OBD2 to OBD1 LT1

[fuzzcar]

I've got a 96 Impala I have put a 94 LT1 into. I have the 94 PCM and knock sensors. The transmission is a T56.

Its my understanding that all I need to do to convert to OBD1 is plug and play. Change the tac signal to a 4 cylinder signal and maybe wire the knock sensors in parallel??

https://buickforums.com/forums/threads/obd2-to-obd1.13374/


The conversion is not hard at all. You obviously need an ODBI computer with a 95 B body tune, because of the PWM TCC in the 95-96 transmission. With a non Impala SS, you only have to worry about the knock sensors. The Impala SS you have to convert the tach signal to a 4 cylinder signal.

The pre-96 B body used 2 knock sensors wired in parallel. The 96 uses 2 knock sensors attached to two adjacent pins on the PCM. The resistance of the knock sensors are different also. The rear O2s and crankshaft position sensor (CKP) are on unused PCM pins, so you don’t have to worry about them.
To fix the knock sensor problem you have two choices. You can change the knock sensors to the 94-95 B-body sensors or add a resistor.
With either choice you must connect the two knock sensors together so they are in parallel.
You have two more choices, modifying the wiring harness or modifying the PCM. Modifying the PCM allows you to switch PCMs back and forth for emission testing, selling the car, etc …
To mod the wiring harness, find the BLUE connector on the PCM and connect the wires at pin 21 (Light Blue) and pin 22 (Dark Blue) together. ODBI only uses pin 22 (Dark Blue). If you did not change the knock sensors, you must attach a 2K resistor to this wire and connect it to ground.
If you can solder you can mod the PCM. Open the PCM and connect pins 21 and 22 (blue connector) together and run the 2K resistor from 21 to ground.

[NomakeWan]

If you have a car that came with a 4-cylinder engine then your tach will expect that signal, yes. You also must modify the knock sensor signal as it says (either by splicing the wires together or by soldering the PCM pins together). The latter would let you swap back to OBDII if necessary without messing with the wiring harness.

[tayto]

Why not convert to a 95 pcm? would be less wiring, if any.

[fuzzcar]

Why not convert to a 95 pcm? would be less wiring, if any.
95 PCM uses the dual signal of the knock sensor?

[Rocko350]

put the motor and the rans in to begin with. Can you tune or are you able to tune the 96 computer? f not. Put the 94 ecm into the car. Repin harness and install the 94 knock sensors. The obd2 port under the dash will need to get changed to the obd1 connector so you can tune and datalog and diag if needed the new engine controller if needed. Tach ouptu is easy. The tach low res periods are adjustable in the obd1 computer. Obviously the crank sensor pigtail isn't used. neither is the AIR. the 96 has a fuel tank pressure sensor the 94/95s dont have. Un pin those from the ecm. Use www.shbox.com for wiring references. Reuse the 96 harness, coil, ignition module. Have a set of the injectors cleaned and flowed. There are plenty of people tha have done this before you. Look also on www.impalassforum.com

[NomakeWan]

Why not convert to a 95 pcm? would be less wiring, if any.
That's...exactly what he's doing. The PCM is the same; it's the program that's different.


95 PCM uses the dual signal of the knock sensor?
No. Only the OBDII PCM does, which is why it's a different part number.


put the motor and the rans in to begin with. Can you tune or are you able to tune the 96 computer? f not. Put the 94 ecm into the car. Repin harness and install the 94 knock sensors. The obd2 port under the dash will need to get changed to the obd1 connector so you can tune and datalog and diag if needed the new engine controller if needed. Tach ouptu is easy. The tach low res periods are adjustable in the obd1 computer. Obviously the crank sensor pigtail isn't used. neither is the AIR. the 96 has a fuel tank pressure sensor the 94/95s dont have. Un pin those from the ecm. Use www.shbox.com for wiring references. Reuse the 96 harness, coil, ignition module. Have a set of the injectors cleaned and flowed. There are plenty of people tha have done this before you. Look also on www.impalassforum.com
Why the 94 computer specifically? Either the 94 or 95 computer will work just fine. The computer is the same--it's the program that's different. Also tuning the '96 is a major PITA, which is probably exactly why he's doing this swap. At some point in the future it won't be that way, but for now, it is.

