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Transmission issues: Same, but different

These three different models came in with problems that appear to be similar, but are quite different.
Tuesday, July 29, 2014 - 07:00
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The resistance readings on SSC, SSD, SSE, SSF, TCC and EPC were all 5.2 to 5.3 ohms, so I’m going to be looking for somewhere around 2.4 amps per Ohm’s Law when I cycle the solenoid.  My actual readings were right at 2.0 amps.  All of the solenoids were the same and consistently had the same reading each time I ran through the test.  At this point I was confident that the solenoids were not the problem and recommended that the TCM be replaced.

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The Last Case
The last vehicle to complete our story was a 2003 Ford Focus 2.0L with a 4F27E transmission.  As with the previous cases, this vehicle also had a solenoid code: P0750 Shift Solenoid A: electrical fault.  This system is similar to the Alero’s, in that it has a remote PCM that controls the solenoid functions.  However, unlike GM, the Ford PCM powers the solenoid and the solenoid itself is self-grounding, according to the schematic.  I verified the code was present by performing a KOEO self test with the scan tool.  This hard-fault tells me there is a problem right now.

The first place I go to test is at the PCM, which is located under the dash on the right side on the A-pillar.  Whenever possible, I like to start my test at or near the computer so I can test the complete circuit from the source.  I run a jumper harness (10 feet of speaker wire with alligator clips on the ends) from the battery to the area I’m working.  For safety, I put an inline fuse on the B+ wire.  I disconnect the PCM and find the pin terminals I want to test.  In this case, shift solenoid A is pin No. 73, and shift solenoid B is pin No. 1.  I will also quickly check the other solenoids while I’m there.

Pin No. 73 (SSA) to ground shows open. Pin No. 1 (SSB) measured 14.8 ohms. Applying a power supply and using the amp clamp I can double-check my findings. Pin No. 73 has no amperage going through the circuit.  Pin No. 1 shows 0.8 amps traveling through the circuit. That’s a good working circuit.

I had verified there is a problem with shift solenoid A circuit, so I went to the transmission connector to do some verification tests.  The first to check are the grounds at the PCM connector.  It’s very easy to do while the PCM connector is disconnected.  I disconnected the transmission connector, attached the jumper wires to the shift solenoid A and B wires at the transmission connector, and I wanted to confirm two things: that there are no opens in the harness from the transmission to the PCM, and that the harness is not shorted to ground.  The harness checked out OK.

Some people might think that the last step I performed (checking for short to ground in the harness) is a little bit of overkill.  It was a step that I normally didn’t do, until a short to ground in a harness ate my lunch.  Now it’s part of the practice, and easy to do when I’m there.

I tested the solenoids at the transmission connector, and had the same readings as what previous in-car testing revealed: shift solenoid A was bad.  Time to advise the vehicle owner and get the repairs authorized.

In each of these cases, the battery and charging systems were tested before the advanced level diagnostics began.  Always make sure the electrical systems are good before testing.  Also, I use the actual battery ground for all my testing.

There are many types of systems and new ones coming out all the time.  Familiarize yourself with the system at hand.  Use the set-ups and tools suitable for each of the different kinds.  As a wiser diagnostician once told me, “if you can’t prove that it’s bad, don’t replace it”.   

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