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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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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:

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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.

A Messy Mazda
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 and 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 to 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 generally will have a higher resistance (10 to 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.

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