Is it the right transmission?

Jan. 1, 2020
We were faced with transmissions having more than three speeds, and torque converters that now had a clutch that applied to lower engine rpms at highway speeds.
Those of you who have been in the automotive and/or transmission repair industry for the past 20 to 30 years most likely can remember the good, old, bread and butter transmissions of yesterday. These were units such as the TH350, TH400, C4, C6, Chrysler Torqueflites and so on. Back then, when it came to swapping out one of these units, you could mix and match just about anything as long as it had the same shift lever, speedometer gear tooth count, same length extension housing, etc.

These were times when you could have an older model Chevy Impala and a Chevy two-wheel drive pickup in the shop and there were no issues with using the same part number transmission for either application. Yes, there were probably some slight differences in the governor weight and or shift valve springs in the valve body, but for the most part, it still would work with a little tweaking here and there.

This was the same for most Chrysler rear wheel drive (RWD) 3 speed transmissions and Ford C4 and C6 units. The interchange on these units was fairly straight forward, and if you didn’t have one with the correct lever you would simply swap it over from the core. As long as the transmission had the same bellhousing bolt pattern, you usually were good to go.

As the years went by, changes took place in the automotive industry that was driven in part by the need to meet new EPA and fuel economy regulations. With the growing challenges automakers faced with the new rules, the transmission industry was forced to take a big turn. We were faced with transmissions having more than three speeds, and torque converters that now had a clutch that applied to lower engine rpms at highway speeds. This was a whole new experience, and frankly, it scared the heck out of the repair industry. The good-old days of interchanging and swapping transmissions between cars was coming to an abrupt end.

This new technology, along with the introduction of computer controls and sensors, brought about the need for transmissions to be more vehicle-specific. This brings me to the point of this article: When it comes to rebuilding or installing a transmission, it is imperative that the correct parts are being used to rebuild the unit, and also that the correct unit is being installed in the vehicle.

In most cases the differences between the units are very subtle, but when the wrong unit is installed into a vehicle, it can and will cause a host of problems that can be a nightmare to diagnose.

The most common problem we see with having an incorrect unit installed usually has to do with ending up with the wrong gear ratio. The computers in today’s vehicles are programmed to know what the specific gear ratio of a transmission is supposed to be, and if the wrong ratio is installed, you typically will see a gear ratio trouble code set during the post-install road test.

A Real World Example You are working on a 2002 Chevy Impala 3.8L equipped with a 4T65E transmission. A reman replacement unit has been installed. During the initial road test, the check engine light comes on and the transmission starts shifting very harshly, all within the first mile.

The scan tool shows a code P0730, which basically is an overall gear ratio code. The problem here was that the tag ID on the core unit was never checked. A 2002 Impala can have two different final drive ratios: tag ID 2LBB has a 3.05 final drive, and a 2LCB has a 3.29 final drive (see Figure 1). The subtle difference in the final drive ratio is enough to set a gear ratio error.

You also will need to be careful if the vehicle has had the transmission replaced at some point in the past. We have seen salvage yard units installed that have the wrong tag ID for the application, and if you order a replacement unit based on that incorrect tag ID, it can send you on a wild goose chase to find the problem. This scenario will hold true with most GM late-model vehicles with the 4T65E transmission.

Another not-so-obvious gear ratio problem comes with installing a transmission that has the wrong final drive ratio into a Subaru with the 4EAT transmission. This one can be a little harder to diagnose since the transmission will seem to operate normally. After driving a short distance, the AWD (4x4) light on the dash will start flashing. A code check with a conventional, non-OEM scan tool reveals no gear ratio codes.

The problem here can also be a case of mistaken identity. We have seen identical Subaru vehicles that have the same transmission tag ID on the unit, but having different gear final drive ratios. In this case, we had a tag ID of TZ1B5LWWAA, but found that one had a gear ratio of 4.11 and the other was 4.44. Therefore, even though you are armed with tag ID info, it may not 100 percent guarantee that you will be able to identify the actual gear ratio. If you any doubts about your application, you may want to consult with your local Subaru dealer.

More Mistaken Identity
In this next case of mistaken identity, we are working a late-model Ford truck with a 4R100 transmission. This transmission comes in two different variations: one with a PTO provision, and the other without. These units are not interchangeable, and when installed into the wrong application, will cause a host of shift concerns in addition to setting turbine speed sensor codes.

The PTO style 4R100 comes equipped with a PTO drive gear attached to the coast clutch drum, and also uses a different turbine speed sensor (see Figure 2). All PTO type units will also have a PTO cover on the driver side of the transmission (see Figure 3).

Although your vehicle might not have an actual PTO bolted to the side of the transmission, it may still have a PTO cover. This unit is internally built with PTO type coast clutch drum and gear. The problem usually starts when you try to install a non-PTO unit (will have no cover on side of transmission) into a vehicle that originally had a cover or actual PTO bolted to the transmission. As I mentioned earlier, the two input speed sensors are not only different because of their physical length; they are also different because of the resistance (ohm value) between the two. The computer will recognize this as a bad signal, which sets a turbine speed signal code. Therefore, you will want to be careful when replacing one of these units.

This really is only the tip of the iceberg when it comes to installing an incorrect transmission into one of these newer vehicles. Although the most common problems we see have to do with incorrect gear ratios, etc., there are multitudes of other things that can cause you grief when the wrong unit is installed. One additional example can be a unit having different electrical solenoids with different ohm readings based on a date range split in a model year. These and similar differences can send you looking for an electrical problem that exists only because of the wrong unit is installed.  

Transmissions are vehicle-specific for many reasons, far-beyond the examples we’ve given. Words cannot express the true importance of having the correct VIN and transmission tag ID information when ordering parts, and/or purchasing a replacement transmission for the vehicle you are working on today. Nobody wants to chase a mysterious ghost. 

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About the Author

Jody Carnahan | Contributor

Jody Carnahan is a warranty technical adviser for Certified Transmission, wher he has worked for more than 25 years. He has seen and handled a lot of situations during this time, and has a wealth of knowledge and experience to share. 

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