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CAR CLINIC: Most gearbox failures linked to ignored ABS failures

Tuesday June 02 2020
ABS

Error sign of High lifters truck on car dashboard. Colourful on dangerous. Close up of car speed meter. PHOTO | SHUTTERSTOCK

By BARAZA JM

I will keep it simple. Time and again I have insisted I am not a mechanic, the best I can do when presented with random cases of vehicular malaise is make educated guesses. This week we feature yet another guest, one Murad Miro (his real name, actually) who breaks it down for us why the Nissan Note seems so prone to transmission problems. Here goes: (verbatim report, for the most part.)

Dear Baraza,
I hear many complaints about how CVT gearboxes are unreliable. It’s new tech and a lot of study has been put into it, therefore it has to be better than conventional automatic gearboxes.

Most mechanics go wrong because of their troubleshooting skills. They are quick to replace gearboxes without finding out the cause of its failure in the first place.

[In] Most cases I’ve handled, I observed that the ABS warning was on. Many mechanics disagree with me that a failed ABS system can cause damage to one’s gearbox, today we shall discuss the relevance between ABS and your gearbox.

So far I have observed that most gearbox failures can be linked to ignored ABS failures.
A vital signal that your gearbox requires is that of the speed. It uses this signal to determine which gear it should be in. We should ask ourselves where this speed signal comes from.

SPEED SENSORS

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Your vehicle is equipped with wheel speed sensors at each wheel, which monitor wheel speed while relaying this information constantly to the ABS and traction control system which are controlled by the car’s ECU.

When the ECU senses an abnormal speed given specific conditions, it engages the ABS or traction control as needed to control the vehicle properly and safely.

Occasionally, these sensors fail or work intermittently, sending inaccurate signals to the ECU. It may be possible that the transmission is receiving a faulty (or no) signal from the ABS system.

When this happens, the ECU commands the transmission module to enter a fail-safe mode. This mode limits the vehicle to one or two forward gears and reverse gear only, often, it will start from 3rd gear.

If the ABS warning is ignored, extended taking off in 3rd gear will slip the clutches, and in no time your gearbox will fail.

Gone are the days when ABS warnings could be ignored with little harm. You can’t ignore an ABS warning in a modern car. I’ve seen some cars whereby a faulty brake switch was responsible for gearbox failure.

Yes, the brake switch is connected to the gearbox transmission as well. How and why? Features like brake assist, a safety feature, are often linked to the transmission in the sense when your car automatically brakes for you in an emergency situation, it also gears down to assist in efficient braking, again, if there’s a fault in the brake switch, it will trigger the ECU to command a fail-safe mode. 

You’ve seen many cars boasting of automatic 4wd. How does this work? Once again the transmission control module monitors speed signals at each wheel. If it detects the front wheels spinning faster than the rear wheels, it instantly engages 4wd, helping you overcome the wheel slip. Often, this happens in a fraction of a second, making drivability very comfortable. In these cases, a fault in the ABS normally causes a 4wd warning to appear on your clock.

There are many other features such as ESP and traction control that are dependent on the speed signal. 

After learning this, it is quite easy to summarise that the ABS system is a very important part in a vehicle and should never be ignored.

CVT GEARBOXES
Now you may understand why Nissans are not loved by many mechanics, it’s because Nissan had CVT gearboxes as early as 1996. Toyota followed suit about 10 years later. So Nissan actually have more experience in CVT.

Having learnt all this today, we need to understand that anything that may affect wheel revolutions will definitely cause transmission issues, this includes having different sizes of tires and having bigger tires than manufacturer specifications.
Please have your ABS scanned for errors and rectified accordingly.
Thank you.
Murad Miro

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