There is a pneumatic device called an "actuator" attached to the front differential that effectively slides the driver side shaft out of the differential. On trucks that have ADD Toyota uses "live hubs" (see hubs section below). This means that both front half shafts and the front diff spin even in 2WD (but not the front drive shaft). There are a set of air bags inside the actuator that control the position of the ADD sleeve. This actuator will stay in it's current position until vacuum is applied to change it's position even if there is no vacuum at all. For 2WD operation vacuum is applied to driver side tube of the actuator and it holds the sleeve in the out position. 

When you engage 4WD a switch on the transfer case changes a set of air valves under the hood. This causes vacuum to be applied to the passenger side tube of the actuator and it pulls the sleeve over to the right and connects the drivers side axle shaft to the differential. Once the splines on the sleeve and the splines on the shaft are aligned the sleeve slides toward the passenger side and the front drive is engaged There are two switches that must be in the closed position for the 4WD light to come on. The first switch is controlled by the position of the transfer case shift lever. The second light is on the ADD actuator and it turns on after the sleeve has moved into the right (locked) position. 

Trouble-shooting is often a processes of elimination, test parts, figure out what works then move on the the next. There are more than 50 individual parts that make up the ADD system and creating a flow chart that includes every possible solution is beyond the scope of the article. For trouble shooting the electrical and vacuum sections of the ADD system you will need the Toyota factory service manual and excellent trouble shooting skills. 

The most common problem is damage, deterioration, or mis-routed vacuum hoses. To ensure the vacuum lines are routed properly, they are marked with either a blue or brown tracer stripe on the hoses. These hoses should match up with the corresponding VSV under the hood. In some cases during engine or collision repairs, these hoses get routed incorrectly or left disconnected altogether. 

When the transfer case gear shift is shifted into 4WD, the ADD system is actuated by the operation of the ADD solenoids located under the hood on the passenger side wheelwell. The solenoids are supplied voltage from the ADD relay which depending on the type of transmission the vehicle is equipped with, has it's ground completed by both a Transfer Position Switch and a Transfer Pressure Switch (V6 with automatic transmissions), or a Transfer Indicator Switch (4cyl.-- manual trans.). These solenoids control the vacuum signals to the ADD actuator which is mounted on the front axle. This actuator engages a sleeve inside the differential via an "intermediate shaft" which is attached to the driver's-side side gear shaft and clutch mechanism. 

If the ADD system does not properly engage the front drive mechanism, the first items to check are the vacuum supply hoses. First, look under the hood and locate the ADD solenoids. Ensure that the vacuum hoses are connected to the solenoids or vacuum switching valves (VSV) as they are commonly called by Toyota. Next, look at the hoses at both the differential and vacuum reservoir located in the R/F wheelwell. Make sure all hoses are connected and not torn, cracked, or otherwise in poor condition. Once the system is engaged or disengaged from 4WD, it will stay in that position. Basically, if the system is not functioning, the vehicle can be placed in either 2 or 4 wheel drive manually with a vacuum source for testing. If you plan to leave the ADD system disconnected you will need to apply vacume from the engine to make sure the front end does not unlock. 

The ADD relay can be inspected for operation as well. This relay is located under the instrument panel on the passenger side of the steering column. Check for continuity between terminals 2 & 6 and 1 & 3 (see diagram). Upon applying 12 volts continuity between terminals 1 & 4. Replace the ADD relay if it fails the above tests. The VSVs or ADD solenoids can be inspected as well.This is another resistance check so a DVOM is needed here as well. Disconnect the electrical connector from the VSV and check to see if the resistance is between 38-45ohms. Replace the VSV if the resistance doesn't meet this specification. A function check of the VSVs will show if the valves are working internally. By applying battery voltage to the connector and blowing air into the lower hose fitting, air should come out of the top fitting and not the lower "filter" on the VSV. Once battery voltage is removed, air should pass through the filter and not the upper hose fitting. A final check can be made of the ADD indicator switch which is located on the ADD actuator itself. This switch acts like a brake light switch. Remove the switch from the actuator. Depress the switch button and check for continuity between the terminals of the connector. If there is no continuity, replace the switch. Depending on the type of wheeling you use your ADD truck for, you may decide to retain the ADD system or eliminate it all together. 

Testing the engagement of the ADD sleeve is relatively easy requiring only a vacuum source. First disconnect the two vacuum hoses from the ADD actuator unit and put the truck in 2WD on level ground. If your truck is equipped with manual hubs, lock the hubs for this test. 

Testing for 2WD: Put a hand vacuum pump on the drivers side and apply 15 lbs of vacuum. This will pull the actuator into the 2WD position. At this time you should be able to turn the front drive shaft by hand with out the front axle shafts turning. The actuator should be able to hold the vacuum without leaking. 

Testing for 4WD: Now place the hand vacuum on the passenger side of the actuator and apply 15 lbs of vacuum. The sleeve should slide into the locked 4WD position. you should no longer be able to turn the front drive shaft (by hand) as the add unit is now engaged. The actuator should be able to hold the vacuum without leaking. 

Live Hubs are found on all ADD equipped trucks. This type of hub can't be unlocked at any time, it is always locked. This is the first hub ever used on 4WD trucks and after all the designs over the years it seems funny to come full circle back to this technology. The Live hub was used on the first 4WD Jeeps and was used for many years until some Jeep dealer in Los Angeles named Warn created a hub that could be unlocked. In the old days when you wanted 4WD you bolted on your live hubs and the front drive was locked. To unlock the front drive for use in 2WD you needed to unbolt the hubs and replace them with a different set of open hubs. I can see why that guy Warn made so much money. 

Automatic Hubs are the type that lock Automatically when you put your truck in 4WD, however you must be stopped when shifting in or out of 4WD. To unlock this type of hub you need to back up a few feet on flat ground. The Toyota version of this hub has proven very unreliable, most notably not locking in when needed, this can be bad if it happens in the middle of a trail ride and you need to get pulled all the way out. This type of hubs is found on non ADD Toyota trucks '86-'89 

If you plan on installing a limited slip unit (LSD) or locker it you should change the front hubs to the manual type. If you are driving in 2WD with the original live hubs and you have one of these units in the front differential then the passenger side axle shaft will turn in the differential. With a traction adding device inside the differential the front drive shaft will turn as if you were in 4WD causing more wear on the front diff, drive shaft, and transfer case, and the traction adding device inside the differential. 

You should also permanently lock the ADD unit to prevent the unit from unlocking (see below). After disconnecting the ADD system and adding manual hubs you can lock the front hubs and still go in and out of 4Hi on the fly (at any speed) 

Permanently locking the ADD unit: There are 2 hoses connected to the ADD actuator. The vacuum line on the passenger side of the actuator puts shaft into the diff and the other side removes it. I removed the vacuum tank and vacuum switches. Then I connected a single vacuum line to the passenger side of the actuator to keep it locked. Then I and caped the driver side tube. This will prevent the ADD system from unlocking. The switches can be found behind the battery on the inside right fender, the vacuum tank is in front of the front right tire under the fender. Be sure to cap the vacuum feed from the engine to the switches or remove the "T" feeding it all together. I have found that the transfer case shifts much easer without the ADD system. 

Special Thanks Andrew Trzynka for his contributions to this article.