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Old 09-21-2008, 11:14 PM   #1 (permalink)
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Code 42

I was driving home today on the freeway and my impala was stumbling today and i am not sure why. I have no MIL on and i was looking through it on my snap on brick and it said EST grounded any idea. I looked through my book today and it said wiring issue i wonder if anything is touching my headers it has started apparently after so. I will look under the car and inspect the wiring underneath perhaps a knock sensor wire is loose. Hit me up with any info please.

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Old 09-22-2008, 06:24 AM   #2 (permalink)
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This is a good read by Ken Hughes that I have bookmarked. It is LONG so I apologize to everyone in advance.

That Annoying GM Code 42
Posted 5/17/1997
By Ken Hughes

This is a common code that is often difficult to diagnose and scan tools can't help. The following understanding of how the electronic spark timing (EST) system works - what is taking place and why - is a big step toward a quicker and easier diagnosis of this circuit. When the engine is turning over, but below the run threshold of 400 RPM, the ECM holds the module's bypass voltage too low (0 volts) to energize its solid-state switch (ignition module). Another way of looking at the solid-state switch is to think of it as a relay. When voltage is applied, the relay is energized. With no voltage applied, the relay is de-energized. The pick-up coil/crank sensor pulses are amplified and shaped by the ignition module. These pulses are used to complete the circuit creating a magnetic field in the ignition coils primary winding that, when collapsing, will induce a high voltage in the secondary winding. Therefore, for every crank sensor/pick-up pulse, the coil is triggered. This is known as the bypass mode or module mode of operation. In this mode, the engine is running on the timing advance that is built into the ignition module.

With the voltage low on the bypass wire, the EST is pulled to ground through a resistor in the ignition module. This keeps the voltage on the EST wire at around 300 mV. When the ECM sees the RPM over the run threshold, it will then apply 5 volts to the bypass wire that will activate the solid-state switch (relay). This in turn will switch the EST from ground to the base of the transistor that controls the primary coil. In this mode, the primary coil winding is being triggered by the altered signal sent out from the ECM. The ECM will alter the signal to the ignition module and control the timing based on the inputs from various sensors. This is referred to as the EST mode.

This is how the system is designed to work! Now let's look at some of the things that can cause problems and set a Code 42. Under 400 RPM, no voltage on the bypass wire, the ECM expects to see low voltage (300 mV) on the EST line during this condition. If it sees 0 volts, indicating an open in the EST circuit - or higher than 500 mV - it sets a Code 42 and stays in the bypass mode.

If the bypass line is open, or grounded, the ignition module will not switch to the EST mode. The ignition module needs the bypass voltage to activate the solid-state switch so the EST voltage will be low over 400 RPM and a Code 42 will set. If the EST line is grounded, the ignition module will switch to the EST - but because the line is grounded, there will be no EST signal. A Code 42 will set.

To check the ignition module to see if the solid-state switch is capable of switching when the 5 volts are applied, the engine should not be running and the ECM connectors should be disconnected. An ohm meter and test light can be used. With the ohm meter on the EST circuit, it should read less that 500 ohms (in some cases, a lot less). Using a test light to battery voltage, probe the bypass wire. With this voltage applied to the bypass wire, the solid-state switch inside the ignition module should switch and the ohm meter on the EST circuit should go over 5,000 ohms. There is also another way to check this with the engine running: by removing the bypass and the EST wire from the module or ECM, run a jumper from the reference wire to the EST circuit. The reference signal is the signal that is used in the bypass mode to trigger the primary coil voltage. Apply 5 volts from one of the 5-volt reference circuits or a test light to battery voltage. This voltage on the bypass wire will activate the solid-state switch, in turn switching the module. On some models, the 5 volts or test light will have to be applied before starting or the motor will stall when the voltage is applied to the bypass. If the module is switching OK, the car will continue to run on the reference signal. If it stalls, the ignition module is not switching properly.

