G. M.’s Misunderstood Codes CODE 42

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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.
Roy



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Sordfish

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I have a 1990 Buick park avenue and have this code 42. I replaced the VSS due to the speedometer and cruise control sometimes not working. After replacing the VSS i got 41,42. I did not check code before replacing the sensor. When the Speedo works then quits it will idle higher. If you remove foot from brake it will move more than normal, I don’t have a tach to see rpm. Can the EST issue affect the cruise and speedometer?
 

grcauto

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I have a 1990 Buick park avenue and have this code 42. I replaced the VSS due to the speedometer and cruise control sometimes not working. After replacing the VSS i got 41,42. I did not check code before replacing the sensor. When the Speedo works then quits it will idle higher. If you remove foot from brake it will move more than normal, I don’t have a tach to see rpm. Can the EST issue affect the cruise and speedometer?
You have more than one problem. I don't think he mentioned in his write-up that the prom can cause this code 42 but let's work on your other stuff.
That code 42 and it's related problem are not causing the speedo or idle problem.
Do you have a scan tool?
You replaced the VSS and it still has a problem, correct? We will need to look at the wiring. The wires go directly from the sensor to the ECM.
As for the idle problem. That uses a digital IAC. Have you tried to clean it and do an idle relearn? These are where we need to start.
A scan tool would be a help if you can get one.
 

Sordfish

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You have more than one problem. I don't think he mentioned in his write-up that the prom can cause this code 42 but let's work on your other stuff.
That code 42 and it's related problem are not causing the speedo or idle problem.
Do you have a scan tool?
You replaced the VSS and it still has a problem, correct? We will need to look at the wiring. The wires go directly from the sensor to the ECM.
As for the idle problem. That uses a digital IAC. Have you tried to clean it and do an idle relearn? These are where we need to start.
A scan tool would be a help if you can get one.
when i got the car it did not stay running, i found a leak in EGR and cleaned it, it ran but when fan turned on after it got warm it ran rough and would die, it would restart. I replaced with a new EGR and it did not die after the fan came on and the idle was not like with the old ear. I hav a jumper plug to send code to engine light and flash. When the speedo gitch it runs different. maybe connections at the ECM. I an going to clean and check grounds on the IM.
 

grcauto

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I have the factory info if you need something let me know.
 

prariedom

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Old thread, yes, but, are we talking OBD 1? I can tell you from experience that a code 42 on an OBD 1 GM with throttle body injection is the injectors failing which in turn burns out the spark chip as it tries to compensate.

I had an 87 Celebrity that mechanics would try to replace just the injector or the ecm to have it fail, repeatedly. Oddly it was a Honda mechanic that tipped me off, and after calling 800 Chevy they verified and it was fixed for $125, ran for many years - but I'd spent $1500 before that.
 
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