Belts, just how important are they? First, if you have lost some weight, they can help hold your pants up. Or, they can just be a fashion accessory. Wait a minute, that is not the type of belt I’m talking about. I was referring to fan and/or accessory drive belts. Losing a belt can be the beginning of a very bad day, for either type of belt.

Engine accessories draw a lot of horsepower and it takes a good drive system to keep all those accessories doing what they are designed to do. The thing about a drive system is it can draw power just to turn. That is, something like a gear train or a chain driven accessory. These drives are somewhat inefficient and require some sort of lubrication but generally have a very long lifespan.

The belt drive system has been around for a loooong time and have evolved through many stages. When I started working on cars the belts were “V” belts. Obviously, the belt was a full circle but the belt had a “V” shape that fit into a “V” shaped pulley. This shape gave the belt two surfaces to contact the pulley, more surface area better grip. The more of a load that needed to be driven, the more belts that had to be installed using multi groove pulleys. These belts also had to have the tension adjusted as the belt wore. To much tension and there may be excessive loading of the bearings in the accessories causing them to fail. Not enough tension and the belt would slip, causing the belt to wear out faster and the accessories not perform as designed.

There were tools that could measure the tension but few mechanics I knew had one. They would tighten the belt and press on the backside of the belt to check “deflection” and call it good. I often wonder how many water pumps, alternators, and A/C compressors failed because of over tightened belts.

Along came the serpentine or flat belt. I believe this was a tremendous improvement over the “V” belt for a couple of reasons. The flat belt is only flat on one side, the other side, the working side, has miniature “V”s, or ribs, running inside the circumference of the belt. I have seen as few as four ribs and as many as sixteen. The number of ribs depends on the load the belt is going to have to drive. The pulleys will also have to have a corresponding number of grooves. The more ribs/grooves there are the more surface area of contact between pulley and belt, the more of a load it can handle.

The flat belt systems utilize a belt tensioner, a spring loaded idler pulley, so there is no more guessing about belt tension. Engineers can determine how much of a load all the accessories will put on the drive pulley and determine if more than one belt is needed, or how wide a single belt would need to be. The tensioner allows faster change time of the belt, typically using only one tool. Anytime there is a serpentine belt used, I have seen somewhere in the engine compartment, a “map” of how the belt is routed around the pulleys and idlers. There may be one, two, or even three flat idler pulleys the flat side of the belt will run against.

The flat side is smooth so there is less resistance against the belt so more of the power from the drive pulley will get to the accessory. This way more accessories could be driven with one belt.

In the video I show how to install a new set of belts on our Detroit Diesel engine. I have found it’s a good idea to replace the belts prior to them needing replaced, as a rule I replace our belts every year.  Ours is a DD13 but I have looked at DD15 belt routing maps and they look the same, as long as they have the same accessories. I do not show how to remove the belts because it was cold outside and I already had them off when I thought this would make a good video. I didn't want to put them back on just to take them off again. Very simply, you can watch the video and work in reverse to remove the old belts.

You can snap a photo of the belt routing before you start or find the routing on the front of the air filter housing. Just refer to the routing and double check that the belts are down in the pulley grooves and the routing is right BEFORE starting the engine. If there is something out of place, the belts could get thrown off the engine and damage the new belts, they will not self-align.


Sign up to get the latest trucking tips and tricks!




Changing Disc Brakes

April 06, 2019


Truck Tow Hooks

March 27, 2019