?)

They refer to certain design features that provide a 'failsafe' aspect.

One of these is the fact that the fuel injection system is triggered directly
by the engine rpm signal. No RPM, no Fuel. (Good thing to keep in mind
if you are intermittently losing fuel delivery, but the fuel filter is fresh and
the fuel pump is rock solid. (!)

****

So an endless flashing Code 12 with the A & B terminals on the ALDL (Assembly Line Diagnostic Link)
plug tied together during KOEO is a good sign. And if you don't get this, then you have to stop here
and figure out how to fix this special diagnostic communicatoin mode as follows:

You must be registered for see images attach

And finally, there's no Code 12 displayed via flashing codes while the engine is running...well, because the engine
is running, right? As a matter of fact, the same 2 ALDL pins shorted together during KOER (Key On Engine Running)
turns the SES light into Open Loop/Closed Loop flashing display. A completely different 'Field Service' mode. (!)

Now in the FSM there was a short blurb in the Diesel section having to do with the possibility of a running Code 12
on a Scan tool due to losing connection to the tattletail RPM sensor. (Obviously the Diesel has no ignition system,
so a cam driven rpm sensor provides a 4 pulse per revolution signal.) But at the same time they specifically stated
that a Code 12 is *never* stored in Diagnostic memory. Again, impossible to see in KOER mode via the flashing lights,
and definitely not anything a 2024 gasoline owner would ever think about again. :0)

Just like missing dial tone, no flashing code 12s = you have to find out what went wrong.

****

So there you have it. Code 12 in Diagnostic Mode simply says that the ECU's error code storage unit is open for business.
And if you are too young to remember ever hearing a landline Dial Tone, then head over to Wiki and take a listen.
(Dial Tones, US, Europe, & Japan)

And if you haven't watched a little of that black and white test pattern while eating your Captain Crunch
and waiting for the Saturday morning cartoons to start, then you haven't lived yet. Ah, the good ol' days.
:0)

 

[A97obs]

Greetings A97obs,

That repeating Code 12 is to the TBI OBD1 system as the dial tone was to landlines
and the Indian Head Test Pattern was to NTSC broadcast television.

In all 3 cases it is a distinct method to let the human know that the system is
up & running and ready for use. Curious about how the gentleman on the s10forum
got the impression that he did, I went straight to the horse's mouth and read up on
what the GM technical writers had to say about Code 12:

You must be registered for see images attach

"A flashing Code 12 does not mean an engine problem; it simply means that the diagnostic mode is working."

So getting into the Diagnostic Mode requires grounding pin B to ground pin A in KOEO, a very specific sequence. (Key On Engine Off)

****

Now here is an interesting blurb about Code 12. Code 12 is
actually triggered when the ECM is powered up (Key On)
but there are no ignition pulses being received by
the ECM from the EST system. (O rpm = Engine Off)

You must be registered for see images attach

One thing you learn pretty quick is that computer system designers play
fairly elaborate "What If" games during the design phase. For example,
you can tell from the design that they absolutely did NOT want to spray
fuel at all unless the engine was spinning in order to consume it.
(Imagine if it were possible for the throttle body injectors just sitting
there spraying WOT fuel into a non-running engine that's parked in your garage?)

They refer to certain design features that provide a 'failsafe' aspect.

One of these is the fact that the fuel injection system is triggered directly
by the engine rpm signal. No RPM, no Fuel. (Good thing to keep in mind
if you are intermittently losing fuel delivery, but the fuel filter is fresh and
the fuel pump is rock solid. (!)

****

So an endless flashing Code 12 with the A & B terminals on the ALDL (Assembly Line Diagnostic Link)
plug tied together during KOEO is a good sign. And if you don't get this, then you have to stop here
and figure out how to fix this special diagnostic communicatoin mode as follows:

You must be registered for see images attach

And finally, there's no Code 12 displayed via flashing codes while the engine is running...well, because the engine
is running, right? As a matter of fact, the same 2 ALDL pins shorted together during KOER (Key On Engine Running)
turns the SES light into Open Loop/Closed Loop flashing display. A completely different 'Field Service' mode. (!)

