Im stumped!!! Please Help!

[1fstss]

looks like i need to start reading the entire thread HAHAHAHA!!!

[meissen]

DTC 42 = Ignition control circuit, short or grounded

Again, when I get home I can give you more info on it - I swear I need to just take a pic of every one of the DTCs in my service manual so I can always reference them.

[dj's ws6]

It's good to hear that you are getting closer to figuring this out.

[Phantom409]

G. M. s Misunderstood Codes
CODE 42

By Roy, BAT Auto Technical,Technical Editor

Probably the most misunderstood code (s) are the ones that deal with quad driver circuit faults like 26, 27, 28, and 29.
These are covered in part 2 of my G.M. ECM RESETS article.

Another code that seems to bring about quite a bit of confusion is code 42 found on most G.M. vehicles except some Cadillac's
and other E-body vehicles that use code (E) 23 as a flag for an Electronic spark timing (EST) circuit fault.
The EST circuit has been on G.M. vehicles for quite a few years. So, EST can be seen on a vehicle with a high-energy ignition
distributor (HEI) or, vehicles that are equipped with a direct ignition system. (DIS) {Distributor less}

Why EST?

To provide improved engine performance, better fuel economy, and to better control exhaust emissions, the computer controls
spark advance (timing) with the EST system. To calculate spark advance the computer uses information from the manifold absolute
pressure (MAP) sensor and the coolant temperature sensor (CTS) as well as engine RPM.

How spark advance is calculated:

Low MAP (low engine load) = more spark advance
Cold engine = more spark advance
High MAP (high engine load) = less spark advance
Warm engine = less spark advance
High RPM = more spark advance
Low RPM = less spark advance

The system.

The EST system consists of the ignition module, computer (ECM/PCM), and the connecting wiring. There are four circuits
to this system.

The circuits and their functions:

Reference.
This circuit provides the computer with RPM information. If this circuit becomes open or shorted to ground the engine will not run,
because the computer will not operate trigger the fuel injectors.

A common saying here is, no reference no run.
Reference ground.

This circuit is grounded in the ignition module and the computer; it makes sure the ground circuit has no voltage drop, which could
affect vehicle performance. I have found an open in this circuit can cause a vehicle no start as well as a false code 42/E23.

By-pass.
The computer applies 5 volts to this circuit to switch timing control from the ignition module to the computer.
The computer is telling the ignition module that it will control the timing. An open or a short to ground of this circuit will set the code and
the engine will run on base timing. (Plus a small amount of advance built into the ignition module.)
EST.
This circuit triggers the ignition module. The computer does not know what the actual timing is, what it does know is when it gets a
reference signal. It uses this reference signal as a base point and advances or retards the spark from that point.
So, if on vehicles where the base timing is adjustable, and the base timing is set incorrectly, the spark curve will be off through its entire
range by the amount that the base timing is misadjusted. An open or short to ground in this circuit will set the code and cause the engine
to run on ignition module timing.

Why the computer flags this code.

What we have here is a failure to communicate. The ignition module and the computer are in constant communication when the
engine is in operation. (Over 400 RPM.)With the computer telling the module the correct timing.
If communication is lost or a circuit becomes infected with false signals (bad grounds, short to battery +, Electro Magnetic Interference
{spark plug wires to close to circuit wiring}, oil contamination of the circuit connectors and wiring) then the computer will turn timing control
back to the ignition module and flag the code.

Possible causes:

EST circuit open or grounded.
Bypass circuit open or grounded.
Reference Ground circuit open or shorted to battery +.
A bad computer to engine circuit ground.
A defective ignition module.

A defective computer.

If you are testing a vehicle for this code check the routing of the spark plug wires making sure they are not to close to the EST
circuit(s) wiring. Check the connector terminals making sure they are not over-sized or oil contaminated. Check the entire wiring of the
circuits for chaffing,
short(s) ground. Alldata has a very good on-line manual system for the DIY s that contains all the information and a trouble tree
chart(s) for the proper method to test the ignition module and computer for your vehicle.

[98blackbeauty]

Wow! Missy did you realize how much you were going to learn when you bought that Z28. LOL

[meissen]

Here's the OEM service manual for code 42 also:

[Phantom409]

Well I think we covered that code

What one is next

[steddy]

I'm just waiting for more DTCs.

[meissen]

Here ya go, Missy - I was on LS1Tech and saw in a thread that someone was linking to thepartsladi's optisparks. Last week in the shoutbox when we were searching for "opti" it only came with the rebuild kits - gotta type out optispark in the search and then her optispark replacements pop up.

Genuine AC Delco for $295+$12S&H
http://cgi.ebay.com/ebaymotors/92-93...d=p3911.c0.m14

[fbodyhottie]

Thanks B! Ill be ordering one this friday hopefully! I cant wait to get her back on the road!