Sometimes life throws you a curve… and those curves get even tougher to hit when the last time you faced this pitcher was so long ago you have a hard time remembering what he looked like. So, when that pitcher starts throwing fastballs, you have to wonder when it’ll stop.
So goes the story of a JR403-E. Many of you may not be able to remember the last time you faced one of these units; others probably see them quite often. This month, I’ve seen five of them with problems.
The JR403-E is a 4-speed Jatco rear-wheel drive transmission used in a lot of trucks, including GM/ Isuzu, Mazda, Mitsubishi, and Nissan. It’s also known as the RE4R03A/ RG4R01A in some applications. Powerflow through the unit is very similar to the 4L60E, which makes it easier to diagnose problems that have similar symptoms to the 4L60E.
The JR403E clutch and band application is the same as the 4L60E. The only differences are some of the names of the individual components (figures 1a & 1b).
There are differences in solenoid and hydraulic operation between the JR403E and the 4L60E. On the JR403E, A/B solenoids are feed controlled: The solenoids are normally grounded; the computer applies system voltage to the solenoid to energize it. The 4L60E A/B solenoids are ground controlled, with system voltage applied to the solenoids all the time.
Both units use shift valves that are controlled by the A/B solenoids: The JR403E uses an overrun clutch solenoid and an overrun valve train to control the overrun clutch and 4th gear circuits. The 4L60E uses the manual valve and 4-3 sequence valve to route the oil to the overrun clutch.
Both units use compound servo designs to operate the 2-4 band. The servo applies oil pressure to the 2nd gear piston to apply the servo in 2nd. In 3rd gear, oil pressure is applied to the 3rd gear area of the servo, forcing the servo to release.
Shifting to 4th applies oil pressure to a 4th gear piston in the servo cover. The combination of the 2nd gear and 4th gear servo surface areas are greater than the surface area for 3rd gear, so the servo reapplies the band. Both units use accumulators to control shift feel.
Now that you’ve had a chance to refresh your memory, let’s look at a common issue with this unit: no 4th gear. The JR403E has a series of pressure taps on the driver’s side of the transmission, including 2nd, 3rd, and 4th servo apply oil circuits.
If you’re faced with a unit that won’t shift into 4th gear, there are two basic areas you should check: the commands to the solenoids (figure 2), and the apply oil available to the 4th gear pressure tap.
Of course, you can check the commands to the solenoids with a scan tool, but that only indicates the TCM is trying to operate the solenoids. It doesn’t mean the solenoids are receiving the signal, nor does it mean the solenoids are operating properly.
Keep in mind that the shift solenoids in this application are feed controlled devices, so backprobing the solenoid circuit while commanding the solenoid on should reveal system voltage is being supplied to the solenoid.
Problems with a solenoid or its circuits can keep the transmission from shifting into 4th gear, but it usually won’t be the only problem: Shift solenoid A receives voltage in 1st and 4th gears, while solenoid B receives voltage in 1st and 2nd gears. So a solenoid electrical or hydraulic failure should cause you to lose two gears; not just 4th.
What’s different between the JR403E and the 4L60E is the use of
an overrun clutch solenoid. When the JR403E shifts into 4th gear, the overrun clutch solenoid is turned off.
This moves the overrun clutch valve, allowing 4th gear oil to reach the servo 4th piston.
If there’s a problem with the overrun clutch valves or solenoid it can prevent the transmission from shifting into 4th gear.
So what should you check when one of these transmissions comes into your shop? Here’s a laundry list of items to check:
No 4th; No 4th Pressure
(See figure 3 for valve locations).
- Is the manual valve all the way into the correct range?
- Is the TCM commanding 4th gear?
- Is the correct voltage/ current available for solenoid operation? Blown fuses and wiring damage have been known to cause issues with this unit.
- Is solenoid A applying oil pressure to shift valve A?
- Is shift valve A sticking?
- Is the overrun clutch solenoid applying oil pressure to the overrun clutch valve?
- Are the overrun clutch and overrun clutch reducing valves sticking?
- Is the shift valve train sticking or the plugs between the valves broken?
- Are the valve body gaskets correct?
- Are the proper holes drilled?
No 4th; Low 4th Pressure
- Is the 4th servo piston broken? (Common)
- Is the 4th servo cover cracked? (Common)
- Is the overrun clutch valve sticking? (Common)
- Are the aluminum plugs between the shift valves broken (Common)
- Is there a massive leak in the accumulator, accumulator circuit, or 4th servo circuits?
So, just like working on a 4L60E, finding the cause for a JR403E with no 4th gear can be a challenge, but it isn’t impossible. Treat this unit like a 4L60E, but keep in mind the electrical and hydraulic differences between the units, as one of those differences may be responsible for your problem.
Just follow these easy steps, and, the next time you’re at bat facing one of these pitchers, you’ll be the one to hit it out of the park!
