Trouble Codes Don’t Tell the Full Story When It Comes to Transmission Troubleshooting

Posted by on Monday, April 21st, 2014

Auto trouble codes

A listing of common auto trouble codes.
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When it comes to transmission troubleshooting, today’s techs are blessed with an abundance of capabilities earlier generations could only dream of. Take trouble codes as an example. The ability to simply ask a vehicle what’s troubling it sounds almost magic. Compared to yesteryear’s diagnostic methods, it offers today’s technicians unprecedented ability to resolve issues quickly, getting their customers back on the road with a minimum of downtime. This is especially the case since the introduction of OBD II codes in 1996.

Like all forms of technology, however, trouble codes have their limits, and sometimes they miss problems. This can leave technicians who rely too heavily on them without a backup option should an issue fail to pop up on the scanner. When this happens, it’s time to forget the high-tech approach and go back to transmission troubleshooting basics: examine the system part by part, looking for anything that doesn’t seem quite right.

An example of how this works was discussed in a recent issue of Gears magazine. A driver brought a 2006 Jeep Liberty into his local shop, complaining that the vehicle stopped running when it got hot. A transmission checkup revealed a mass of clutch material in the pan that was clogging the filters. The crew pulled the old tranny, rebuilt it, and reinstalled it along with an updated torque converter. Problem solved, right?

Well, sort of. The new gearbox had an annoying habit of doing a “double bump” when going into drive. A test run revealed no problems with up or down shifting. Reverse worked fine, as well. The shop staff hooked up their trusty Chrysler DRB3 scanner and ran a trouble code search; nothing came up. The only clue was that the engine idle speed was a little low. This wasn’t enough to trigger a trouble code, but it did provide the insight that led to the answer.

The crew decided to adjust the idle speed to see if it had an effect on the problem. Sure enough, when they kept it a little high, the “double bump” engagement issue disappeared. So they went through the engine and fuel system, finding that the throttle body was dirty. They gave it a good cleaning, idle speed picked up all on its own, and the transmission experienced no more problems.

What happened here? Well, the low idle speeds delayed the transmission pump just enough to cause a problem when shifting into drive. But, because all the readings were within specs, the issue didn’t show up on the trouble codes. This points out an inherent limitation with onboard diagnostics: they can only measure what they’re designed to detect. Should something fall outside that range, then transmission troubleshooting problems can easily go undetected.

 The takeaway for techs everywhere is:

  • Trouble codes are a useful tool, but they can’t take the place of good, old-fashioned knowledge of how transmissions work.
  • Sometimes the solution to a problem comes from unexpected sources – like a dirty throttle body.
  • So, the rule of thumb should be to rely on scanners as a snap-shot of the transmission’s functions, but not as a complete image of everything that’s going on. In the end, it’s still a technician’s brains and experience that resolve transmission troubleshooting issues, and that will never change.

If you’re looking for a quality remanufactured transmission, then you won’t do better than ETE REMAN. That’s why we offer a three-year/unlimited mileage warranty on all our products. Browse our online catalog and contact us today.

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