Check the Ratio - Transmission Digest

Check the Ratio

The wrong differential was installed when the accident repairs were made two years before. The problem never surfaced until the car was driven more than 30 miles at one time. Since this was a second car, it took two years for the problem to show up.

Check the Ratio

TASC Force Tips

Author: Ed Lee, Deltrans

TASC Force Tips

  • Author: Ed Lee, Deltrans

A) A 1997 Buick LeSabre equipped with a 4T60-E transmission was brought to a transmission shop. The customer said he had complained to his regular mechanic about a Check Engine light and no 4th gear or lockup. The mechanic told him that the 1870 code displayed meant he had an internal transmission problem and sent him to the transmission shop.

The transmission technician verified the 1870 code and found that excessive TCC slip was causing the code. The vehicle had only 53,000 miles, and the technician thought that rebuilding the transmission and replacing the torque converter would solve the problem. When he disassembled the unit and found a worn TCC regulator-valve bore, the technician felt even more confident that the problem would be fixed.

When the rebuilt transmission was installed, the vehicle was taken on a 12-mile road test and performed flawlessly. The customer paid his bill, exchanged pleasantries and was on his way. This should have been the end of the story, but two weeks later the customer returned with the Check Engine light on and no 4th gear or lockup. And yes, the dreaded 1870 code had returned.

The technician verified that excessive converter slip was causing the 1870 code. At this point he was having second thoughts about his own work and bad thoughts about his converter rebuilder. He had found some previous front-end damage, and common sense told him to investigate the repairs before confronting his converter rebuilder. Although the repairs did not appear recent, he reasoned that someone might have installed an incorrect computer when the repairs were made.

Thinking that the 1870 code might be a result of the accident repair, he checked with the local dealer on the part numbers of all the related components. Although the part numbers checked out OK, one important clue did surface. In 1998, the vehicle was issued a salvage block rebuild title. This told the technician that the original damage was worse than suspected.

The technician was reviewing what had been done during the transmission rebuild and found that the differential had been replaced. Since the original parts were still in the shop, they were compared with the replacement parts and found to be exactly the same. Both had a 2.84 ratio. A check of the ATSG white seminar book, which is an excellent source for this information, revealed that the vehicle should have had a 3.33-ratio differential.

The wrong differential was installed when the accident repairs were made two years before. The problem never surfaced until the car was driven more than 30 miles at one time. Since this was a second car, it took two years for the problem to show up.

A problem that lies dormant for two years is the kind of thing that makes you want to convert your transmission shop into a Starbucks coffee house. The original technician overlooked an important clue. The converter slip was in direct proportion to the vehicle speed: the higher the speed, the greater the slip. When you see this condition, you can be sure that the differential ratio is incorrect.

B) A 1997 F150 Ford pickup equipped with a 4R70W transmission was brought to a transmission shop. The customer said all the upshifts felt normal, but every 10th stop, when the transmission was downshifting, it felt as if someone were hitting him in the back with a 2×4. A road test verified the condition, and a scan check showed no codes. The customer said the vehicle had been to three other transmission shops and that everything related to the transmission had been replaced. This included the transmission, torque converter, wiring and computer.

A quick check of the differential revealed a 3.55 ratio, but the door sticker said the vehicle should have a 3.33 differential ratio. These ratios are close enough that the driver had not noticed that the speedometer was off. At this point you can either change the differential or change the calibration to compensate for the differential ratio.

Checking the differential ratio of a rear-wheel-drive vehicle is easy. Chock the front wheels properly, raise one rear wheel off the ground, mark the wheel and rotate it exactly two full turns while counting the revolutions of the driveshaft (see Figures 1 and 2). The number of times that the driveshaft rotates tells you the ratio; e.g., a little over three full turns is a 3.08 ratio.

If you can’t rotate the wheel with one wheel off the ground, you have forgotten either to release the parking brake or to put the transmission in neutral, or else the vehicle is equipped with some sort of limited-slip differential. If the latter is true, you still can check the ratio. Simply raise both wheels off the ground and rotate one wheel exactly one complete revolution while counting the revolutions of the driveshaft. The number of rotations of the driveshaft tells you the differential ratio.

Special thanks to Dave Van Aernam of Utica Hydra-matic Service for his technical assistance in writing this article.

The TASC Force (Technical Automotive Specialties Committee) is a group of recognized industry technical specialists, transmission rebuilders and Sonnax Industries Inc. technicians.

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