obd1 bypass mode

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07-20-2010, 08:15 PM
rally_scort

obd1 bypass mode

94 Silverado with 4.3TBI-5.7TBI swap and a 4L60

Short version.....Am I able to set my base timing by jumping the A and B terminals in the diagnostic port?

Longer version.....I picked this truck up, it was priced decent since it ran like dog poo.The timing was way off, the owner thought the motor was coming unglued.It made the drive home, but was severly underpowered and had a very irratic idle.I got it back into town and stopped by work, and reset the timing by ear, i was lacking a timing light.It picked up a good amount of power, but still has the lopey idle when letting off the gas, but will pick it self back up to a smooth but high idle.This truck was a 4.3TBI truck, and was swapped very poorly.Lots of stuff unhooked, some wires are cut, but the truck gets me to work and back.I searched for the Tan/Black wire to unhook to set the base timing.I found one that looks to be the right one, under the dash near the blower motor.The truck doesnt respond when i unhook it, so im failing at setting the timing.I plan on ripping this truck apart little by little and cleaning it up, but i want to get the timing set.Its running very rich, and still seems sluggish, with the irractic idle.I read somewhere, the timing can be set if the A and B terminals are jumped in the diagnostic port.This would put the truck into bypass mode, as though i were pulling codes.Is this accurate?

BTW, I really like this truck...id love to find somebody that would be willing to take a look at this thing, and let me know the best route in cleaning the wiring up.Im not even sure what im looking at, but id bring some beer for a teaching session :thumbsup
07-20-2010, 10:11 PM
Coops Brother

That is where the timing connector is. I know for a fact that the ECM's are the same part numbers, however when swapping a 5.7 in place of a 4.3 you need to replace the PROM in the ECM. This might be the problem, check the PROM ID with a scan tool and call a GM dealer and have them look up a PROM, there will be a couple of choices and the parts guy should be able to see which one is for which engine.
07-20-2010, 10:26 PM
rally_scort

i appreciate the info.Was i on the right track, being able to set base timing by jumping the A and B pins?
07-20-2010, 11:33 PM
Reverend Cooper

I would listen to my bro,he is one smart mother trucker
07-20-2010, 11:39 PM
rally_scort

Quote:

Originally Posted by Reverend Cooper

I would listen to my bro,he is one smart mother trucker

im all ears, ill check up on the PROM number tomorrow and work from there...im starting to think this truck might have a baby cam, causing the MAP to freak out.If i cant figure it out somewhat soon, ill just carb swap it and be done with the bullshit.
07-20-2010, 11:41 PM
Reverend Cooper

i bet the prom repl. fixes it
07-21-2010, 10:50 AM
outlawgibbs

I second that. All the wires are the same from the 4.3 to the 5.7 and if your lucky I may have a spare computer laying around for a 5.7 that you could swap to see if it works.
07-21-2010, 06:26 PM
rally_scort

Quote:

Originally Posted by outlawgibbs

I second that. All the wires are the same from the 4.3 to the 5.7 and if your lucky I may have a spare computer laying around for a 5.7 that you could swap to see if it works.

im assuming the computer is behind the glove box?
07-29-2010, 07:45 PM
rally_scort

I researched the service number on my ECM and its supposedly the correct computer for the truck.The service number is 16197427.Its listed as being used in 94-95 light duty trucks and some heavy duty applications.Im stumped now, the truck isnt running any better or worse since ive bought it.Id hate to start throwing parts at it, id like to get the timing figured out first.
07-29-2010, 08:32 PM
Coops Brother

If it is right for the truck, then are you saying it has a 4.3L prom in it, which is what the VIN would show when the parts dept. entered it. EDIT- "The service number is 16197427." If you got that number from the outside of the ECM....that isn't the PROM ID, in fact for sure it is not, as the PROM ID numbers are only 4 digits. There are 2 screws holding a 1"x3" cover down, open it there is the blue PROM, there will be a sticker on it with a 4 digit number, that is what you need to verify is for a 5.7L. Have GM look up a PROM for your year truck, when the ask what engine tell them a 4.3L, when they find it, ask if they can tell you the ID numbers of all of them, if one matches what you have...there is the problem. Have the guy look it up for a 5.7L and pick one....not the CA. emissions one though.
07-29-2010, 08:34 PM
Coops Brother

Quote:

Originally Posted by rally_scort

i appreciate the info.Was i on the right track, being able to set base timing by jumping the A and B pins?

