No code diagnostics

July 1, 2015
From time to time a vehicle will come to our bays with drivability concerns, and there is no check engine light and no diagnostic trouble codes stores in memory. But just because the problem didn’t store a code doesn’t mean the problem can’t be found.

From time to time a vehicle will come to our bays with drivability concerns, and there is no check engine light and no diagnostic trouble codes (DTCs) stored in memory. An even worse situation is when the vehicle has no check engine light and the complaint is something that only happens intermittently. What do you do when something like this is sitting in your bay? Just because the problem didn’t store a code doesn’t mean the problem can’t be found.

1995 Buick LeSabre; 3.8 engine, Automatic transmission, 135,000 miles

Many times, scan data can be used to find a direction on driveability problems. This scan data is an example of using fuel trim data to find drivability issues.

When we think of drivability problems, we tend to think of an engine that doesn’t run properly, but driveability problems can also be an engine stall, a transmission that doesn’t shift properly or even a poor power complaint. My question to you is where do we start when these kinds of concerns show up in the shop?

Many in the business will tell you a good interview with the person who drives the vehicle is always the starting place, and I totally agree. On the other hand, there are times when this interview will get some of the information we need, but there is also a chance that the most important information will be lost in translation between the vehicle driver and the person doing the interview. Technicians do not always do a great job of handling these interviews since we tend to think and talk car, and most vehicle owners and drivers do not speak our language. I find that asking the correct questions — who, when, why, where and to what extent — will draw out the needed information. Keep it simple and speak in terms your customer can understand.

For the technicians accustomed only to OBD II, we have had the luxury of DTCs to gain a direction. For those of us who’ve been turning wrenches longer than OBD II has been around, we can remember when we didn’t have this luxury and had to actually think our way through each diagnostic process. I embrace the self-diagnostic capabilities of the electronic modules on the vehicles we see in our shops today, but sad to say, there are times when a fault that can cause a drivability problem will not code. When this happens, what do you do? Problems like this can and will test the grit of technicians. Sad to say, many times a vehicle with a no-code drivability problem will get backed up to the parts store and loaded up. If that doesn’t make the problem go away, the vehicle will get passed from shop to shop until 1.) the owner runs out of patience or money; 2.) trades the vehicle in for another, or 3.) takes the vehicle to a shop that has a great drivability technician .

Before we can start on a diagnostic journey, we need a direction. Many times, this direction is obtained from our interview, other times the direction comes from a test drive. No matter where it comes from, we must have a direction before we start using up our valuable diagnostic time.

Labscope capture of a relative compression test. I would use something like this if the engine had a misfire. The relative compression test done with a labscope is a very fast and accurate way to find a cylinder or cylinders with low compression. 

A labscope is great for testing an action/reaction of a component. In this case, the scope is capturing the action of the 3x and 18x CKP sensors, and is also capturing the reaction of the DREF and the ignition coil current. When the engine stalls, all 4 signals remain correct. The problem is not found here.

Labscope capture of the 18X CKP (blue channel) 3X CKP (red channel) DREF-fuel control signal between the ICM & PCM (green channel) Ignition coil current (yellow channel). The engine hiccupped during this capture. The scope data shows the engine speed slowed down, and the ignition primary current dropped a little, but none of the signal traces went away. There was no problem found with this trace. Time to move on.

Gaining direction
When I think of no-code driveability problems, I try to break them down into two different categories, mechanical problems or electronic problems. Mechanical problems could be something like an intermittently sticking valve or a camshaft timing problem. Electronic problems could be things like a misreporting mass airflow (MAF) sensor sensor or an intermittent electrical problem with an engine control module (ECM). Problems like these still require gaining a direction before the diagnostic process starts.

To get a diagnostic direction, I always start with a scan tool to check for any stored DTC. If no information is stored, then it’s time to move on to other things. Since I live and die by fuel trim information, I always take a look at engine data to see if there are any clues here. It is amazing how many driveability problems can be found with this data. Of course, to get fuel trim data, the vehicle needs to be taken out on the road for a test drive.

If the direction is not found with a scan tool, then it’s time to pull out a Digital Storage Oscilloscope (DSO) and start testing. But wait — poking around with a DSO scope can take you on a scope safari. The art of diagnostics is to get the problem to come to you. In the case of an engine drivability concern, using your DSO to run a relative compression test might be a great way to get this direction, or even a starting point to get this elusive problem to come to you.

In every diagnostic process I use three easy steps.

1.     Verify the customer’s concern

2.     Learn how the system works

3.     Apply the learned knowledge to the problem (do the testing)

Before you start any testing procedure, always ask yourself, “What do I expect to find with this test?” And “Why am I doing this test?” Don’t start testing things in hopes of finding something, and don’t blindly follow the troubleshooting flow charts every manufacturer has. Always use a testing plan and follow a direction with your testing.

An example
The vehicle is a 1995 Buick LeSabre equipped with a 3.8 liter V6 engine with 135,000 miles on the odometer. The vehicle owner concern is an intermittent engine stall. When the engine stalls, it can be restarted immediately. In the last year or so, several attempts have been made at fixing this stalling problem. A set of plugs and wires, three new ignition coils and a new Ignition Control Module were all installed with no fix for the problem.

I started by checking to see if any DTCs were stored. None were found, so the vehicle was taken on a short test drive. After driving the vehicle a short distance, the stall was experienced. This test drive only took two miles before the engine stalled as it fell to an idle while turning around for the return trip to the shop. While pulling the vehicle into the shop, the engine stalled two more times with all engine stalls happening with the engine at idle. These stalls felt like someone was turning the key off and smelled of something electrical, rather than mechanical or fuel related. Time to read up on the ignition system’s operation.

