There are several different reasons that one of these units may not get fourth gear-reasons both inside and outside of the transmission. However, one of the stranger causes of no fourth gear is insufficient line pressure, or no line pressure rise with throttle. This has to do with the function of shuttle shift valve S in the valve body, but more on that later.

Your first task, as with any transmission problem, is to establish whether the cause of the problem is inside or outside the transmission. Two procedures will usually answer this all-important first question.

1. Check for retained trouble codes in the transmission controller's memory. This is something easily done from the driver's seat, and can lead you straight to the cause of the no-fourth-gear problem. This procedure is covered in Gears Magazine, Fall/1993 issue and again in Issue #10, July 1994-as well as many service manuals.

2. If you get no trouble codes, the next step is to find out whether or not the computer is commanding fourth gear. Hook up the signal monitor to the two shift solenoids (usually the yellow and green wires at the transmission), and find out if solenoid A turns back on at the right speed for fourth gear.

If not, there's a control system-usually bad signals to the computer from a sensor, making the computer think that it's normal not to request fourth gear.

If the solenoids are getting the fourth gear command from the computer at the right speed for fourth gear, the problem is in the transmission-not a control system problem. There is, however, one unlikely exception: a bad pressure control signal.

Testing the line pressure control solenoid signal
To check the signal to the line pressure solenoid, hook a voltmeter (positive probe) to the red wire at the transmission, then ground the negative meter probe. Start the engine, then shift to D, and hold the brakes on hard. You should see about 2.5 volts on the red wire at idle. As you step on the throttle, the voltage should drop smoothly as you press on the throttle-until it shows 0.5 volts at wide open throttle. If the voltage does indeed drop with the throttle, the line pressure control signal is working properly. If not, check out the TPS signal. This is one of the more common causes for a bad line pressure control signal.

No fourth gear, even though the computer commanded it
Now we need to look at shuttle shift valve S. This valve has the ability to control the overrun clutch control valve, which can keep the overdrive apply oil from getting to the servo-even when the shift solenoids and shift valves ask for fourth gear. Shuttle shift valve S is normally held in this blocking position by its spring (Figure 1).

Figure 1

For this valve to move into the unblocking position, line pressure must rise above minimum line-typically over 100 psi. This increase in pressure moves shuttle shift valve S against its spring, shifting the overrun clutch control valve, which will now allow fourth gear apply oil to reach the servo, and shift the unit into fourth gear (Figure 2).

Figure 2

This is why it's so important to know how line pressure rises with the throttle when diagnosing a no-fourth-gear condition.

Remember-if there's no fourth gear, but there is an electrical signal for it, you may have a line pressure rise problem to correct. And the most common cause of a line pressure rise problem is a bad line pressure control solenoid.

NOTE: You may find that the unit will shift into fourth gear when you use jumper wires on the yellow and green wires to shift the unit manually. This is not unusual, since with the wires disconnected from the computer, the unit is typically at maximum line pressure. It isn't unusual for a bad line pressure solenoid to achieve minimum and maximum pressures, yet be unable to modulate properly between the two extreme values.