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Gear ratio error codes slipping away

While DTC signal trouble, they do give you a good deal of information on your problem-solving journey.
Friday, August 29, 2014 - 07:00
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Diagnostic Trouble Codes are no doubt quite helpful when diagnosing a malfunctioning vehicle. Sometimes, when the scan tool reports a never ending list of trouble codes like the ones seen from a 2005 Jaguar S-Type (X200) V6-3.0L in Figures 1 through 4, it might be a bit overwhelming to know where to start. In scenarios like this, my first approach is to know the history of the vehicle, if possible.

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I do a cursory view of the electrical system, looking for fatigued wiring harnesses, pinched wires or wires melted on the exhaust or manifold. I inspect connectors looking for water intrusion and then conduct power and grounds tests. I repair any system voltage concerns before proceeding any further. Hopefully by this time I’ve knocked out a few problems, but if not, the sleeves roll up and step by step diagnostics are performed based on which codes I believe might be causing other codes to set.


Through the years, it is interesting to see how the types of codes have developed with the advancement of technology and the variety of transmission types we now areseeing. One such example is gear ratio codes being replaced with solenoid performance codes. As the title of the article goes, gear ratio errors are “slipping” away.


For years, we would see a generic ratio code P0730 for any gear error ratio fault and/or a P0731 for a first gear fault, P0732 for second, P0733 for third, P0734 for fourth and P0735 for fifth. P0736 was for reverse and P0737 would be a generic TCM Engine Speed Output Circuit 

fault. Now, when you look up a code list for a six speed transmission, the generic P0730 series codes are no longer there. You might see a code list where it jumps from P0723 to P0741, no P0730 series codes listed at all.


Let’s consider GM’s front wheel drive 6-speed transmissions. Torque rating wise, they come as small as a 6T30 (1.2L) to as high as a 6T75 (3.9L). You will not find gear ratio error codes as much as you will find solenoid performance codes instead. Take for example a 6T30/40/45/50 series transmission (GF6). These transmissions have the computer mounted on the valve body inside the transmission (Figure 5). Thecombination of the TCM, solenoids and valve body together as an assembly has been called a Mechatronic unit with many European vehicles. GM uses what Delphi calls the Transmission Electro-Hydraulic Control Module or TEHCM (Q8) for short (Figure 6). This TCM/Solenoid body TEHCM assembly is mounted onto the valve body.

There are seven solenoids mounted into the TEHCM housing used to control shift feel and shift scheduling for six forward gears, converter clutch apply and one reverse gear. There is a line pressure control solenoid (Line PCS), a TCC PCS, PCS’s 2, 3, 4 and 5 and one On/Off shift solenoid.

Typically, when a TCM/PCM would command a specific gear, it would observe the input and output speed of the transmission with which it could calculate the current gear ratio. Once calculated it should match the commanded gear and if it did not, a gear ratio error code would set for that specific gear.

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