Several points were brought up in a seminar presented at the Automechanika Chicago show in April about the frustrations that come with diagnosing transmission concerns. To determine if the cause of a malfunction is related to an engine management problem, an ABS issue or an actual transmission malfunction can be at times quite challenging. If the malfunction has been determined to be a transmission problem, the next step typically is to decide if it is a minor or major repair. Of course it is always nice when a cause to a problem has been discovered and is passed along to others saving them the grief it took to resolve it.
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Through many years of experience diagnosing transmission concerns in shops and on ATSG’s technical help line, a general diagnostic approach has been developed to find that silver bullet as quickly as possible. The list begins with “Information,” meaning we look first to see if there are any bulletins or articles already covering the subject. The available diagnostic equipment as well as the ability to use the equipment is considered if handling the problem over the phone.
Understanding the codes received, checking power and grounds, doing a cursory view of the wiring and connectors while looking for cross connect possibilities are on this list. Checking into the four main critical PCM/TCM inputs (Engine load, Speed Sensors, Gear Shift Position Switches or Sensor and Temperatures) as well as how the signal is generated and delivered to the controlling computer and computer strategies (ABS, torque reduction, sport and economy modes, adaptations, manual tap shift controls, start/stop technology or neutral controls, etc.), transmission misapplication, transmission fluid and aftermarket programming, products and parts round out the list. Having this comprehensive overview goes a long way in determining which avenue one needs to take based on the problem at hand to get to the cause as quickly as possible. Of course it doesn’t completely eliminate every possible headache, but it sure can eliminate many of them.
One reason we are seeing frustration levels increase diagnosing transmission concerns are programming issues. The complexity of today’s vehicles brings a whole new dimension of diagnostics to the table. Shift strategies are becoming reliant on a multitude of inputs that if they become skewed, affect transmission operation. Put that together with a program not written well and you have malfunctions that cannot be totally corrected. Sometimes malfunctions are discovered and corrected through reprogramming, which is why the first step in our general diagnostic list is information. Nothing can be more frustrating than trying to fix a problem that can only be achieved via reprogramming.
A simple example of this was a call we took from a shop that decided to put its toe in the water to fix a CVT transmission for the first time. It was a 2007 Nissan Altima that arrived to the repair facility with a vehicle speed sensor code P1722. The shop had diagnosed it down to a problem with the tone ring and wheel bearing rather quickly. After repairs the code was cleared and they drove the vehicle. All was perfect for the exception of an approximate five-second delay into gear, both forward and reverse. Once engaged, the transmission drove flawlessly (as far as a CVT is concerned).
This particular CVT utilizes a torque converter allowing a forward or reverse clutch to apply like any other conventional transmission. This means all that is required to engage the clutch is to move the manual valve into the Drive or Reverse position. A select control valve in the valve body would then control the engagement of the clutch. Not only is the intention of the select control valve's function to provide a nice comfortable garage shift into gear, it primarily is used to prevent a damaging jolt to the drive pulley assembly.