There are some other checks that can be made when working with an intermittent Code 42. When using a digital volt ohm meter (DVOM), there should be 5 volts on the bypass wire. On the EST wire, with the engine running, there will be close to 2.3 volts. The EST is a 5 volt on/off digital signal, so with a volt meter you will see the average of the high and the low. With the EST signal being a digital on/off signal, if you have a meter that will read in Hz, you can also pick up a Hz reading on the EST wire. Normally, you will see about 28 Hz with the engine idling and will increase with RPM. If the EST circuit would become momentarily grounded, or open, the engine will cut out. The voltage reading will jump down to as low as 1.3 volts and the Hz reading will jump from as low as 17 to 128 Hz. If there is a problem in the EST circuit, the voltage on the bypass wire will not change. If the bypass wire is momentarily grounded or open, the reading on the EST wire will be the same as if it was grounding or going open, but unlike a problem on the EST circuit, the bypass voltage will be pulled low if it becomes grounded or goes open. If this happens fast enough, the engine will cut out. The voltages will vary, but the SES light will not come on and no codes will be set.

With the EST circuit open, engine running, the voltage will stay low on the bypass wire and the ECM will not put out the 5 volts. With the EST shorted to ground, the voltage on the bypass wire will be high (5 volts) for a very short time (three seconds or less) then the ECM will remove the 5 volts. Using a min/max on the DVOM is the best way to see this voltage. With the bypass open, the EST circuit OK, the ECM will put out the 5 volts. By using a DVOM on both the EST and bypass wire, you can tap on the ECM, wiggle and tug on the wiring to see if the voltage or Hz reading will change. If it does, this is an indication that you have located the problem area.

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Old 09-22-2008, 07:35 PM   #3 (permalink)
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Very nice article there moneypit. That is the most detailed explaination I have read about how the ignition system works on the newer GM cars.


1977 Chevrolet Impala Sedan, 305, TH200, 2.56:1, 110K miles, 37 Lifetime Warranty Parts.
1989 GMC C1500 Sierra ECSB 350, HM290, 3.08:1, 338K miles, 15 Lifetime Warranty Parts.

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Old 09-23-2008, 09:40 PM   #4 (permalink)
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Hey i was going thorugh my snap on and i pulled codes 16 and 36 high and low resolution problems. I was also looking through the recipts and it was replaced with a jegs setup and it was 249.99 and my buddy had one and it took a crap after two months. Anyways im thinking that it could be cap and rotor problems because it starts and runs but it is irratic sometimes. I am familiar with the opti-spark system and i am going to school for mechanics but im stuck in a rut any ideas? Anyone else have problems with the jegs setup?

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Old 09-23-2008, 09:50 PM   #5 (permalink)
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I answered your PM with this...hopefully some others can chime in.

Stay away from all of the optis except the GM's. The best place to get them is "partsladi" on ebay....very cheap and authentic parts.

For code 16 and 36, it could be the opti harness....those are known for going bad and causing this code...

Code 16- Low resolution pulse (distributor). ECM is not detecting the 4X pulse from distributor. Check wiring connections, especially the pigtail connector from the optispark to main wiring harness. This is the black/red wire that goes to terminal "A" on the pigtail connector. On the 1992-93 ecm, this is wire C5 (green connector). 1994-95 ecm, this is wire B2 (black connector). If wiring checks out, you will have to replace the optical sensor module in the distributor.

Code 36- Distributor ignition system. Faulty high resolution (360X) pulse or multiple low resolution pulses detected. Check pigtail connector from distributor to main harness for any opens or shorts. Verify wiring is okay before replacing distributor.

Get in there and look at the harness to make sure wires are in good shape. It might be worth it to replace that first before swapping out the opti again. When you do switch opti's, don't fall for the hype and go gm. Let me know what you find....

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Old 09-23-2008, 10:50 PM   #6 (permalink)
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Do the jiggle test on all of the wires to the opti pigtail and coil, including grounds. Let us know what you find out.