Now in the FSM there was a short blurb in the Diesel section having to do with the possibility of a running Code 12
on a Scan tool due to losing connection to the tattletail RPM sensor. (Obviously the Diesel has no ignition system,
so a cam driven rpm sensor provides a 4 pulse per revolution signal.) But at the same time they specifically stated
that a Code 12 is *never* stored in Diagnostic memory. Again, impossible to see in KOER mode via the flashing lights,
and definitely not anything a 2024 gasoline owner would ever think about again. :0)

Just like missing dial tone, no flashing code 12s = you have to find out what went wrong.

****

So there you have it. Code 12 in Diagnostic Mode simply says that the ECU's error code storage unit is open for business.
And if you are too young to remember ever hearing a landline Dial Tone, the head over to Wiki and take a listen.
(Dial Tones, US, Europe, & Japan)

And if you haven't watched a little of that black and white test pattern while eating your Captain Crunch
and waiting for the Saturday morning cartoons to start, then you haven't lived yet. Ah, the good ol' days.
:0)

yep I get it I get it case closed. Yo on the 12.
I guess at that point you got to wonder how do you get a solid SEL light
“while running” and then apparently no codes when you go to check it

 

[Schurkey]

1. Short to power on the wire harness to the MIL, and/or faulty wire harness from diagnostic connector to computer.

2. Faulty computer.

3. Faulty scan tool.

 

[Road Trip]

I guess at that point you got to wonder how do you get a solid SEL light
“while running” and then apparently no codes when you go to check it

You know, I'll bet you a dollar that if a small speaker would have been cheaper than a single
bulb in the dash, they would have had the ECU burp out Morse Code instead of the
flashing light codes that we have to work with.

Morse Code only works when it is able to spring from silence. Q: Does your SES light
come on during KOEO, and then once the engine starts, go out for ~10 seconds and
then come back on? Proving to you that the computer has full control over the Lit/Not Lit
state of the SES lamp? Or is it always on right off the bat? (This is the way for you the
user to differentiate between a valid SES light in normal driving mode vs an electrical/electronic
failure that forces the light to be always on when the engine is running?)

See below:

You must be registered for see images attach

If the above isn't the problem, then you have to decide whether the error code memory
storage section of the ECU is working or not. Have you tried disconnecting the MAP
sensor, getting the light, shutting off the vehicle, and then jumpering the ALDL connector
and seeing if you can retrieve the correct code for this failure? (ie: a diagnostic performance
test via forcing a known failure for the ECU to generate/save a failure entry so that it persists
across a power cycle?)

If the ECU can't do this, then it is also suspect in other functional areas. (ie: What else is it
having trouble doing?)

A possible workaround if the ECU can't share what's wrong via the blinky light subsystem
would be to try and employ a live data scan tool. Possibly this communication path is still working?
(Don't know if you have that capability on hand?)

As Schurkey correctly explained earlier, the Code 12 is the one code that can't be stored in memory.
If you lose the Reference Signal while driving, the injectors are prohibited from firing, so No Fuel,
No Worky. IF no other codes after paper clipping, then we are supposed to assume that there's
a problem with a missing Reference signal. (!) Sounds like you are experiencing this?

Finally, I scanned this FSM for all instances of Code 12, and came up with 34 different
instances. I could type all night & still not do it justice. Instead of me turning this into the
Error Code Telephone Game, a much more efficient method might be for you to read the FSM
yourself and then ask diagnostic-related questions based off of that?

This doesn’t pertain to a gmt400 but my 91 s10 which all the same obd1 methods of reading codes

You can find the '91 GMT400 manuals here. (MANUALS) I have no idea how much of the GMT400 diagnostic
section is the same as the same section in the S-10 manual, but at least it's a start.