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1992 GMC: K1500..a 4.3 V-6, 5-speed manual transmission..start fine

Resolved Question:

HI, I have a 1992 GMC K1500 pick-up truck with a 4.3 V-6, 5-speed manual transmission. I recently noticed missing of the motor while driving, it would idle and start fine. The last few trip out with it the shift light would flash on the dash while driving. This light has never come on before, I didn't even know I had one. I have noticed the engine would only miss when the shift light would flash. The shift light would never stay lite, only flash.The last time out, the check engine light came on. I clutched and started looking for a place to pull over and the check engine light and shift light went out and the truck ran fine for about 5 miles. I do have a haynes manual for this truck, and I pulled the code from the ALDL and I had two, a code 42 and a 45. The 42 is the Ignition Control Circuit. The book says to replace the ignition module or the ECM. I don't like replace parts without being sure. can you help me on this. Thanks
Submitted: 9 years ago.
Category: Chevy
Expert:  AMEDEE replied 9 years ago.



Would you like some more information about these codes?

Customer: replied 9 years ago.
I need to know if there is any way to be sure the ECM is bad or if the ignition moddule is the problem.
Expert:  AMEDEE replied 9 years ago.

You bet!


The best way to do this is run some tests! Usually, the computer is the last thing that gets replaced. THey do go bad, but its not very often.





Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.


Code 42 means the ECM has seen an open or short to ground in the EST or bypass circuits. This test confirms Code 42 and that the fault causing the code is present.
Checks for a normal EST ground path through the ignition module. An EST CKT 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.
As the test light voltage touches CKT 424, the module should switch causing the ohmmeter to "overrange" if the meter is in the 100-200 ohm position. Selecting the 10,000 to 20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
The module did not switch and this step checks for:
EST CKT 423 shorted to ground.
Bypass CKT 424 open.
Faulty ignition module connection or module.
Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423 or 424


Circuit Description:
The ECM supplies a voltage of about .45 volt between terminals "D6" and "D7". (If measured with a 10 megohm digital voltmeter, this may read as low as .32 volt.) The Oxygen (02) sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down through about .10 volt if exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below about 315°C (600°F). An open sensor circuit or cold sensor causes "Open Loop" operation.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.


Code 45 is set when the Oxygen (02) sensor signal voltage or CKT 412:
Remains above .7 volt for 60 seconds, and in "Closed Loop."
Engine time after start is 1 minute or more.
Throttle angle greater than 5% (about .2 volt above idle voltage).
Diagnostic Aids:
Using the "Scan," observe the block learn values at different rpm and air flow conditions to determine when the Code 45 may have been set. If the conditions for Code 45 exist, the block learn values will be around 115. Code 45 will be a hard code on 4.3L CPI engine. The "SES" light will remain "ON" until the ignition is turned "OFF," due to possible high fuel pressure.

An oxygen supply inside the 02 sensor is necessary for proper 02 sensor operation. This supply of oxygen is supplied through the 02 sensor wires. All 02 sensor wires and connections should be inspected for breaks or contamination that could prevent reference oxygen from reaching the 02 sensor.


Fuel Pressure - System will go rich if pressure is too high. The ECM can compensate for some increase. However, if it gets too high, a Code 45 may be set. See: Symptom Related Diagnostic Procedures\A - E Charts & Related Test Procedures\Chart A-6 Fuel System Diagnosis
Check for fuel contaminated oil. See: Symptom Related Diagnostic Procedures\A - E Charts & Related Test Procedures\Chart A-6 Fuel System Diagnosis
HEI Shielding - An open ground CKT 453 (ignition system reference low) may result in EMI, or induced electrical "noise." The ECM looks at this "noise" as reference pulses. The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing system to go rich. Engine tachometer will also show higher than actual engine speed which can help in diagnosing this problem.
Canister Purge - Check for fuel saturation. If full of fuel, check canister control and hoses.
MAP Sensor - An output that causes the ECM to sensor a higher than normal manifold pressure (low vacuum) can cause the system to go rich Disconnecting the MAP sensor will allow the ECM to set a fixed value for the MAP sensor. Substitute a different MAP sensor if the rich condition is gone while the sensor is disconnected.
Pressure Regulator - Check for leaking fuel pressure regulator diaphragm by checking for presence of liquid fuel.
TPS - An intermittent TPS output will cause the system to go rich, due to a false indication of the engine accelerating.
CTS - Check for a shifted sensor that could cause a rich exhaust but set Code 15.

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Customer: replied 9 years ago.
Also am I on the right track?Is this problem related to the ECM? I hate to replace electronic parts without being sure.
Expert:  AMEDEE replied 9 years ago.

Well you are close. But I would say not to replace the ECM.


I would start by replacing the o2 sensor. This is a very important input to the computer and must not be overlooked.


If that does not help it, I would replace the module next. The ECM is the last thing that I would replace for these codes.