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Old 09-23-2008, 11:56 PM   #7 (permalink)
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i will do so tomorrow or today what have it i am frustrated right now because there are no codes anymore and as soon as i am to drive it anywhere something will happen to itjust because it can always right before work lol. Regardless i love the car and i wont throw the towel in because one i am not going to be defeated by it, two bring it to a shop there wont be anything left of the tires well there is not much left as it but there is still tread.

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Old 09-25-2008, 08:52 PM   #8 (permalink)
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The outcome.

Alright, after doing alot of head scratching researching and cussing swearing and everything else. I bid adoo to the old optispark and to the new. Kinda ryhmes, anyways if you are the unlucky soul of the code 16, code 36, code 42 and or code 42 i wish that you would not have to deal with it. Anyways On my ss i was having problems with my car fallign flat on its face and just plain ol out dying spitting sputtering and all the other common signs of optispark failure. I bought my car with a Supposed new one and it was however low quality one at that, from jegs. So first thing do not buy one from jegs in the first. Anyways to start the adventure of changing the optispark, Chances are if you do get the check engine light and have the luxury of a scanner then use it. Find out what codes you have. If a 42 or 42 set with the code 36 check all wiring. If all seems good then chances are your OPTISPARK took a dump.
Alright here is what all the codes mean

16 Low resolution- The PCM detects 720 high resolution timing pulses before any low resolution pulses are detected.

36 High Resolution- 40 low resolution pulses occur before any high resolution pulses are detected

42 Electronic Spark Timing- If DTC 42 is detected, the PCM will disable the fuel injectors to prevent flooding of the engine. DTC 42 will be stored in the PCM memory but will turn "ON" the Malfunction Indicator Lamp

Also on a lighter note, If these codes are intermittent chances are you have a bad cap and rotor. That is what happened to my car but i went ahead with replacing the whole assembly because if the cap and rotor went bad after 15k then i dont trust the optical part of it either why do all the work twice.

After you have determined your codes and what has went south follow the appropriate action. If it is determined that your optispark has went out then, one you are going to be pissed off, two be prepared to spend some money, three you are going to be irriatated, and four after it is all done you will be happy you did it.

Expect about a 4/5 hours to complete.

Tools required.
-Elbow Grease
-3/8 Drive Ratchet
-Harmonic balancer puller
-Drain pan
-10,13,14,15,17 mm sockets and a 3/8 socket
-Inch pound torque Wrench
-Foot pound torque Wrench
-random sizes of pliers

Torque Specs.

Water-pump 37 Foot Pounds
Distributor 106 Inch Pounds
Cap screws 25 Inch Pounds
Rotor Screw 6 Inch Pounds (apply LocTite)

Well after all that noise, Here is what you do.
1. disconnect your battery- people with the factory radio make sure you shut off the theft lock and if you didnt you just made your day that much longer.
2. Remove "home base" and all of the intake tubing (more work room)
3. Get a drain pan and remove all heater hoses, radiator hoses, and let it all drain out.
4. Remove air pump, water pump(there are six bolts instead of the 350 standard of four)
5. Remove the harmonic balancer 17MM bolts and get a puller to remove it off the snout.
6. Now the fun part it you are replacing the whole assembly disconnect the OPTI harness connector on the distributor, IF not then dont bother for a cap and rotor replacement. There are three 10mm screws holding the whole assembly on and it will come off as a unit if you do just remove the three screws. If you are going to replace the cap and rotor just remove the four screws and it will come off.
7. Pay attention to exactly how it comes off so you put the new one on correctly.

Installation is the exact opposite of the directions.

This may deserve a sticky.

Two photo shows the cap burn through
Attached Thumbnails
Code 42-optispark-replacement-002.jpg   Code 42-optispark-replacement-004.jpg   Code 42-optispark-replacement-005.jpg   Code 42-optispark-replacement-006.jpg   Code 42-optispark-replacement-007.jpg  

Code 42-optispark-replacement-008.jpg   Code 42-optispark-replacement-013.jpg  

Last edited by notacapricecapala; 09-30-2008 at 10:18 PM.. Reason: pictures!
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