Over the years, I’ve learned a few ways to make quick work of the tedious job of tracking down electrical faults. Anytime we see a solenoid circuit code, there’s a list of possible causes. The computer, connectors, terminals in the connectors, wire harness, and the solenoid make up that list. First, we determine the possible causes, and then from that point on, it’s a process of elimination. One of my early mistakes was that I used to start with a resistance test. Doing this sometimes gave me the false impression that everything was good in the circuit. I would do the test, and if the resistance was correct, I thought the circuit was in good condition. So, I would condemn the computer and then wonder why a new computer didn’t fix the problem. I found out the hard way that a resistance test would show problems with damage to solder joints, the wires, or even loose terminals.
My next lesson was load testing a circuit using an amp meter and the vehicle’s battery. I was taught to disconnect the battery and the computer. Supply power from the battery to the solenoids power wire in the computer connector on the wire harness, then use an amp meter between the ground wire for the solenoid at the computer connector and the battery. This is probably the best way to test circuit integrity, and it has yet to let me down, too.
A quick test for the computer is to take a good solenoid and wire it directly into the connector, bypassing the wire harness and solenoid in the vehicle. If the code comes back right away, there is a good chance you have a bad computer. If the code goes away, it’s outside the computer.
We recently had a 6R140 with a shift solenoid A (SSA) circuit code come in that had been to another shop. They replaced the solenoid, the internal wire harness and were ready to condemn the computer. After finding out the price of the computer, the customer decided to get a second opinion and brought it to our shop. They told us what that previous shop had checked the external wire harness and the other work they did and recommended a computer. They requested for us to check the vehicle to see if we came to the same conclusion.
Our initial impression was that it was probably a computer issue, so we figured we’d wire a solenoid directly into the computer to see if the code reset right away. We disconnected the harness from the computer to inspect for water intrusion, which is very common on these (Figure 1). We removed the terminal from the connecter (Figure 2) and inserted a test wire with an OE terminal to tie into a solenoid, then connect the other side to battery power.
After installing the solenoid, we cleared the codes, restarted the vehicle, and the code did not return. So now, along with the internal wire harness and solenoid, the computer was also off the list of possibilities, leaving us with external wire harness and terminals as possible causes.
Next, we load tested the circuit, providing battery power to pin 32 and grounding the solenoid through an amp meter at pin 42 (Figure 3). The solenoid should be 5.5 ohms resistance, and the battery voltage was 12.5. Ohms law states that volts divided by ohms equal the amperage the circuit should carry. This circuit should show about 2.27 amps when loaded. Ours was showing less than half an amp. This meant the circuit could not carry the proper load and indicated a loose connection or damaged wire. We repeated the test at the case connector to verify the solenoid and internal wire harness were good; it passed. We checked for corrosion, pin damage, and pin fit throughout the wire harness; everything appeared good. There was only one thing left. There had to be damage to the wire harness.
After removing the harness from the vehicle, we found a broken retainer for the wire loom. This allowed the loom to rub against a pinch weld on the vehicle’s body. A few wires had the insulation rubbed off, and the SSA wire was almost cut in two. Since the wire was damaged and not a complete break, this would have passed a resistance test but failed a load test. We repaired the wire and sealed everything up with some shrink tubing. We reconnected everything, cleared our codes, and drove the vehicle to see if any returned. After a few runs around the block and no codes returning, we confirmed the fix and returned the vehicle to the customer, who was very happy to spend a couple hundred instead of a couple thousand.
A majority of the time, when wire harness damage sets a circuit code, it will be smashed or cut, leaving an open circuit. A tool I’ve had good luck with tracking down problems like this is FF310 short finder (Figure 4). It uses a transmitter you connect to one end of the wire to send a radio signal and a receiver to track the signal. When there’s no more signal showing on the wire, you have located the break. Wire harness damage can often be one of the most difficult things to track down. Minor damage can lead to major headaches with performance issues, circuit codes, and intermittent problems. However, just like diagnosing hydraulic problems in a transmission, with time and experience, it gets easier.