The scope leads have been moved around. Ignition control module supply voltage (blue channel) DREF-fuel control signal (red channel) fuel injector #6 current (green channel) primary ignition current (yellow channel)

The green trace, #6 fuel injector current, shows something that looks like the fuel injectors quit pulsing. I want to see the current from all six fuel injectors. Since the fuel injectors are pulsed only once every 720 degrees of engine rotation, too many things can happen between injector pulses to be accurate in this analysis. 

The scope leads were again moved. This time the blue channel is a current probe hooked to all six fuel injector control wires at the PCM. Green channel is the DREF signal from the ICM to the PCM. Red channel is the 3X CKP and yellow is primary ignition current.

When the engine stalled, the PCM quit pulsing the fuel injectors. The labscope waveforms show the PCM has all the needed inputs; 18X, 3X and DREF to fire the injectors. Time to pull the trigger on a new PCM. 

Using my service information system, the theory of operation for the 18X and 3X Crankshaft Position (CKP) sensors reads:

3X reference PCM (Powertrain Control Module) input

From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The PCM compares pulses on this circuit to any that are on the Reference Low circuit, ignoring any pulses that appear on both. The PCM also uses the pulses on this circuit to initiate injector pulses.

18X reference PCM input - The 18X reference signal is used to accurately control spark timing at low RPM and allow IC operation during crank. Below 1200 RPM, the PCM is monitoring the 18X reference signal and using it as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using the 3X reference signal to control spark timing.

Since the engine runs strong at cruise and acceleration, I have ruled out a fuel supply problem as the cause of the stall. One more thing to think about — since the stall happens at idle, as if the ignition key were turned off, I want to know if the stall is caused by a lack of spark or a lack of fuel. Knowing the theory of operation for the dual CKP sensors, it seems as if there is a problem with the 18X CKP sensor since the engine stalls when it comes back to idle.

This ignition system uses a Distributorless Ignition System (DIS) that is controlled by an Ignition Control Module (ICM) with three ignition coils piggybacked on top of the module. The ICM is controlled by a dual CKP sensor (3x and 18x) and a Camshaft Position (CMP) sensor. The 3x signal has three interrupters spaced 1, 20 & 30° apart. The 18X signal has 18 identical interrupters spaced equally around the 360° of the reluctor. The changing relationship between the 3x and 18x CKP signals allows the ICM to quickly identify the correct ignition coil to fire within the first 120º of crankshaft rotation and allows for quicker starting. If the 3x signal is lost while the engine is running, the engine will continue to run, and the fuel injection will continue to run in a sequential mode. Loss of the 3x signal will prevent the engine from starting. 

Most engine management systems need some sort of ignition feedback signal sent to the PCM to operate the fuel injectors. In the case of this GM system, the service information refers to this signal as the “fuel control” signal. In the days of the GM distributor systems, this signal was referred to as the distributor reference (DREF) signal. Without the DREF, the PCM will not pulse the fuel injectors.

When using a DSO to analyze a problem like this, we need to hook to the signals that support both the ignition system, and fuel injector operation.  My logic behind the test is that the two CKP signals are needed to operate the ignition coils. The DREF signal is needed by the PCM to pulse the fuel injectors, so the scope was set to watch both CKP sensors, ignition coil current and the DREF signal.

The engine was idled until it stalled and these waveforms were captured. No problems were found with the ignition, but there is some good information gathered. The ignition system is not the cause of the stall and neither is the ICM, since the coils are being fired. But wait, as the engine dies, the ignition coil current trace changes. Is this possibly the cause of the stall?

Knowing where to connect your scope starts with the circuit diagrams. In the case of the Buick, the PCM is inside the car and away from possible interference from other underhood electronics. We’ll connect to the circuits we need there.

One more try with the same data — a idling for about five minutes, the engine faltered for a second or two, then went back to idling smooth. The data was captured on the DSO. With this new data, all it has done is raise more questions.

Is the problem possibly a poor power supply to the ignition module?

DSO tip! When scoping electrical components and using current to analyze a problem, it is also important to use voltage along with the current. The next scope capture was taken with a voltage lead and a current probe hooked to the power supply to the ICM. This will prove if the voltage to the ICM is the problem. I also hooked another current probe around the #4 fuel injector to see if the cause of the stall is the fuel injectors being shut off.

The fuel injector is not a good place to get this information. The engine turns two revolutions between each fuel injector pulse, and too many things can happen in two engine revolutions. There is also a lot of electrical noise under the hood, and a current probe set at a low current is capable of picking up that noise. The PCM is mounted under the glove box and is easily accessed with the removal of one small panel. The wiring is even hanging down where all six fuel injector control wires can be grabbed with the current probe. This puts the current probe inside the vehicle where the air is clear of electrical noise. This also allows monitoring the current from all six fuel injectors.

With this information on the scope screen, it is now easy to see exactly what is happening. The ignition system is working well, the ICM is sending the DREF signal back to the PCM so the fuel injectors can be pulsed at the correct time, but the PCM has stopped pulsing the injectors. In this case, the PCM needs to be replaced. The PCM was tap tested and twisted, but no problems were found with cracked solder joints or crack circuit boards.

This problem was on an old archaic Buick, but these diagnostic principles can be applied to most any type of ignition/fuel system built today. Understand how the system works, get a diagnostic direction, then do the proper testing.

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