Other Articles - October/November - 2015

Basic Test Procedures: Isolate No Reverse on a Jatco JF506E

One of the most common problems with the Jatco JF506E transmission is when they won’t go into reverse. This can be all the time, intermittent, or only when the unit’s cold. It’d be easy to assume the problem’s something internal, so you might want to pull it out and get it on the bench. But, as most of us have learned over the years, that’s never a good idea.

Before going any further, always verify the customer’s complaint. At the same time, one of the most important questions to ask is, “Has anyone worked on your car recently?” If someone’s been there before you, you need to find out whether the problem was there before, or did it show up right after they worked on it.

If the problem only showed up right after someone worked on the vehicle, the most common cause for losing reverse is when someone mistakenly removes the band anchor bolt, thinking it’s a fill plug.

This band is used in reverse, and first through fourth; but in forward gears, the reduction sprag will hold, even if the band isn’t anchored or it’s broken.

If the customer’s been driving the car for awhile since that service, chances are the band rotated, wedging it between the case and drum. By now it’s probably worn out; nothing you can do will fix it without removing the unit and replacing the band.

But if the car was towed in without being driven very far, you might be able to fix it in the vehicle: Remove the servo to release tension on the band, and use a pick through the band anchor hole to reposition it.

If no one worked on the vehicle recently, start by checking the fluid level and condition, and connect your scan tool. Check for codes in all systems, and examine sensor inputs and solenoid operations.

One thing to keep in mind when any vehicle loses reverse is whether the transmission is capable of inhibiting reverse (figure 1). Many transmissions have this capability to prevent damage caused by engaging reverse while the vehicle is moving forward.

On those systems, the vehicle’s computer monitors the output speed sensor during reverse engagement. If it sees movement, the computer will command solenoids on or off to prevent reverse from engaging. If you lose reverse because of an output speed sensor problem, it’ll usually be accompanied by forward shift pattern issues.

Here are a few possible causes for false signals on the output speed sensor signal:

  • Alternator failure, causing electromagnetic interference
  • Faulty computer or sensor grounds
  • Aftermarket electronics installed improperly or incompatible with the vehicle’s computer system
  • Ignition system problems, such as faulty wires, plugs, or waveform irregularities

For Land Rover, Jaguar, and Volkswagen, the computer turns the low clutch timing solenoid on to inhibit reverse (figure 2). Mazda computers turn the neutral shift solenoid on to inhibit reverse (figure 3).

Even if the computer isn’t commanding the solenoid on, it’s still a good idea to check whether the solenoid is being energized. Use a current clamp, backprobe the solenoid wire, or simply disconnect the case connector and see if the solenoid turns off.

Solenoid failure can cause also prevent reverse all the time or only when cold. These solenoids sometimes stick on when cold and begin operating properly after they warm up. So if you’re missing gears during cold operation but the transmission works okay after it warms up — or vise versa — check the solenoid application chart and start with that solenoid.

When checking solenoid operation on the bench, use a lab-type power supply that can adjust current to the solenoids to prevent damage (figure 4). If the failure occurs when the solenoid’s cold, place the solenoid in a freezer for a couple hours.

If it fails hot, use a heat gun, or place the solenoid in a pan filled with ATF and heat it on a hotplate to warm the solenoid up. Use an infrared thermometer to monitor solenoid temperature, and retest the solenoid when it reaches normal operating temperature.

If the solenoid passes the bench test, your next check should be the valve that the solenoid controls, to see if it’s sticking.

One thing to consider when checking for a sticking valve is that some valves will rotate in their bore as they stroke. And some valves can stick in one position, while they move freely in all other positions. This is usually because of debris embedded in the valve or bore.

Another check for isolating a sticking valve is to remove the suspect valve, spring, and end plate from the valve body. Bolt the valve body — minus the valve assembly — to the case and torque it down. Then try to reinstall the valve.

If the valve moves freely with the valve body off the transmission, but won’t slide freely once the valve body’s bolted to the transmission, suspect a warped valve body or case.

If the valve still strokes freely with the valve body bolted to the case, try placing the valve body and valve in a freezer or use a heat gun, depending when the problem occurs.

And as always, vacuum test the valve body: Like all other aluminum valve bodies, these are prone to wear. Minor leaks may pass a road test, but burn up clutches in just a few hundred miles. A vacuum test will isolate those leaks that are right on the edge.

When clutches are burnt, refer to a hydraulic diagram to verify which valves operate that clutch to narrow down which valves to vacuum test.

The real key here is to follow the diagnostic procedures, and examine every part of the system when faced with a failure. A little extra time on the bench can help you avoid a transmission comeback and an unhappy customer.