Downshifting can damage automatic transmission

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This article was published 24/04/2015 (4106 days ago), so information in it may no longer be current.

QUESTION: I have a 2003 Focus with automatic transmission. Sometimes when I am braking, instead of immediately applying the brakes, I will shift from drive to 2, then 1. I only use this procedure at low speeds between 50 to 70 kilometres. Will this damage my engine or transmission? Please advice.

ANSWER: If you shift to a lower gear while at higher speeds, the engine could possibly go above its rpm limits, usually called the redline because the face of the tachometer gauge will typically be marked in red at the engine r.p.m. limits. This can cause engine damage. However, almost all automatic transmissions will not shift to a lower gear even if you manually select one.

But, any time you shift down manually, you are placing an extra load on the engine and transmission. If you are driving in a spirited manner, you may not care, but if you want to maximize your economy, you are better to use the brakes to slow the vehicle and just leave the transmission in drive.

There are exceptions to this. When driving on long downhill grades, such as you find in the mountains, you may wish to shift to a lower gear to help keep vehicle speeds down, especially if you are towing or hauling a heavy load. By shifting to a lower gear, you are able to use the engine compression to help brake the vehicle, and this helps reduce the possibility of overheating the wheel brakes during a long brake application.

Another exception could be when trying to stop on glare ice. Shifting to neutral in some instances and using only the brakes to stop the vehicle will sometimes give you better vehicle control during stopping, because the engine has no effect on the drive wheels. Be prepared to shift back to drive if you need to drive your vehicle away from a potential collision.

When you manually select a lower gear in an automatic transmission, you are increasing transmission hydraulic pressures and often applying additional clutch packs. This minimally increases wear in the transmission if done occasionally, but if done all the time, it will wear out the transmission faster. Repairing brakes is cheaper than repairing transmissions, so I suggest using the brakes, unless you need the performance of a lower gear.

Q: I have a 2003 Pontiac Sunfire. I lost all the gauges in the dash, such as speedometer, gas, r.p.m. and temperature. This seemed to happen when the battery went dead. It was jump-started. Would that cause the gauges to quit working? Is there anything I could check before taking a lot of things off or apart? Could it be the ground wires or fuses?

A: The instrument panel cluster (IPC) is actually a computer module. The four gauges you have indicated and some of the warning lights are operated by the IPC using information they get from the body control module (BCM) and the engine control module (ECM). It is possible the data line between these modules and the IPC is broken, but it is wired in a “loop” system, much like an old telephone party line (so all the modules can listen to the data conversation), and it would take two breaks in the loop before data communication to the IPC would be lost.

The fastest way to verify if IPC is getting data is by using a scan tool to communicate with the IPC. Before doing that, you should check the fuses for the IPC. There are two fuses in the fuse box inside the left end of the instrument panel. Remove the cover and look for the 10-amp cluster fuse and the 10-amp BCM/CLU fuse. Remove these two fuses and inspect them for opens. If they are good, reinstall them after about one minute. This allows the computer module to power down and reboot.

If the other warning lights are working, the IPC ground should be good, but you could check Ground G110, which is bolted to the top of the transmission mounting bolt on the engine. If the power, ground and data communication are good, then the problem is with the IPC. A voltage surge during the boosting can damage computer components, and you may have to replace the IPC.

james.kerr@sasktel.net