Toyota 4Runner 1984-1995: Transmission Diagnostic Guide

Is your transmission giving you fits? This guide will help you track down the source of the problem and get that tranny shifting smoothly again.

By Brett Foote - February 17, 2016

This article applies to the Toyota 4Runner (1984-1995).

Your Toyota truck is a durable ride, but it is not infallible. Transmissions can plague any vehicle, and they tend to feel the most daunting of any problem. Most people are not equipped to do substantial repairs to their car's transmission, so when an issue occurs, it's a big deal. Fortunately, not all repairs have to be daunting. From mild to wild, here are the common transmission-related issues that can plague your 4Runner, and what you can do about it.

Toyota 4Runner Transmission Diagnostic Guide

Materials Needed

  • Jack and jack stands
  • Pry bar
  • Multimeter/voltmeter
  • Transmission fluid

Step 1 – Vibrations while driving at speed

Worn transmission mounts can cause vibrations while driving at speed.

There are a lot of things that can cause vibrations while driving at speed down the highway: bad suspension bushings, uneven tire wear, a bad alignment, etc. But another common cause is worn transmission mounts. Inspect your mounts and make sure they are both secure and in good shape. If not, it might be time to think about replacing them.

toyota tacoma 4runner transmission problem diagnostic fix slipping overheating
Figure 1. Make sure your transmission mounts are secure and in good condition.

If your transmission mounts are in good condition, move on to Step 2.

Step 2 – Whiny noises coming from the transmission

Your bearings could be shot, but these kinds of noises are normal.

Whiny transmission noises, which usually get worse with age, are annoying but completely normal. Sometimes you will hear them only when you're in a specific gear or two, or maybe you hear them in every gear. You'll definitely want to make sure your transmission fluid is changed out, and using a heavier gear oil can help muffle the sound a little. Eventually, your bearings will simply be worn to the point where they will need to be replaced, but for many noisy transmissions, this is a long time and many miles down the road.

toyota tacoma 4runner transmission problem diagnostic fix slipping overheating
Figure 2. Changing out your transmission fluid/gear oil is never a bad idea, especially if you have a whiny transmission.

If you don't have any whiny transmission issues, move on to Step 3.

Step 3 – Transmission temperature warning light comes on

Warning lights can be an indication of a serious (and sometimes not so serious) problem.

A transmission temperature warning light is supposed to come on when the transmission fluid exceeds 300 degrees, which generally means that your transmission fluid is toast. But if you're not getting a warning light or code, the problem could be a faulty sensor. Either way, it's a good idea to flush the transmission at this point and get some fresh fluid in there. Older 4Runners had a tendency to downshift a lot when driving up steep inclines, causing fluid temperatures to rise significantly. Many people have had luck installing a transmission cooler, which might be a good idea if you live in a hilly part of town.

toyota tacoma 4runner transmission problem diagnostic fix slipping overheating
Figure 3. A temperature light indicates that your fluid temperature has exceeded 300 degrees.

If you don't have an issue with transmission temperature, move on to Step 4.

Step 4 – Transmission won't shift into gear or slipping

A transmission that won't shift into gear could be a sign of a number of issues.

If your transmission won't shift into one or more gears, the most common cause is a faulty solenoid. Oftentimes, with age, the solenoid will get hung up and simply refuse to work. You can check each solenoid by supplying 12 volts to them, one at a time, and verifying that they click into place. If not, replace them.

In some other cases, the clutch refuses to disengage, keeping it in constant contact with the flywheel. This means that your transmission won't disconnect from the engine, which prevents you from being able to shift into gear while the engine is running. Check the clutch fork and verify that it is moving, then check your master and slave cylinders (the most common cause of this problem). If those components check out OK, you will probably have an issue with the clutch itself.

toyota tacoma 4runner transmission problem diagnostic fix slipping overheating
Figure 4. A stuck clutch can prevent you from shifting into gear while the engine is running.

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