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That was too easy

Tuesday, March 19, 2013 - 11:47
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I recently diagnosed the cause to a vehicle’s electrical problem in a relatively short order of time. I am not patting myself on the back as to how quickly I nailed it, but rather how easy it was to diagnose it because of the vehicle’s configuration. It was so easy it was pleasant.

The vehicle was a 2007 Ford Explorer 4.0L (VIN E) with a 5R55S RWD transmission. It arrived to the shop with a customer complaint of a sudden no up shift slipping condition. Upon the initial road test, I quickly determined that it was in a no shift failsafe condition. The higher gear launch in the Drive position (fourth gear only) felt like it was slipping to the owner of the vehicle. After clarifying the customer’s complaint, I retrieved the codes.

It was loaded with solenoid electrical fault codes. This reminded me of GM vehicles having shift and torque converter clutch solenoid electrical codes because of a faulty ignition switch. These faulty switches would cut power to the solenoids, which the computer would pick up as an electrical fault on the controlling ground side of the circuits.

This prompted me to pull a wiring diagram to see if this Explorer might be experiencing a similar condition. When the PCM commands the Power Relay to close via an ignition signal, the relay supplies power to various circuits in addition to the transmission solenoids through 15-amp fuse number 38.

Clearly the possibility of a detective ignition switch can be ruled out, as an inoperative PCM power relay would cause several other problems in addition to these solenoid codes and there were none. But a blown fuse No. 38 becomes a likely candidate causing a loss of power to the transmission solenoids.

As a reminder, I have only road tested the vehicle, pulled codes and consulted a wiring diagram at this point, nothing really hard or time consuming. But here is where the actual diagnostics became easy and enjoyable due to the vehicles configuration. The fuse box and PCM are conveniently and readily accessible under the hood. With the ignition on, I quickly tested both legs of fuse 38 for power. To my surprise, I had good KOEO voltage on both sides of the fuse, so I pulled it out to inspect the condition of the fuse blades and terminals. They looked good also.

Honestly, at this point I would have hoisted the vehicle to take a look at the transmission case connector, but a lift wasn’t available at the time. So while waiting for one, I thought to myself that because the PCM and fuse are so easily accessible, I would make some additional solenoid circuit checks.

Another pleasant aspect of this vehicle in terms of doing some transmission diagnostics is that not only is the PCM easily accessible, but also the upper most connector is the C175T connector where all the solenoid ground wires are located. There is no need to lie on your neck trying to probe PCM wires deep under the dash with this vehicle. And as with most connectors, the terminal numbers conveniently are embossed into the connectors for circuit identification in conjunction with a wiring diagram.

Because I already pulled the fuse out, I located the circuit from the fuse leg that feeds voltage to the solenoid body, turned the ignition off and carefully inserted a test lead. I then modified a paper clip end to be inserted into the PCM connector and started doing a resistance check of each solenoid. Pressure Control Solenoid A (PCA) at terminal 11was the first solenoid I checked. The range for this solenoid is 3.3 to 7.5 ohms. Once again, to my surprise, I was within range with 6 ohms (Figure 12). The same occurred with checking all of the other solenoids with the exception of the TCC solenoid. This solenoid has a range of 9 to 16 ohms, yet this one measured 0.513K ohms.

I then poked the fuse back in, turned the ignition to ON to see if voltage was present at the PCM connector for PCA and there was. In fact, all of the solenoid ground circuits showed similar voltage readings. With these results, I still am not quite seeing what is causing all the solenoid codes yet. 

Because a lift had yet to open, I decided to do some amp checks. This was easy enough to do, as it only required changing the meter setting to amps and moving the positive lead to the appropriate jack. I saw unusual results when I performed this test procedure. Amp readings for all the solenoids fluctuated in and out of range, sSome worse than others. I managed to capture Pressure Control Solenoid A as it was fluctuating as low as 1 amp.  No doubt something is very wrong.

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