Street Smart - December - 2016

A Logical Approach to Solenoid Circuit Testing

A common call to the ATRA HotLine has to do with solenoid circuit problems. Most of you have been dealing with these types of problem for years. Many of you have the answers seconds after you check for codes.

If you’re one of those technicians, this article may not be for you. But if you’re a technician who appreciates a new look at a common problem, never dealt with solenoid codes, or are new to the industry, keep reading.

For this scenario, we’re going to use a 2003 Hummer H2 6.0 engine with a 4L60-E; the MIL is flashing and the transmission is in failsafe.

You check for codes using a scan tool and find DTC P0758: 2-3 shift solenoid B electrical problem. So you put the Hummer on the lift and drop the pan.

The DTC says that shift solenoid B has an electrical problem, so you install a new shift solenoid B, a new filter, and bolt the pan back up.

After you fill the transmission, you’re ready for the road test. But before you can back out of the shop, the MIL is flashing again. You reconnect your scan tool only to discover the same 2-3 solenoid trouble code. How can that be? It’s a new solenoid!

Let’s look at that code definition again: 2-3 shift solenoid B electrical problem.

One of the features of the ATRA HotLine is that we have a wide selection of factory manuals. In the factory manual, DTC P0758 reads 2-3 shift solenoid circuit electrical. Ah ha! Almost the same, except for one small word: circuit.

That brings a whole new picture into play; or should I say circuit?

Let’s look at the 2-3 shift solenoid circuit (figure 1). Keep in mind that, with most solenoid electrical codes, any problems in the circuit can cause the code. This includes power, ground, wiring, connections… and even the computer.

With all these possible problems, the question of where to start comes up. Remember, it’s always easier to diagnose one thing at a time. So, let’s break down each part of the circuit.

IMPORTANT: Never use a testlight to diagnose electrical problems on computer circuits. Testlights will light up with as little as eight volts; some even less. That’s well below the voltage needed for a shift solenoid to function properly. Always use a voltmeter. 

POWER SIDE TESTING

Power for this circuit comes from the ignition switch through the ING 0 fuse. This is a 10-amp fuse in the I/P fuse block, and it provides GEARS December 2016 15 voltage to the case connector at the transmission. From there it feeds the TCC, TCC PWM, 1-2, 2-3, and 3-2 shift solenoids.

Since the power circuit feeds five solenoids, you’d think a power feed problem should set DTCs for all of them. But in many computers, as soon as one solenoid code sets, the computer stops checking for other circuit problems. So never assume that the power circuit is okay just because there’s only one DTC in memory.

One thing to keep in mind is that the power at the ING fuse comes from the ignition switch. Those of you who’ve been doing this for some time know that bad contacts in the ignition switch can cause an intermittent power problem.

The place to start is the fusebox.

With the key on, check for system voltage on both sides of ING 0 fuse. If there’s no voltage on either side of the fuse, look for a bad ignition switch or a problem in the wiring between the ignition switch and the fuse.

Because an ignition switch can cause intermittent problems, it’s a good idea to wiggle the key around while you’re checking for voltage.

Let’s move on to the transmission connector (figure 2). Pin E receives voltage from the ING 0 fuse and should have system voltage with the key on. Inspect the connector for corrosion, loose pins, and oil intrusion. Monitor voltage at pin E while the code sets. If voltage is good, you can eliminate a power-side problem.

Another way to eliminate a suspected power problem is to bypass the original power circuit; here’s how:

  • Tap into the original wire that feeds pin E.
  • Jump this wire to battery positive through a 10-amp fuse (figure 3).

If the problem goes away, you’ll need to test the power circuit for loose connections, breaks in the wire, or additional resistance.

If the problem is still there, remove the bypass and check the ground side of the circuit.

IMPORTANT: When you tap into pin E with a jumper wire, you’re basically hot-wiring the vehicle. You won’t be able to shut the engine off until you disconnect the jumper wire.

GROUND SIDE TESTING

The computer supplies ground to the 2-3 solenoid through pin B in the case connector (figure 1). One way to diagnose the ground side is to disconnect the PCM (computer), turn the key on, and check for voltage at pin 47 of the C2 connector. Current travels from the ignition switch, through each solenoid, and to the computer, seeking ground. So you should see system voltage on pin 47 of the C2 connector.

This is where a lot of the problems show up, especially after the transmission has been out of the vehicle. What happens is the installer pinches the wiring harness between the bellhousing and the engine block, crimping the wires. The wires break, causing the circuit to open, or even short against the block.

During the ground check, this shows up as no voltage on the ground side of the circuit. That’s what happened here.

The fix was simple: The technician loosened the transmission and slid the wiring out of the bellhousing. Then he repaired the broken wires and the transmission worked like new: No more code P0758.

But what if that’s not the problem? What if the current measurement is correct?

In that case, since you’ve verified the circuit is complete, your next step would be to ground pin 47 through an ammeter to identify any problems in the circuit.

Solenoid resistance is 20 to 30 ohms and system voltage with the engine off should be about 12.6 volts, so a good circuit should develop about 0.63 to 0.42 amps of current flow (V/R=A).

If the circuit current flow is incorrect, it means something’s wrong in the circuit: power, ground, or the solenoid itself. Assuming the circuit passed the other checks, you’re probably looking at a resistance problem: either too much or not enough.

If current flow is correct, it means the circuit is good and you may have a computer problem. But before you spend hundreds of dollars replacing the computer, there are a few final tests you should perform:

Before you replace any computer, always check the circuit in question; we’ve already done that for this example.

You should also check all power and ground circuits to the computer. Use a schematic to identify the power and ground circuits in the computer connectors.

Figure 4 shows a view of both computer connectors with the power and ground pins labeled for this vehicle.

Check these circuits by backprobing the computer pins with the connector plugged into the computer and the key on.

All power inputs should equal system voltage and all grounds should have less than 0.10 volts.

BYPASS TEST

If everything else checks out okay, there’s one more test you should per-form before condemning the computer: The Bypass Test.

  • Cut the wire at the computer that controls the circuit you’re testing, leaving 1 to 2 inches of wire on the computer connector. In this example, it should be pin 47 of the C2 connector.
  • Connect the wire from the computer connector directly to one of the terminals on a known good 2-3 shift solenoid.
  • Run another wire from the positive post of the battery through a 10-amp fuse to the other terminal on the 2-3 solenoid (figure 5).

By doing this, you’ve created a test circuit for the computer to control. If the code still sets with this circuit in place, either you have a bad computer or there’s a problem with computer grounds or power inputs. Since you should have already checked the powers and grounds, it’s probably the computer.

As you can see, there are many possibilities in the solenoid circuit that can set a 2-3 solenoid electrical trouble code, and there’s more than one way to test the circuit. You can test a wide variety of solenoid circuit problems using these same procedures.

Using logic and a proper wiring diagram, you, too, can tackle solenoid circuit electrical codes with this simple bypass test, and take the guesswork out of your diagnosis. And that’s not just smart, that’s street smart.