94 GMC K1500...can't read data stream OBD1

[Comp268]

Hello everyone, new guy here. I was referred here to hopefully get some help trying to figure out some issues with my 16 year olds newly purchased truck.

Long story short...someone in the trucks past life had swapped in a built vortec 350, with a GMPP vortec TBI intake, headers/duals, cam, etc. It ran ok, but the engine ended up being in need of a total rebuild...I'll leave out all the gory details, but lets just say a valvetrain "noise" snowballed into an engine swap.

We swapped the trashed engine with the bone stock L31 vortec engine out of my old '99 Suburban 3/4 ton. Used the vortec TBI intake and TBI unit, and headers/exhaust...otherwise it is still a stock '99 vortec 350 long block.

The engine is in and running...starts right up and idles at 675 rpm perfectly fine. Transitional throttle response is pretty bad though...it acts quite lean, and the spark plugs confirm this. It stumbles on light throttle tip in and lightly pops out the exhaust.

I have a Bosch OBD1300 scan tool that I have used on many vehicles over the years, and never had any issues with it, until now with this truck. This thing is OBD1, and my scan tool does not see the ECM. I have an old DLC plug in device for GM OBD1 vehicles that is supposed to make it flash any trouble codes...it also does nothing. And jumpering between the A and B ALDL posts does nothing either. There is no "check engine" light, but there is a "check gages" light that is always on and never changes or flashes.

I have been troubleshooting this for several days now, but I'm not getting anywhere. Here are the pinouts of the ALDL connector and what I am seeing with the key on, engine off...

Position A...0 ohms to ground
B...5.05 volts
C...0 volts
D, E, and F not used
G...0 volts
H...9.68 volts. I believe this goes to the ABS system
J...0 volts
K and L not used
M...5.01 volts. I believe this one is supposed to be the data stream.

I have no idea what, if anything, has been done to the computer in this thing. There is no EGR. With the timing set at 0 with the spout connector disconnected, it idles at 16* timing with the spout connector plugged in, but hits maximum timing of 22* by 1000 rpms and doesn't advance any further. It very well may be a stock TBI computer and chip. Maybe it's in some sort of limp mode? The computer is a 16196395 unit, BJLJ.

Where do I go from here? I'd like to at least be able to see the data stream and check O2 sensor function, fuel trims, etc. Could it simply be the computer is bad?

 

[Schurkey]

Saw your similar post on Speed-Talk.

1994 350 TBI questions - Don Terrill’s Speed-Talk

Aside from the usual Vortec-to-TBI issues (cooling system bypass/water pump, EGR, etc.) I'd be wondering if your scan tool is capable of reading an OBD-1 vehicle. SUPPOSEDLY, it is suitable for GM OBD-1.

Bosch OBD 1300 | Bosch Diagnostics

www.boschdiagnostics.com

Are the pins recessed in your diagnostic connector, so that the scan tool can't connect with them? "I guess" the computer could be faulty and not outputting a data stream...but that seems unlikely. Can that scan tool connect to another '94 K1500 similar to yours?

Does the MIL light up during bulb-test?

The "Check Gauges" light comes on with low oil pressure, or high engine temp. Perhaps with low voltage. Do all the gauges work? Do they show reasonable readings?

Download the service manual set for your vehicle from the links in the "sticky threads" section of this web site.

My '88 K1500 uses a Vortec short-block, but with aftermarket heads and a heap of intake-manifold modifications. Stock TBI, stock computer, stock programming but with additional fuel pressure and 6 degrees of initial timing advance.

 

[Comp268]

Hi Shurkey. Yes, that was me on Speed-Talk. Unfortunately this is getting in a lot deeper than is typically done on ST...especially with a stock EFI system. I will say that you and Carl (In-Tech) have been a huge help so far.

The diagnostic connector is good. I did go in and tighten all the pins just to be sure, but it didn't change anything.

Do the gauges work? They seem to, although everything seems to read a bit low except for the fuel gauge. The tach reads about 200 rpm low at idle, voltage reads about .7 volts low, engine temp only goes up to about 140-150 driving or idling but I haven't got a mechanical gauge hooked up yet. I have had this problem in the past with vortec head swaps...engine runs too cool...so I'm not sure if this is an issue or not yet.

I do not know anyone else that has an OBD1 anything. That would certainly help if I could at least confirm that this scan tool and connector works. It works perfectly fine on multiple OBD2 systems and Toyotas (which have their own diagnostic port adapter).

The only dash lights I have seen is the check gages light, and the ABS light on initial startup. I cannot get anything else to light. The check gages light is always on as long as the key is on...engine running or not.

Carl sent me the files for the particular computer that this has. It shows a whole lot more timing range than what I am seeing.

How did you increase the fuel pressure...adjustable FPR and higher volume fuel pump? I'm going to get a fuel pressure tester adapter here shortly to check that.

 

[tayto]

there is a CEL, i suspect someone pulled the bulb probably because they never got a chip/tune done on the previous setup.

once you get the mechanical stuff all done this thing will need a tune. This place still does chips and probably has something on hand for your setup:

HOME | Wester's Garage

www.westersgarage.com

 

[GoToGuy]

So your not getting any data, and no codes? And you used the external power supply ext cord, obd 1 does not supply device power. When you insert key, turn to " ON " , wait... check all instrument warning lights work in test,? Before turning key to " start " ?
Do you have the early rectangular ALDC or the later tapered OBD 2 ALDC, that operates on OBD 1 ?

 

[Comp268]

Regular old OBD1 rectangle ALDL.

When you turn the key on, without starting the engine, the ABS light comes on for a minute then goes off. The check gages light comes on and stays on. Nothing else comes on at any point, running or not.

 

[Schurkey]

How did you increase the fuel pressure...adjustable FPR and higher volume fuel pump?

Stretched and shimmed the OEM spring in the OEM regulator. Fuel pump has been changed to a higher-pressure unit, but the OEM fuel pump should accommodate the pressure I've got.

Better check the bulb and wiring for the MIL. You'd really want that working.

 

[GoToGuy]

Whenever you insert key and turn to on, all warning lights should light up for self test, then off with only the lights you see with eng off , key on. ( oil press, alt or gen, check eng ? )
OBD 1 does not power scanner, OBD 2 can power a scanner.
Are you using the OBD one plug with ext' power for Bosch scanner? Does the scanner handbook have guidance for GM OBD 1? So even jumping pins A and B gets no reaction ?

You must be registered for see images attach

 

[Jeepwalker]

I agree...check for the CEL bulb. Someone definitely could have removed it. Someone removed ...actually disassembled & desoldered the LED from a newer-ish vehicle I bought once. I had to solder in a new LED and straighten out a bunch of issues (kind of like you).

Yeah, on my older snap-on 'Brick' scanner, ya have to supply power (a lot of guys use the lighter 12v plug adapter). But I'm pretty sure my newer scanner supplies it's own power. So worth re-reading your scanner manual.

 

[Comp268]

My scanner has multiple ways to power it... internal batteries or a cigarette lighter adapter. This trucks cigarette lighter is missing. I guess I need to go look at the manual again but I seem to remember it saying it would diagnose OBD1 with just the internal battery power.

Jumping A and B does absolutely nothing. Sounds like I need to open up the dash and investigate