GT dieseling, code 42

[Zuus]

Not being much of a mechanic, my Typhoon buddy suggested I look to the forum for help.
On the way to work, truck lost all power. When I pulled over to attempt a restart, the motor was dieseling, and a push on the throttle didn't change anything. So I turned the key off and everything shut down. Soon as I tried to start, it fired right up running fine with the check engine light on.
Got to work and stuck a jumper wire in and got a code 42. Typhoon buddy thinks it's the spark control module, but we're checking to see if anyone here has a different idea?

Thanks in advance, E

 

[jeepruby04]

Re: GT dieseling, code 42

Code 42 comes up as a rich condition or O2 sensor not crossing from rich to lean. Could be bad O2 sensor?

Not sure what site I got that info off of, but it was absolutely incorrect and I am sorry I didn't double check first before posting.

 

[Zuus]

Re: GT dieseling, code 42

Very possibly. I have not replaced it and doubt if previous owner did.
Thanks

 

[Osmosis]

Re: GT dieseling, code 42

Code 42 is an Electronic spark fault code. This code usually come up when you disconnect the brown wire when adusting the timming. Check and see if you have power to the brown wire with the Key on Engine Off and when the truck is running. If you have power to that wire, disconnect the battery for 30 seconds so the computer resets. Reconnect the brown wire, then connect the battery.

If the truck is still having issues, check the igntion control module.

 

[Zuus]

Re: GT dieseling, code 42

I checked for power at the brown wire and no juice, either running or in ON position. Indicate a bad spark control module?

 

[Osmosis]

Re: GT dieseling, code 42

Code 42 is not a lean/rich condition. Code 42 is an electronic spark fault code that is commonly seen when properly adjusting the timing. To adjust your timing properly, you must disconnect the brown wire wihch will throw a code 42. Usually you just have to disconnect the battery for 30 seconds and the code goes away. Now, it sounds like he has not messes with anything and this code came up. If you are not getting power to either side of the brown wire while the car is running, you need to find out why no power is there....or if there is supposed to be power there while the engine is on?

Can anyone answer this puzzle???? I"m not sure if the ignition module is connected to the brown wire or not? Or, is power ever running to the brown wire, or only when the timing is advancing or retarding??? Anyone know? I don't:roll:

 

[The Bronze]

Re: GT dieseling, code 42

I checked for power at the brown wire and no juice, either running or in ON position. Indicate a bad spark control module?

Code 42 sets for a couple of reasons:

1) While the Bypass (Brown to you, Tan/Blk to the rest of us ) is low (below 400 rpms), the ECM monitors the EST line (white). If there are reference pulses present (Would/Should be ~5V squarewave if present) then code 42 sets. The ICM should be pulling the white wire to ground at this point preventing this signal from occuring.

2) When the Bypass (Tan/Blk) goes to 5V (over 400 rpms), the ECM looks to see that there are reference pulses (~5V squarewaves) present. This would occur because the ECM sees that we are over 400 rpms on the reference wire (purple/white) and applies the 5V to the Bypass and the module then releases the ground on the white wire so that the ECM's 5V ignition control signal can trigger the module when to fire.

So, Key-On, Engine-Off (KOEO), there is no voltage on the Tan/Blk wire (Bypass) and the White wire (EST) is grounded. The Purple/Wht (Reference) has no voltage as well (No Engine RPM)

Key-On, Engine-Running (KOER), (BELOW 400 RPM), there is there is no voltage on the Tan/Blk wire (Bypass) and the White wire (EST) is grounded. The Purple/Wht (Reference) has a 5V square wave going to the computer (Engine RPM information).

Key-On, Engine-Running (KOER), (ABOVE 400RPM), there is 5V present at the bypass line (comes from the ECM) and the White wire has a 5V squarewave pattern that is the combination of the reference and the desired ignition advance/retard.

 

[The Bronze]

Re: GT dieseling, code 42

The answer is the module, wiring or ECM can be bad.

Assuming wiring isn't shorted/open:

At normal idle, If the white wire and tan/blk wire aren't grounded by the ICM and there isn't 5V at the bypass connector (connected), then the ECM is bad. (SETS CODE 42)

At normal idle, if the white wire is grounded and there is 5V at the bypass line then the ICM is bad. (SETS CODE 42)

At normal idle, if the white wire isn't grounded by the ICM and there is 5V at the bypass connector (connected), then the ECM is bad. (SETS CODE 42)

At normal idle, the white wire shouldn't be ungrounde unless there is 5V at the bypass line. (NORMAL OPERATION)

Someone check my logic. It should be sound.

 

[Osmosis]

Re: GT dieseling, code 42

So, where would one put a test light or ohm meter to trouble shoot this problem.

At which connection between the brown wire and white/brown striped wire? Brown wire side or white with brown stripe wire?

 

[The Bronze]

I don't know what he is trying to measure. I don't ohms test, so the bypass connector would be connected and I would be looking at voltage. I would recommend putting down the test light and spend $20 at Sears for a voltmeter. I don't stick a test light into a electronic circuit and neither should you. You will want some impedance to protect the circuit during the testing. That means a meter. Then he gets a wiring diagram, some "t" pins from the sewing/hobby store and access to the PCM.

 

[Osmosis]

Re: GT dieseling, code 42

Sorry, I was meaning to say a volt meter. Where should we test the wires with a volt meter and what exactly are we looking for?
Should we take the postive & negative to opposite sides of the brown and tan wire where it connects? Or at the ECM, if so at which pin?

Thank you for the help!:tup:

 

[The Bronze]

Re: GT dieseling, code 42

Sorry, I was meaning to say a volt meter. Where should we test the wires with a volt meter and what exactly are we looking for?:

I would start with the white and the tan/black (brown if it is on the GT) wires. Not to be coy, but we are looking to see what voltages are there.

Should we take the postive & negative to opposite sides of the brown and tan wire where it connects? Or at the ECM, if so at which pin?

The process would just measure the difference in voltages between the two points, which in your scenario would be (almost) useless. Keep in mind, that when you are testing with a meter using the "volts" function, you are simply measuring the potential difference between the two probe tips.

Ensure the bypass connector is secure. The test should take place (IMO) at the ECM. The negative lead needs to go to a solid ground. The positive lead will be what you are connecting to the white and tan/blk (brown) wires. You should have alligator clips on the meter. Use the t-pin to get into the ECM connector. Bend the pins so they are going different directions (so you don't touch the two together). Just shoving the probe in there will likely result in intermittent connections.

Check the voltage on both the wires under the 3 states I posted below (KOEO, KOER (crank), KOER). The cranking state will work better if you disable fuel so it won't run, or simply remove and ground the coil wire on the distributor side.

Testing the white wire for ground requires the meter to go the other direction, with the positive lead on the power supply (battery) and probe with the negative lead. A system voltage reading while done this way will indicate a ground (good enough for the electronic circuit anyways) present on the circuit under test.

If it were working correctly, you would see 5V at the bypass (tan/blk) and a steady voltage on the white. The "steady voltage" would actually be a squarewave that is the ECM spark command. It probably averages around 1.5V at idle. If you were curious, you could use the "duty-cycle" mode on your meter to see the duty-cycle. That is how you would verify it isn't a steady 1.5V signal on the cheap.

There is more to it, and variations of what needs to be done and where. Start there.
Report back.

 

[Zuus]

Re: GT dieseling, code 42

I fixed it. Replaced the spark module in the distributor. (By the way, the 2 screws that hold it in are 7/32" head). $60 at the local parts store.
Thanks for all your help.
E