breathment

Wut happends if i start my car with the two diagnostic terminals jumped? (A&B?)

------------------
88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

Chris Etemadi

It will fry your computer!!!!!!
I hope you didnt already do it

------------------
91 Z28 5.7l Camaro
T-56,McLeod Street Clutch, Cross Drilled Brakes, KYB's,AFPR, 3.73 Gears, Borla Adjustable Cat Back, Hypertech Chip, Hooker Headers.
more to come....
My 91 Z28

whiteroc

...and how do you know this? Is there anything published about this?

SillyWabbit

LOL!!!!! That's great!

But really, if you start your car with terminals A&B jumped, it should put the ECM into field service mode where you can see whether you're running in closed or open loop, and once you're in closed loop, whether you're running rich, good, or lean. You can probably tell other stuff too, but I don't know how to read it, so, bah.

breathment

SO REALLY WUT WILL HAPPEN? i heard that it can screw ur computer, but i did it to my friends camaro when his EGR was causing his car not to start (stuck open), and i knew that jumping that would bypass all the sensors, and it started right up. so how should i do it? im confused.

------------------
88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

rezinn

NOTHING, other than driving around with annoying flashing lights.

IROC5.7TPI

Put down the crackpipe

prepz28

it doesnt do anything. i think the SES light just flashes

JETHROIROC

Wrong guys. It can damage the ECM to start the car with the diagnostic terminals jumped. For field service mode, you must jump the diagnostic terminals AFTER the car has been started.

------------------
1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, Accel coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley AFPR, ported plenum,
Ruger P95DC, hot wife, new oil filter, thick rubber floormats, no cats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

whiteroc

Well...? I started my car 3 months ago with the a-b connected. I have not had any problems (no SES codes) since. So what is suppose to happen now? Fried ECMs tend not to work so good. Don't you think GM would have thought someone would make the mistake of doing this even at the Dealership Service level and made a failsafe integrated into the system? Are we guessing here or does someone have something to back up this Fried ECM Theory?

JETHROIROC

Okay, so you won't take my word for it but maybe this previous post from Vader will clear things up:

"DON'T attempt to start the engine or even crank it with a jumper in the ALDL diagnostic terminal. You can possibly cause ECM damage. Instead, start the engine, allow it to run for a moment, then install the jumper.

Field Service Mode

On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is
running.

WARNING! This must be done after the engine is running. This is called "Field Service
Mode" and will not harm the ECM.

If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per
second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.

A 3.9K resistor installed in ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel
Mode. This mode forces the ECM to use predetermined fuel calculations in the ECM
PROM instead of the learned inputs in active RAM. This is commonly referred to as
the "Limp Home" mode of operation."

Notice it says POSSIBLY cause ECM damage, which means you got lucky .

whiteroc

Needless to say I won't be starting it with the jumper in the ALDL terminal anymore. I forgot I had it hooked up when I started it. I knew you could have it running with it hooked up but not at start up. Thanks for the heads up guys. ...glad I got lucky.

breathment

well, on my friends car, it wouldn't start unless u jumped A&B terminals. and then once it started, i unpluged my jumper, and the car ran in closed loop (i think). whatever mode is doesn't use any readings from the sensors.

------------------
88' GTA 350 MODS---> air foil, K&N, Shift Kit, 180* therm, TB bypass

drive it

iTrader: (1)

This is a direct quote from the factory service manual. " NOTICE: To prevent ECM damage, the key must be "OFF" when disconnecting or reconnecting power to the ECM (for example battery cable, ECM pigtail, ECM fuse, jumper cables, etc.)."
That is the only reference to ecm damage in the FACTORY service manual(they should know). I've personally started it numerous times with A and B jumpered without any damage when checking things before I bought diacom.



------------------
86 406

gravitar

I've started up with A&B jumpered lots of times. I'll have to check again, but I'm pretty sure my '85 S-10 factory shop manual tells you to start it with the jumper if you want to enter field service mode! Granted its a different ECM, but I'm sure the same procedures would apply.

Grim Reaper

Diacom actually warns you to NOT start Diacom until the engine is running. But I have MANY TIMES started my engine while Diacom is running.

I think a lot has to do with what type of ECM you have. The newer TPI 7730s (and probably 165s) are not as critical.

Yes, jumping will put you in Service Mode which causes the ECM to use different settings than the eprom. This is something I point out to users of Diacom to MAKE SURE they are in Normal Mode as Diacom defaults to ALDL Mode and actually overrides certain settings in the eprom. With the jumper, the ecm is not operating properly.

Svelte_SS

Aren't you supposed to have these jumped when you set minimum idle while resetting your IAC motor???

Correct me if I'm wrong but I thought the procedure was
1 - jump A & B terminals
2 - turn ignition on for 30 seconds
3 - disconnect IAC
4 - start engine
5 - set idle to 450 (or whatever)
6 - shut off engine
7 - reconnect IAC