Fan Belt Adjustment

An incorrectly adjusted fan belt may shorten the life of the belt or the generator pulley leading to repairs on the side of the road. Are you prepared to replace a pulley or fan belt when traveling away from home?
L1020382 On the way home from getting my car back from the shop after the engine rebuild the red generator light lit up. I pulled off at the next opportunity and saw the belt was fine, so I was still cooling the engine. I pulled the generator and, again thanks to near-by AutoLectrics, I had the generator back in a day and shortly received a new voltage regulator from Will at CSP.
L1020386 A few days later I needed to test my own advice to see if my car could be repaired by the side of the road, or would I be stranded when the fan belt destructed with a bang?

Why did this problem occur?

Usually the problem with belts or pulleys occurs because the sheaves are not installed correctly. This can lead to broken sheaves or belt.
030208-1722 I had a spare belt in the car.

I also carry in the emergency bag in the trunk new pulley sheaves and a new hub. The recommendation is to change all three because often pulley problems are caused by the hub. Back when these things were important, the guarantee on the pulley sheaves was voided if a new hub was not installed at the same time as as the pulley halves.

I decided to change pulley halves with the new belt, in part because we seldom see powder coated pulleys. A friend told a story that they had experienced multiple belt failures attributed to powder coated pulleys.
L1020395 When I got home, the pulley was running hot and the belt tension was loose. Hmmm. It was tightened as I always do.

A careful inspection showed the hub was not fully seated against the shoulder on the shaft. It would tighten properly, then under use, the hub would move closer to the shoulder and the belt would be loose again.

The shaft has a woodruff key that prevents the hub from turning relative to the shaft. When you remove the old hub, it is possible to drop the key. A magnet may help find it, or you can loosen the lower shield if you can get a screw driver on all 6 screws that hold it in place. I now carry a long screw driver and a magnet in my tool kit in the car.

I removed that hub and put on the new one, this time tightening the assembly with a 36 mm socket until the hub was seated firmly against the shoulder on the shaft. This was done with all the washers but not the belt. An impact wrench is good for this step so you do not bend the rear pulley sheave using a screw driver in the notch to keep the generator shaft from turning.
L1020399 Now we need to install the belt with the right tension. Be sure that the pulley hub is seated against the shoulder on the shaft.

The belt tension is determined by how close to gether the sheaves are contolled by the number of washers. Spare washers are saved under the nut. The nut must be tight enough so there can be no movement of the sheaves. This tension must be contolled by the washers and not the belt. A trial assembly without the belt will show how much of the shaft protrudes past the nut. With the belt on the same amount of protrusion must be seen.

Here are the steps you can use to set the tension:

  1. Tighten the nut until the washers are firmly held between the sheaves as discussed above.
  2. Start the car and run for a bit to allow the belt to move to its natural position.
  3. Check the tension. If too loose, move a washer from between the sheaves to the storage position under the nut. If too tight, move a washer from under the nut to between the sheaves.
  4. Repeat until you have 5/8 in. to 3/4 in. of slack when pushing the belt with your thumb. The belt can be twisted about a quarter turn when the tension is correct.
L1020388 The belts will stretch with use. Check and adjust again after 100 miles of use.

Check the temperature of the pulley after a short drive. If the belt is working too much because there is not enough belt tension or the nut is not tight and the sheaves are wobbling, then the pulley will get very hot. This will lead to premature belt failure.

Carry an extra belt in your emergency kit. I carry two. Belt and pulley problems are some of the most common reason for road side repairs. If you are prepaired you should not need to have the car towed home.
Last modified: Sun, 30 Jan 2005


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