LT1 Bench Harness

[In-Tech]

Hi Guys/Gals,
I am getting ready to finish up my bench harness interface and wanted to add the LT1 stuff to it. Here's the pinouts I have so far. It's been so long since I did any LT1 bench stuff I can't remember what I did and can't find my old cable from years ago :( I sort of remember it only using one connector to the PCM though. I like to have the fuel pump sense on my bench setup besides just the MIL cuz if the fuel pump runs and shuts off like normal, pretty much everything else works. Could someone check the two fuel pump pins I noted and let me know for sure? I appreciate any help and/or suggestions.

LT1 OBD1/2
Red Pin 2 Ground
Red Pin 7 Fuel Pump enable to relay +12v out
Red Pin 25 Fuel Pump enable VATS
Black Pin 6 Analog Ground
Black Pin 15 12v Constant
Black Pin 16 Analog Ground
Black Pin 28 5v
Black Pin 29 5v
Black Pin 30 12v Switched
Black Pin 31 12v Constant
Blue Pin 1 Ground
Blue Pin 3 12v Switched
Blue Pin 7 Serial Data <<<<< OBD2
Blue Pin 9 MIL
Blue Pin 20 Output/Field Service Enable (Diag Port)
Blue Pin 29 Serial Data <<<<< OBD1
Blue Pin 30 Serial Data <<<<< OBD1

[kur4o]

For ground use red 2 and blue 1.
Black 6 and 16 are for sensor grounds.

Mil is blue 9

and red 25 is vats signal to pcm.

All other looks good.

[sherlock9c1]

While we're talking pins, OBD2's ALDL port has port 4 & 5 as ground, 9 is data link to all the modules and 2 is a second data link that appears to go directly to the PCM. Which one is the one to use on an OBD2 plug to talk to an OBD1 PCM? (test car is a '96 B-body)

[In-Tech]

While we're talking pins, OBD2's ALDL port has port 4 & 5 as ground, 9 is data link to all the modules and 2 is a second data link that appears to go directly to the PCM. Which one is the one to use on an OBD2 plug to talk to an OBD1 PCM? (test car is a '96 B-body)

These might help, from John at TunerCat.

[In-Tech]

For ground use red 2 and blue 1.
Black 6 and 16 are for sensor grounds.

Mil is blue 9

and red 25 is vats signal to pcm.

All other looks good.

Thanks for catching that typo on the mil and the other stuff(corrected in original post too). Seems like I had a 1 connector bench harness back in the day. What are you guys doing for simple bench harness?

[steveo]

http://fbodytech.com/wp-content/uploads/2020/03/IMG_1329.jpg

[In-Tech]

Wow steveo :happy:

If this is grounded does it flash the MIL with codes on obd1 and obd2? Blue Pin 20 Output/Field Service Enable (Diag Port)

[steveo]

ive never tried but i've always been told that it does not

[sherlock9c1]

OK so this explains why I could never get a data connection when I installed an OBD1 PCM into a '96 OBD2 B-body when I tried it.

OBD2: the data line (pin 2) goes to PCM Blue 7.
OBD1: ALDL Pin 2 says not connected, Pin 9 goes to PCM Blue 28.

So now I'm super confused. All of the OBD1 conversion cars out there... how do they datalog or tune through the OBD2 ALDL port?

[steveo]

by swapping pins at the ecm and aldl port?
personally i prefer to chop the wire for ALDL and wire a serial interface directly to that, and tuck a usb cable up in there somewhere

[In-Tech]

I wrote: "If this is grounded does it flash the MIL with codes on obd1 and obd2? Blue Pin 20 Output/Field Service Enable (Diag Port) "


ive never tried but i've always been told that it does not

If anyone here has a 96-97 LT1 and could jump pin 6(paper clip works) and pin 4 or 5(both ground) of the OBD2 port to check if it starts flashing codes on the MIL, I would sure appreciate knowing. Code 12 is normal(1 long and 2 short flash) then other codes after pause. 1 long and 3 short = code 13 and so on.

[Tom H]

http://fbodytech.com/wp-content/uploads/2020/03/IMG_1329.jpg

I looked at this thread a second time and thought perhaps I would post a suggestion. I think a bench setup should address grounding between boards a little better. There are no grounds through the flex/ribbon cable between boards. GM used a spring clip with solder resist voids to connect ground. If ground wires through the connectors are used, the ground path and loop area make it ripe for errors and depending on currents analog errors. I have been using some braided ground strap with spring clamps (like you use for a large stack of paper) to do the grounding.

There was another post (I lost it somewhere on this site) where a PCM was found bad because of VPP errors. Grounding could be an issue there as well, along with the ribbon connections. I had problems with my ribbon after moving the boards around a bit. The insulation displacement design is not too rugged.

-Tom

[steveo]

i agree with your theory, but in reality, i have left it overnight programming in a loop with zero errors, and resistance between the two boards at the factory ground point is zero when just grounded through the connectors in this way. i did alligator clip the big ground trace between boards but when i took it away there was no difference.

[Tom H]

i agree with your theory, but in reality, i have left it overnight programming in a loop with zero errors, and resistance between the two boards at the factory ground point is zero when just grounded through the connectors in this way. i did alligator clip the big ground trace between boards but when i took it away there was no difference.

Great that it's working for you. The resistance may be zero, but what about the impedance? As frequency goes up those longer grounds start to look inductive. Depending on activity it is possible the grounds can shift. When working there will be no difference other than noise immunity by adding grounding. Most things have a good margin though and will work (?)

The way I would set my test bench is just slightly different. The cards can easily be stood up in a fabricated wood frame such that solder sides face each other: just like in the stock case. This will place the grounds between the connectors close to each other. Then a short braid ground can be used. This is what I plan.

Another good thing about doing it this way is that you don't need to unbend the ribbon cable. Mine failed after just a little handling. IMHO IDC connections should not have been used...

-Tom

[steveo]

it's mounted flat as i had to swap those chips a billion times and this way was best, otherwise i would have just left it in the case. i added an additional ground wire so you can sleep at night, but i hope you realizes that this puts us way over budget.

[Tom H]

way over budget. :yikes:

I guess when flashing, there isn't too much going on, though VPP and the SPI cross that cable. In any case, working = working

-Tom

[kur4o]

When I was testing some stuff with boards off the case I put some thick paper between 2 boards to prevent possible short issues and connect ground to each board from the connectors. a2 and d1 pins do the trick with grounds.

[steveo]

i had a ton of beer and resocketed this thing with a propane torch. greenwired it with fluxed up bailing wire when one of the traces ripped. the plywood that the ecm is screwed to is warped but i just cranked it down anyway. the screws were too long and it was squeeking the glass so i just threw some scrap foam under it. works fine. fbodytech is a drunken basement operation. time spent making things pretty just takes time away from coding.

edit: should also mention that that screw terminal block looks solid but none of the ring terminals i had in stock were the right size, so i just cut the things off. they're barely in there.

[In-Tech]

Hiya,
Does anyone know if the hardware of the obd1 and obd2 pcm's are the same? My 94/95 never showed up. Just wondering if I can socket and just swap chips so I can continue experimenting?

[steveo]

no that wont work