While The Symptoms Are Different, Electrical Diagnosis Remains The Same

By Randall Peterson, Diagnostician

peterson-randyRandy has been employed by Certified Transmission since 1992. He is an ASE and ATRA certified technician and works as our Gladstone, MO shop’s diagnostician.

There are periods of time in which several days may pass without any need to perform some in-depth testing, and even more seldom are the days when you have multiple opportunities to do so.  This week our shop had several cases that required just that, and each one presented a different type of system to test. Technology changes so fast, but there are many fundamentals that remain the same.  Electricity behaves the same way in all cases, thank goodness.

Our first test case was a 2000 Oldsmobile Alero 3.4L with a 4T45E transmission.  Code P0758 was set: 2-3 shift solenoid circuit; electrical.  The code was not a hard code, and I had to drive this vehicle a bit of a distance to get the code to set after clearing it.

Since this was a circuit-related code, I printed out the transmission electrical schematic and reviewed it.  From experience, I knew that this vehicle’s PCM grounds the solenoids on the control side, and power is supplied through the “Auto Trans” fuse in the under-hood junction block.  Keeping that information in mind, I gathered my tools and started the pinpoint process.  For this application I am using my Tech II, scope, low-amp probe, and a fuse-buddy with a 10A fuse.

First, I tested the fuse to make sure we had B+ on both sides of it (this was a redundant step; there would have been no codes if there were no power to the transmission. Needless to say, it tested okay).  Next, I checked for B+ at the transmission harness on Pin “E”.  Again, this tested okay.  At this point, I put a battery charger on the vehicle, because sometimes testing with the key on can run a battery low in short order and can create false readings.

The solenoid in question is an on/off solenoid and should read 0.4-0.6 amps (19-36 ohms) when commanded on.  I installed my fuse buddy in the fuse box, connected the amp clamp to the fuse buddy, fired up the scope and set up the Tech II to use the bi-directional commands.


I first tested the solenoid that was working to see if I had everything set up and displaying properly.  The 1-2 shift solenoid was working correctly, and the current ramp looked good:



Next, we needed to test the solenoid in question.  Since we were using bi-directional controls, the power feed is common to the shift solenoids so we didn’t have to move any leads to test the other solenoids.



Wow, look at the difference in the scope patterns.  There was definitely a problem there.  This is why using a scope will find a problem much faster than trying to perform a resistance test.  After letting the solenoid operate for a couple of minutes, the solenoid failed altogether:


With bi-directional controls and a scope, this style system is very quick and easy to check.  Performing the test on all the solenoids, except the EPC, are available at the click of a button.  I use this setup on all types of GM vehicles.

To wrap up this case, the owner of the vehicle was advised of the fix and authorized the replacement of the solenoid.


The next case is a 2006 Mazda 6S 3.0L with an AW6A-EL transmission.  Code P0979 was set; SSC circuit low current.  The diagnostic approach on this car is very different from the Alero, because the TCM is mounted directly on the transmission so there are no wires from the TCM to the transmission to back-probe or put an amp-clamp on.  The solenoid in question is a duty-cycle type solenoid, as opposed to the on/off solenoid in the Alero case.

After studying the electrical schematic and researching the set strategy for this code, I planned my diagnostic course of action.  I was testing without power (KOEO), so no need to hook up a battery charger this time.  To start, I needed to get a voltage reading from the battery so I had a reference voltage to do the math with.

Shift solenoids ‘C ‘, ‘D’, ‘E’, ‘F’, plus EPC and TCC on this vehicle are all duty-cycle type solenoids.  The measured resistance on these types of solenoids is generally on the low side, at 3-6 ohms.  Amperage, however, will be relatively high, so we don’t want a large amount of current running through it for a long period of time.

The TCM handles both the power and the ground to these solenoids.  Shift solenoid ‘A’ and ‘B’ are on/off solenoids, and therefore they will generally have a higher resistance (10-15 ohms).  In order to check these solenoids, the TCM has to be removed to access the case plug.



After locating a diagram of the pin outs on the case plug and the circuit schematic, I’m ready to start testing.  I resistance tested each of the solenoids and all were within specs.  Since the solenoids passed the resistance test I needed to see if they performed ok when powered.


These solenoids are duty cycle solenoids, so I don’t want to just hook power to them like I would an on/off solenoid.  I made a momentary switch that I connect with a 3A inline fuse to momentarily turn the solenoid on and off to get amperage readings.


The resistance readings on SSC, SSD, SSE, SSF, TCC and EPC were all 5.2-5.3 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.


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.  That way 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 #73, and shift solenoid ‘B’ is pin #1.  I will also quickly check the other solenoids while I’m there.

Pin #73 (SSA) to ground shows open:


Pin #1 (SSB) measured 14.8 ohms:


Applying a power supply and using the amp clamp I can double-check my findings. Pin #73 has no amperage going through the circuit:


Pin #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 okay.


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”.