The automatic transmission in your vehicle is undoubtedly the most complex component of your entire car, and yet it’s sole purpose is very simple. Transporting you from one spot to another comfortably and using the engine output to it’s maximum efficiency. When you want to learn how to troubleshoot your transmission, it’s a good idea to what parts you are actually working on and how they relate to the car.
Most transmission related failures will require removal of your transmission, so this guide on how to troubleshoot your transmission is meant to help you eliminate possible causes. Some of these steps may help, but if your vehicle has high mileage chances are you will need some serious repairs. Doing some investigative work will go a long way in you both troubleshooting as well as preventing future problems your transmission.
Many times a small solenoid or switch may cause your transmission to fail, or slip or even give the impression of a faulty transmission. Knowing how to troubleshoot your transmission can also stop you from making an expensive mistake or overpaying for a job that you could have done yourself.
This how to troubleshoot your transmission guide will help you troubleshoot your problems and find the cheapest solutions before turning to the professionals. Even if you end up having to go to a repair shop and pay for a large repair, you’ll at least know you’re getting a more accurate diagnosis or if something seems off.
Transmission Fluid
Many times simply flushing your transmission fluid will do more than enough to help alleviate any issues. Dirty transmission fluid can cause clunking during shifts, or make the shifts much harder than they need to be. When you feel your car shifting harsher or the familiar “clunk” during upshifts or downshifts, try checking your transmission fluid.
Having it serviced and flushed or even just replacing the transmission fluid is a much cheaper alternative than an overhaul on your gearbox. Replacing your transmission fluid will remove metal sediments, deposits and debris that may be contaminating your transmission.
Be careful when your transmission fluid has been sitting in your transmission too long, because changing the fluid can cause problems (see Service your Transmission section)
How To Properly Fill Your Automatic Transmission
If you find the transmission dipstick is slowing a low level of transmission fluid, put your emergency brake on and the vehicle into PARK. Using a funnel you can try pouring into the transmission dipstick funnel in small increments. Give time for the fluid to travel down the tube and into the transmission before checking the level again.
Do not overfill or pour too much transmission fluid into the transmission, or you may be facing a leaking gasket or seal problem shortly.
Check for OBDII codes
The OBD-II diagnostic system in your vehicle not only monitors the engine vitals and performance, but the vitals of the transmission as well.
Make sure there’s no stored DTC’s in your PCM or ECU, and take the proper steps to diagnose or test your transmission solenoids. If your vehicle is manufactured after 1995, you should be able to detect codes indicating a solenoid or sensor failure.
Transmission Shifting issues
When your transmission or shifter does not want to shift, or you have problems removing your key, you may have a mechanical issue in your shift cable. This may be an issue with your shift cable being damaged, or your shift interlock causing your ignition key to stick.
Checking this is much easier and cheaper than removing and overhauling your transmission, if you are having a problem with your shifter, check your cable and / or shift rods.
Because this cable or rod connects your shifter to the linkage attached to your transmission, damage to this cable can impede your ability to shift.
Service your Transmission
If the fluids inside your transmission seem clean and levels are good, hope is not lost. Try servicing your transmission as part of our how to troubleshoot your transmission DIY guide, this means you should have your filter and / or pan gaskets removed and replaced. Make sure you know that your solenoids and various traction control related devices are in proper working order before taking this step.
If it’s been a long time since you last changed your transmission fluid, changing it may cause a leak as deposits from the oil transmission fluid build up. These residues build up around the seal, and when new fluid is poured in it can cause these sediments to break away, causing leaks.
Vacuum Modulator
Older automatic transmissions can also have something called the vacuum modulator. These units can suck up fluid and burn it, causing your vehicle to smoke excessively.
When this thing fails the vehicle will stay in the lower gears and not upshift. Knowing how to troubleshoot your transmission will also include replacing your vacuum modulator to prevent a major transmission rebuild.
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