[tayto]

I guess my point was '95 uses an OBD2 port even though it's an OBD1 PCM. If fuzz only has an OBD1 cable then I see the reasoning for wanting to switch to an OBD1 connector. I am merely suggesting the least work possible. You can throw a resistor inline and not have to remove the '96 knock sensors if you are so inclined. I believe that is the only change, software aside.

[NomakeWan]

I guess my point was '95 uses an OBD2 port even though it's an OBD1 PCM. If fuzz only has an OBD1 cable then I see the reasoning for wanting to switch to an OBD1 connector. I am merely suggesting the least work possible. You can throw a resistor inline and not have to remove the '96 knock sensors if you are so inclined. I believe that is the only change, software aside.
Wrong again. Again, please read the first post much, much more carefully. The OBDI PCMs wired their knock sensor(s) into a single pin in the PCM, while the OBDII PCMs wired them into two separate pins. You can either join the wires at the harness (which will make it so you can't easily swap back to the OBDII PCM), or join the pins inside the PCM (which would let you easily swap PCMs again).

Don't focus so much on the physical connector for the scan tool. Its shape is irrelevant.

What matters is the computer, and there are significant differences between the 94/95 and 96/97 computer. And since programming the 96/97 is currently a major pain in the ass unless you feel like spending $400+ on software, it's more than understandable that OP would want to run an OBDI computer instead.

[fuzzcar]

I am running the 94 PCM because I have it and it matches the engine. I'm getting ready to attempt to fire without any tuning what so ever. A few more bolts to turn then I give it a go. I have all the cables and adapters needed. I have OBD1 and 2 tuning software. I just want this car to run as simply as possible.

[NomakeWan]

Best of luck to ya then! If you followed the directions you quoted in the first post, you'll be fine.

[tayto]

the pins get tied together with a 3.9k resister to ground. this modification can also be done INTERNALLY in the PCM. why change the obd2 port when he already has a cable is beyond me. sounds like more unnecessary work. (you are aware that 95 had an obd1 pcm but had an OBD2 port right?)

I never mentioned anything about 96/97 PCMs nor did I ask for clarification, thanks.

[tayto]

most of the websites (with pictures) with this information are gone. here is a forum post with pictures: https://www.z28.com/threads/obd1-resistor-mod.141119/

[fuzzcar]

Thank you for the responses. I am wondering if I use a 94 motor with 94 knock sensors and a 94 PCM why do I even have to mess with resistors and wiring?

[NomakeWan]

Thank you for the responses. I am wondering if I use a 94 motor with 94 knock sensors and a 94 PCM why do I even have to mess with resistors and wiring?

If you do use the 94 knock sensors, you do not need to do that resistor mod, period.

The wiring change is necessary because your OBD2 wiring harness separates the two knock sensors onto individual wires, while the OBDI computer has them wired together to a single pin.

[Rocko350]

That's...exactly what he's doing. The PCM is the same; it's the program that's different.


Pcms are not the same. Im kinda tired of explaining this.



Why the 94 computer specifically? Either the 94 or 95 computer will work just fine. The computer is the same--it's the program that's different. Also tuning the '96 is a major PITA, which is probably exactly why he's doing this swap. At some point in the future it won't be that way, but for now, it is.

reread first post. It is what he has to install.

[NomakeWan]

reread first post. It is what he has to install.

I suggest you take your own advice. It states any OBDI PCM (because they’re the same), with a program from a ‘95 to handle the PWM TCC.

The only difference in OBDI PCMs is between Corvette and non-Corvette (the Corvette ones added additional silicon related to OBDII testing). The year does not matter, and since he doesn’t care about the OBDII stuff, neither does Corvette vs non-Corvette.

[Rocko350]

I suggest you take your own advice. It states any OBDI PCM (because they’re the same), with a program from a ‘95 to handle the PWM TCC.

The only difference in OBDI PCMs is between Corvette and non-Corvette (the Corvette ones added additional silicon related to OBDII testing). The year does not matter, and since he doesn’t care about the OBDII stuff, neither does Corvette vs non-Corvette.


Why would he need the pwm trans control?

[NomakeWan]

Why would he need the pwm trans control?
You're right, he wouldn't--it's only an issue on the auto trans. So actually he can do whatever, good point. Even less of a reason to be picky about 94/95.