In part 1 of this series, we explored how to use fuel trim to help determine whether a problem is in the engine or transmission. In part 2 we focused on engine breathing and associated engine implications.
In part 3 we’ll “shift gears” and examine a testing technique that’s useful for both engine and transmission issues: solenoid testing.
Solenoids, regardless of what automotive system they’re used in, share some common testing techniques. Of course you can listen for a “click” when the solenoid turns on and off. You can also measure their resistance if you have its specifications.
Each of these tests only provides minimal information to analyze. The resistance test, for example, is a static test that can only indicate whether a solenoid is definitely bad. If it passes, you still don’t know if the solenoid is good. A much better test would be to energize the solenoid and view voltage and current on an oscilloscope.
That being said, let’s explore a scope technique you may already be D4 (0/D) N-P ON (Closed) ON (Closed) using for transmission solenoids that also applies to fuel injectors and other engine-related solenoids. We’ll start by analyzing a shift solenoid in a 2004 Nissan Maxima that has a late, and sometimes harsh, 1-2 shift.
The first step is to check the solenoid chart (figure 1) to see which solenoid changes state to shift the transmission into 2nd gear. In this application, shift solenoid A is energized in first gear and de-energizes to shift the transmission into 2nd. So we’ll focus on shift solenoid A.
The second step is to use the wiring diagram (figure 2) and make the proper scope connections. To check a solenoid, connect your scope lead between the load and the switch. In this case the load is shift solenoid A and the switch Is In the transmission control module (TCM). You can probe for voltage anywhere on the red wire with the yellow stripe.
To measure current, clamp a low current probe around any wire in the shift solenoid A circuit. Because current is the same everywhere in the circuit, It doesn’t matter where you connect the clamp. On this vehicle, we made both connections at the TCM connector in the passenger side kickpanel.
The scope capture (figure 3) shows voltage as the red trace at the top of the screen. Because this solenoid Is power-side switched, the Image may appear upside down compared to other solenoids. This is completely normal and you can identify the same information as if it were ground-side switched.
Point A shows the transistor in the TCM turning on. The corners should be nice and clean. This indicates the switch or transistor is turning on as designed.
Point B shows the transistor turning off; the signal corners should also be clean. In addition you can see the inductive spike at point C. A solenoid builds a magnetic field when energized. When current in the solenoid turns off, the magnetic field collapses and induces a high voltage, just like an ignition coil. The presence of a strong inductive spike indicates that a magnetic field did build while the solenoid was energized.
The scope capture also shows solenoid current represented by the blue trace at the bottom of the image. This also shows when the transistor turns on and off. But the current capture provides some additional information that the voltage capture doesn’t.
For example, look at where the current peaks (point D). When the capture levels out, current is 2 amps. Now you can use Ohm’s Law to calculate solenoid circuit resistance accurately while the solenoid is actually doing its job.
This is a great dynamic test, as opposed to the inferior static test of measuring resistance with an ohmmeter. Using Ohm’s Law, you can use peak current and system voltage to calculate resistance while the solenoid was loaded.
An additional advantage to observing solenoid current is a mechanical indicator. By zooming in where the transistor turns on (figure 4), you can see the current ramp up, with a dip near the top (point E). This dip is known as the pintle bump.
Sometimes you can see the pintle bump in the voltage waveform, just after the transistor turns off, when the inductive spike bleeds back to nothing and the solenoid closes. The current capture is a better place to look for the pintle bump because it’s more obvious and you don’t need both to be visible. If the solenoid opens repeatedly, it must be closing, and vise versa.
In our Maxima, we now know that the solenoid, its wiring, and the TCM are good electrically and the solenoid is operating mechanically. By process of elimination the late shift has to be either a plugged solenoid (that can’t be determined with a scope) or an internal transmission problem.
These principles apply to engine-related solenoids, too. Figure 5 is a scope capture of a saturated driver fuel injector. To measure voltage (blue trace), the same rule applies: connect between the load and the switch.
Most fuel injectors are ground-side switched, so the voltage starts at system voltage and pulls down to zero when the transistor turns on. When the transistor turns off, you again see the inductive spike. In this example the pintle bump (point F) appears in the voltage capture when the pintle actually closes.
You measure injector current (red trace) the same way, too. This allows you to calculate resistance and see the pintle bumps. But, since most vehicles today are sequentially injected, the current check provides additional information.
Because you can connect a current probe anywhere in the circuit, and usually all of the injectors receive power from one or two fuses, you have an easy access point to all of the injectors without having to probe each one. This can be a major advantage when injectors are nearly impossible to reach without some disassembly.
Figure 6 shows a voltage and current capture from a 4-cylinder Kia with a misfire and a P0303 misfire DTC. We connected the scope to the cylinder number one injector (red trace). The engine firing order is 1-3-4-2.
The blue trace is the current from all four fuel injectors, captured by replacing the injector fuse with a fused jumper wire. If you use injector number one voltage as your sync and plug in the engine firing order, you can see all of the injectors on the display at once.
For reference, point G is the number one injector voltage and point H is number one injector current. Note how voltage and current align with each other on the time scale, from left to right. In this example, notice there’s no current flowing through injector number three.
Zooming in on the other injectors would show pintle bumps and, even without zooming, injector resistance. In this case, a broken fuel injector driver wire caused the misfire.
Regardless of what solenoid you’re testing, engine or transmission, the same basic electrical laws apply. Whether you’re a seasoned transmission technician trying to expand into the realm of engine diagnostics, or a longtime driveability technician expanding into transmission diagnosis, you still play by the same rules and can use the same diagnostic techniques.