THE DRIVE SHAFTS AND U-JOINTS: the jeep drive shafts are all 7/8″ diameter steel bar stock with machined in keyway slots. Woodruff keys index the u-joints to the slots in the shafts. The shaft lengths vary by application. To circumvent typical obstacles (crossmembers, exhaust, clutch linkage, etc), a common application will have two shafts, three u-joints, and a center support bearing. For example the early CJ uses a 27 3/8″ front shaft and a 22″ rear shaft. The jeep truck, jeep wagon, or jeep sedan delivery uses a 31.5″ long front shaft and a 17.75″ rear.

Three different jeep universal joints are used in each setup. The front u-joint has one 7/8″ opening with a 3/16″ keyway and one 7/8″ opening with a 1/4″ keyway. The center u-joint has one 7/8″ opening with a 1/4″ keyway and one 7/8″ opening with a 1/4″ keyway. The rear u-joint has one 7/8″ opening with a 1/4″ keyway and one 1″ opening with a 1/4″ keyway. Note all shafts use 1/4″ keyways, but the winch uses a 3/16″ keyway and the PTO, which uses the 1/4″ keyway has a larger 1″ output shaft.

Here are the steps to take to perform a voltage drop test:

1. Connect your DVOM negative lead to the battery’s negative post.
2. Connect your DVOM positive lead to the battery’s positive post and measure its voltage. Record this reading.
3. Connect your DVOM positive lead to the positive side of the load being tested, as close to the load as possible.
4. Operate the circuit, and record your measurement.
5. Connect your DVOM positive lead to the negative side of the load being tested, as close to the load as possible.
6. Operate the circuit and record your measurement.

Let’s take a closer look at these steps. First, we want to test the integrity of the ENTIRE circuit so placing your test leads at the battery is a must. You can make an extension lead for your DVOM so you can always connect to the battery ground terminal with your negative meter lead. Second, it is important to know the source voltage available. If the battery is weak to start with your circuit is already working with a handicap.

In the third step, you are making sure all that source voltage is getting to the load component. Getting as close to the load as possible insures you are testing the entire path. On the positive side of the load you should read within 0.50 volts of your source voltage, i.e. if you started with 14.56 volts you should read no less than 14.06 volts. This applies to most basic circuits. If you are diagnosing a PCM/ECM related circuit you may want to see even less than that.

In step #5, you are looking for unwanted voltage drops in your load downstream because all the voltage should have been used by the load. If you get a voltage reading of 0.50v or more here there is something else taking voltage from the primary component. This unwanted source may be ultimately found downstream of the load maybe even at the battery connection itself.

Some voltage measurement on the ground side of a working load is normal. If an open to ground exists you will measure source voltage on both sides of the load. If the ground side measures 0.0 volts and the component is receiving correct source voltage then the load likely has an internal open AKA you have a failed component.

A common mistake is to use a chassis ground under the dash when testing inside the car.; but don’t forget that chassis ground point still has to make it back to the battery typically through a few more connections before the ground path is complete.

If done properly a voltage drop test will quickly isolate the problem and save you time and money in the process.