Ford Mustang V6 1994-2004: Transmission Diagnostic Guide

This article applies to the Ford Mustang V6 (1994-2004).

Your Mustang's transmission was built to last, but it is a hard working piece of your car. So after awhile, it can need your help. Any time you drive your Mustang, the transmission is working. The fluid is the lifeblood of any transmission and it may surprise you what issue it can cause. If it will not shift out of "Park," we will look at that. If it shudders when shifting, we will look at that, or if your's slips, we will look at that and lots more. The following are a few simple things you can do to get your Mustang happily cruising again.

If your car will not come out of "Park," Step 1 is for you.

Step 1 – Check the brake lights

Say what?

To shift your automatic out of "Park," you step on the brake pedal to turn on the brake lights and actuate the shift interlock. If the switch will not actuate, then you are not moving anywhere. It is an easy thing to check, but replacing the switch is not unless you are a very compact and flexible person. First thing though, check the fuses. A blown fuse is a lot easier to change. The wiring going to the switch can get damaged too, but check the fuses first. If the wiring is burnt, there is a short somewhere that you will have to trace down.

The shift interlock switch could be bad if the brake lights work, but it is rather uncommon. It is in a really bad spot on the top of the steering column under the dash.

If you can get your car out of "Park" but it is shifting poorly, see below.

Step 2 – Check trans fluid

Trans fluid makes it all happen. For manual transmissions, there is a drain and fill plug on the passenger's side. Remove the top plug that is for filling and check that the fluid is to that level. If it is not, add or even change the fluid. A small fluid pump will help get the fluid in the trans.

With an automatic trans be sure the car is sitting level, running, and warmed up. Pull the dipstick and check the level. If it is not at the recommended level, add Mercon V until it is.

If the fluid level in your auto trans was OK or adding some did not help, all is not lost.

Step 3 – Change auto trans fluid

Brown is bad.

If the trans fluid was brown and smelled burnt then when you checked it, now is the time to change it. Dead trans fluid can cause your car to shudder when it shifts or even to not shift. There is an ongoing feud over whether power flushing or dropping the pan and changing the fluid as well as filter is best. It is a fantastic idea to get all of the old fluid out and replace it with fresh, but many trans pros say no to flushing. It is also expensive. Dropping the pan and changing the filter as well as fluid removes anything in the pan or stuck in the filter. You can go a step further and blow the cooler lines out, plus drill and plug the converter to drain it. C4 and C6 converters actually had drain plugs. If you change the fluid and filter, several times most of the fluid is cycled out.

The pan in Figure 6 looks really good. If yours has metal chunks in it or the fluid is brown, you are doing the right thing by changing it.

If your car has an auto trans fluid and it just doesn't want to shift, see below.

Step 4 – Tune up your engine

Your engine needs to pull strong.

A rough running engine gets amplified through the transmission. Tune up your engine so it pulls strong and is smooth. If your plug wires are old, replace them too. If your car revs and won't come down, a bad thermostat has caused this for others. Be sure the engine is warming up properly.

Before you rebuild, look into a shift kit to prolong the transmission service life.

Step 5 – Install shift kit

Fast crisp shifts are best.

When Ford designed your transmission, there were compromises. Not everyone thinks a crisp shift is OK. When it comes to wear though, a crisp shift means that clutches engage quickly with less wear. You don't have to have tire chirping shift either, just crisp ones. A number of companies offer these.

Related Discussions

• Won't Shift Out of Park - MustangForums.com
• Automatic Transmission Shifting Oddly - MustangForums.com
• Transmission Slipping - MustangForums.com