monsolo2

I have an 86 coupe and just picked up the corresponding factory service manuals last week. I went to read about resetting my minimum idle speed and noticed something about the FSM that has me bothered. It says in plain black and white on line 6 to start the engine and THEN on line 7 to remove the diagnostic grounding wire from the ALDL socket. That is NOT the way every Corvette forum I've been on dictates... people tell you NOT to start the engine with the ground on because it will do something bad to the ECM. Is it possible the FSM is wrong OR is it the "bible" and I should just follow it?? I've attached some pics for reference...

gregs86

That's ignition on not running

gregs86

Sorry read that wrong

playsdixie

you can start the engine with A&B grounded.....this is field service mode....do a search...you can tell if your in closed or open loop by frequence of flashes and also if fuel is rich or poor...should be in your fsm somewhere......

Cliff Harris

If you short A&B and then turn on the ignition it will NOT go into field service (open/closed loop rich/lean) mode. You have to short A&B AFTER you turn on the ignition.

I know that's weird, but it's the way my car works.

The FSM says, "If the Diagnostic terminal is grounded with the engine running, the system will enter the Field Service mode".

monsolo2

Playsdixie, I'll check that out... I do remember seeing something about "field service mode". Thanks!

monsolo2

Cliff, what you're telling me is not the way the FSM specifically states the process to be. Understand that I'm an engineer and must always have a process! LOL! Is what you're describing the way YOUR car functions or are you recommending that I do it this way?

I guess to address my original fear, the ECM won't be harmed if I ground the A-B ports and start the engine, correct?? If I can be assured that nothing bad will happen, then I'll gladly try the process according to the FSM. I just don't want a blown ECM, that's all...

Cliff Harris

I quoted the FSM. What more do you want?

I'm a BSEE myself. I'm saying that if you short A & B before turning on the ignition then you won't go into field service mode. That's simply a fact. See my quote from the FSM that says the engine must be running before shorting A & B. That's not 100% true, as the ignition can be on and the engine not running.

It is VERY difficult to damage the ECM. It has over voltage and surge protection on every input. The circuit that ALDL pin B connects to is a simple ADC (Analog to Digital Converter) voltmeter. It compares the voltage on pin B with a reference voltage from a 10K resistor to 5 volts.
Nothing bad will happen.

monsolo2

Not trying to argue with you, Cliff... just being logical, that's all. Thanks for the help and I'll give it a try this weekend.

DanZ51

I just gotta say the FSM looks brand new. Mine looks more like a Tech Manual back from a 10 year deployment. Clear packing tape protects the covers and the binding was rebound by Staples like ten years ago

By the way, Staples did a really great job re-binding the service manual. And it was pretty cheap money IIRC.

Cliff Harris

Mine had large sections falling out so I bought a new one from HELM. It wasn't until AFTER I threw the old one away that I found out that the electrical diagrams in the new book were black and white instead of the color in the original FSM. Sucks.