How does electronic timing advance work on a TBI distributor?

[MANIAC]

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Yeah TBI's got by without speed density or at least mine does but on another note I love the jokes and memes some of you guys post they are hilarious and a good laugh definitely dials down the frustration but you guys are awesome

 

[skylark]

Yeah TBI's got by without speed density or at least mine does but on another note I love the jokes and memes some of you guys post they are hilarious and a good laugh definitely dials down the frustration but you guys are awesome

You have to try to have some fun!

 

[Road Trip]

...also I can't seem to find the diagrams like the ones you guys have sent, is there a specific website where I can locate the diagrams specific to my truck?

Yes there is. It's actually in here, thanks to the shared efforts of fellow GMT400 forum members.

If you follow this link you can download the exact same .pdf files I've been using for
this thread. ('88+ GMT400 Factory Service Manuals)

When working on vehicles of this vintage, the more self-help you can muster the more
successful you will be in the old truck hobby.

NOTE: Some folks find using the actual printed manuals is easier for them, while others
find scanning the .pdf files is their preferred method. (For my own '99 I cheat and use
both methods. See attached for an example of how some find used factory manuals
to scratch that ink on paper itch.)

****

These manuals are not perfect, but they do get rid of at least 94% of all guesswork
on these old brutes. :0)

 

[scott2093]

These manuals are not perfect, but they do get rid of at least 94% of all guesswork
on these old brutes. :0)

I'd give mine a 93%..lol
I think I had to look in the 1990 wiring to find the closest resemblance to my ac circuit.... still need to revisit that mountain one day.. but everything's working good enough for now....

 

[MANIAC]

Ok so my ADL port is showing 7.5v today and I unplugged every sensor one by one and the voltage on pin B did not budge so I guess I'm gonna get in contact with rock auto and try to figure out if it's a dud on they're end

 

[MANIAC]

I'm kinda wondering if I should just go pull a spider injected Chevy dash and get rid of this stupid TBI system because I don't have 1500$ to go efi but I have no idea how to get a ECM tuned for that sorta thing because you never see spider injected k2500 Chevy's at pick n pull here in Missouri unfortunately

 

[MANIAC]

I'm kinda wondering if I should just go pull a spider injected Chevy dash and get rid of this stupid TBI system because I don't have 1500$ to go efi but I have no idea how to get a ECM tuned for that sorta thing because you never see spider injected k2500 Chevy's at pick n pull here in Missouri unfortunately

Okay so now that I've disconnected and reconnected the sensor now I have a fuel but no spark issue this is getting dumb I'm about to try putting the original ECM back in and see what that does

 

[Road Trip]

Ok so my ADL port is showing 7.5v today and I unplugged every sensor one by one and the voltage on pin B did not budge so I guess I'm gonna get in contact with rock auto and try to figure out if it's a dud on they're end

Hello Maniac,

If I had a TBI PCM truck in my driveway I'd check to see what the 'awaiting input'
voltage is on Pin A14? (If any TBI owner out there is reading this and has made
this voltage check, could you please share what you have observed?)

Of course, going by the FSM documentation, Pin A14 is listed as sitting at "5v", and
given my understanding of computer systems running on regulated 5V supplies,
that I would expected to see 5v +/-5% or so?

So as a troubleshooter, the questions at this point becomes:

A) What is the probability that a the PCM power supply fails to a 7.5v reading?

B) What level of confidence do we have with the test equipment (voltmeter) in use?
B1: What does this voltmeter read on the battery with the engine off? (~12.6 volts?)
B2: What does this voltmeter read with the engine idling a couple of minutes after
startup? (~14.0-14.5 volts?)

C) When performing the voltage check at Pin A14, what is the voltage reading
when the negative lead is held on either G108 or G109? Versus checking
Pin A14 with the negative lead on the battery terminal?

D) Would it be possible to borrow a friend's voltmeter and get a confirmation
on your meter's accuracy?

****

The point I'm trying to make is that I've had test equipment lie to me, and I cheerfully
followed it down the garden path. All I am advocating for is for you to build confidence
in your test results, and assuming you can't make any further progress, then proceed
with contacting the seller about what to do with the PCM?

NOTE: Can you apply a ground to Pin A14 and at the same time measure the voltage
showing? In other words, can you drop the voltage to ~0 volts? For me, that's
what matters, for this is what the designers implemented so that the mechanic has a
way to signal the PCM to go into Diagnostic mode. If due to some internal failure we
can't make this happen, then we're essentially at a standstill until this can be resolved.

By the way, the questions above are not questioning your process. Instead, this
is the conversation that we would be having while shoulder to shoulder under the hood.
And since I've been lied to by unknown test gear so many times in the past, I tend to
be extra-cautious about accepting test readings at face value. I trust you, it's the
test gear that I have trust issues with. :0)

Sometimes I wish that overpressure or leakage in the land of electricity was just as
visual a failure as when I'm working with heater hoses. Instead, we have to rely upon
our handheld meters to tell us what's going on in the (mostly) invisible world of electricity.

Please make the above confidence checks with your voltmeter, and if they pass, then
I want to know what voltage is between A14 and G108/G109 when you have grounded it.

****

One more thing? Do you still have the original PCM, or did you already turn it in for the
core charge? If you still have it, I for one would be curious about what voltage level is
sitting on Pin A14? 5.0v? 7.5v? 0v? Something is amiss here.

 

[MANIAC]

Okay so all the wires tested ok I'm good there so now I'm suspecting a potential fried prom so looking into that now but I will let you guys know as soon as I find out also if anyone knows what prom codes might be compatible with the BNKM letter code on the original prom or knows where I might find that info id appreciate it.

 

[PlayingWithTBI]

anyone knows what prom codes might be compatible with the BNKM letter code on the original prom or knows where I might find that info id appreciate it.

www.repairpal.com/gm-obd-I-code-chart

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