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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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There are periods of time in which several days might 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.

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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.

Because 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 OK.)  Next, I checked for B+ at the transmission harness on Pin E.  Again, this tested OK.  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.

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