How to Find Out Why Transmission Downshifts

Posted 12/11/2008
By Jim Watson

A Chevy truck is pushed back into the shop. The box containing a rebuilt transmission just showed up. Time to bolt it in and get it out the door! Things are going good, everything is tight, the fluid is topped off, and there are no leaks. Time for a test drive! After 15 minutes on the road and at cruise speed, all of a sudden the malfunction indicator light (MIL) comes on and this fresh transmission just downshifted to third gear. What happened?

"After 15 minutes on the road and at cruise speed, all of a sudden the malfunction indicator light (MIL) comes on and this fresh transmission just downshifted to third gear. What happened?"

After the sinking feeling runs its course, you now have to fix this truck. Where do you start? Let's look at what is going on. DTC 68 is described as an "overdrive ratio error." The powertrain control module (PCM) is looking at the ratio of the gear set inside "the box" when the transmission is in fourth gear. The overdrive or fourth gear ratio on this 4L80E is 0.75 to 1.0. The PCM will be able to read this based on two sensors: the turbine input shaft speed sensor (TISS) versus the transmission output shaft speed sensor (TOSS) or the vehicle speed sensor (VSS). This ratio value will be displayed on the scanner so you can see it.

This is all fine until you think about the fact that if the ratio is not correct, the PCM is going to downshift to third gear and not allow fourth gear to happen. The trick is to be in fourth gear to see if the transmission is slipping, or to see if the PCM has lost its mind or maybe one of the sensors is going nuts.

To evaluate the issues, go to the PCM (or TCM on a diesel). Remove the shift solenoid wires and you can "be the computer." The wires will be light green for the A or 1/2 shift solenoid and yellow/black for the B or 2/3 shift solenoid. Actual pin assignments vary based on application. Removing these wires from the connector, plug the PCM back in and attach one jumper wire to each of the wires. The jumper wires need to be long enough to comfortably ground them while sitting in the front passenger seat. Next, you need a driver and the scanner plugged in and set up. Ground the light green wire, and you are ready to go. Use the overdrive (OD) or fourth gear position on the shifter just like when the computer was in charge. There is no need to worry about shifting the wires in park, reverse or neutral. The fact is, the PCM is electrically ready for first gear in any of the ranges. Because of the manual valve, all three gears are mechanical/ hydraulic. Shift the transmission by grounding and ungrounding combinations of the wires. At the same time, while in the individual gears, watch the ratio value on the scanner to evaluate the "rebuilt" transmission. Don't forget the "seat of the pants" ... what do the individual gears feel, smell and look like?

For first gear in the overdrive range, the light green wire will be grounded, so look for a ratio of 2.48 to 1. Once you have developed some speed and have a good feel for first gear, unground the light green wire. You just shifted to second gear. The ratio should now show 1.48 to 1. Adding more speed and approaching 25 to 30 mph, ground the yellow/black wire. Now you are in third gear. The ratio should now be 1 to 1. Now comes fourth gear, and with some more speed, leaving the yellow/black wire grounded, ground the light green wire. You are now in fourth gear and the scanner should indicate the 0.75 to 1 ratio.

With all this information, now what is broken? If the ratio is OK in all gears except fourth, the DTC is setting due to a slip inside the transmission. If the ratios are floating around in first and fourth gears, look for a 1/2 or A shift solenoid leakage. If the ratios are floating in third and fourth gears, look for a problem with the 2/3 or B shift solenoid. If the ratios are all over the place, what is the speedometer doing? A failed VSS or wiring issue will screw up the ratios and the speedometer. If all of the ratios are erratic and the speedometer is OK, look at the TISS sensor. As with any other electrical issue, don't forget about wiring, connections, powers and grounds.

If the vehicle shows "slip in the box," share this ratio info with the transmission rebuild guys. Being more familiar with the hydraulic circuits may help them to help you.

Jim Watson is an IDENTIFIX GM specialist. He is ASE master and L1 certified.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2011 IDENTIFIX. All Rights Reserved.

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