No
07-29-2010, 08:45 PM
rally_scort

I pulled the ECM out, and looked up the Service number on the sticker.It says here http://tbichips.com/ecminfo.htm. The service number on the case is listed as being used in 94-95 light duty trucks.Im not completely positive that this truck was originally a 4.3, but just an assumption based on the half'assed bracketry on the intake manifold/wrong a/c lines/unbolted motor mounts/unplugged connectors in engine bay...i could be wrong.As for the tan/black wire, its located under the glovebox on my truck.I tried unplugging it, but it still allowed the timing to jump all over the place when trying to set the timing.That is why i was assuming either incorrect ECM, or possibly shotty or missing wiring that occured during the swap.Its sad that i dont know the story on this truck, nor does the previous owner(bought it from his buddy that isnt around anymore).He dumped the truck, because it was running like complete dogshit and didnt want to mess with it.I was told that TBI can be a real nightmare in diagnosing if anything is missing or incorrectly wired.It seems like you know your shit, so im throwing everything ive read out the window.I will be pulling a complete TBI intake off my boss's parts truck.Im going to attempt to grab every vaccum line, breather hose, sensor, etc that i can and hopefully piece this truck back together.
07-29-2010, 10:30 PM
Coops Brother

The service number is not the PROM ID number, you need to remove the cover to look at it, or hook it up to a scan tool to see the PROM ID.

The service number is the part number of the ECM, not the prom that is why the ECM is used in the V6 and V8 trucks....the difference is the PROM. Get it?

Did you look at the reluctor wheel on the distributor shaft? They come loose and cause the timing to go all over the place...
07-30-2010, 06:05 AM
BadAzzGTA89

Can you get any flash codes out of it?
# Turn ignition switch to OFF position
# Locate Data Link Connector (DLC), see image below.
# Jumper the A&B terminals(a paper clip or a jumper wire work ok)
# Turn Ignition switch to ON position.
# Count the flashes on the "CHECK ENGINE" or "SERVICE ENGINE" light.
# FLASH, pause, FLASH, FLASH = code 12
http://www.troublecodes.net/gmobd.gif
http://www.troublecodes.net/GM/
07-30-2010, 07:14 AM
rally_scort

its throwing codes 33(MAP sensor) and 43(Knock sensor)...im going to check over the wiring and make sure both sensors are plugged in and trace the wiring back to the ECM.Im leaning towards the knock sensor being the culprit, i havent heard any pinging but that would explain why the timing is all over the place and it seems to fall on its face in the mid-range.
07-30-2010, 11:12 AM
BadAzzGTA89

Quote:

Originally Posted by rally_scort

its throwing codes 33(MAP sensor) and 43(Knock sensor)...im going to check over the wiring and make sure both sensors are plugged in and trace the wiring back to the ECM.Im leaning towards the knock sensor being the culprit, i havent heard any pinging but that would explain why the timing is all over the place and it seems to fall on its face in the mid-range.

DING DING DING the knock sensor due to the amount of KR it's pulling on the timing!
Some times its easy to over see the little crap:thumbsup
07-31-2010, 03:53 PM
rally_scort

I read about 2345625 more threads about TBI troubleshooting and found a major issue.The MAP sensor needs to be fed off the rear fitting on the TBI, mine was routed to the front.I swapped it with my EGR vent lines and now it idles perfect and drives smoother.The only other issue i want to resolve is the knock sensor code.I noticed that mine is hooked up, but the kid ran some new wiring that routes up and over to the driver's side of the harness.My experience with knock sensors, taught me that the wire needs to be the shielded style, to reduce any noise from giving off a false reading.Is this true for this application? Im going to start tracing the wire to see if its tapped into the correct wire for knock.Hopefully its just a wiring mess and i can get this thing code-free by the end of the weekend.Thanks guys for the help, i appreciate it :thumbsup