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Would you stake your life, right now, on the condition of your equipment?
NEWS | Jan. 10, 2023

HEMTT A4-PLS A1-HET A1: Engine Cranks but Won't Start

BLUF: There’s a possible issue with some C15 and C18 engines on HEMTT, PLS and HET trucks cranking and not starting due to a damaged front cluster gear.

Photo by Staff Sgt. Steven Colvin

If you’ve got a HEMTT A4, PLS A1 or HET A1 truck with a Caterpillar® C15 or C18 engine installed, TACOM advises that the engine could crank but not start due to a damaged engine cluster gear. The gear typically fails due to a broken woodruff key which hinders correct rotation of the camshaft gear or fuel pump drive gear.

Maintenance personnel then have to spend a great deal of time and effort troubleshooting the problem. It’s incredibly labor-intensive to remove all the covers to visually inspect the cluster gear.
It’s also important to note that there are several gears that are collectively referred to as the engine timing gear. You’ll see all the gears, along with a related legend below:

Image of cluster gear
C15 and C18 engine timing gears with cluster gear (5) highlighted
Engine Timing Gears
1 - Timing marks
2 - Camshaft gear
3 - Adjustable idler gear
4 - Idler gear
5 - Cluster gear
6 - Timing marks
7 - Crankshaft gear
8 - Oil pump gear

The easiest way to check for a damaged cluster gear is first to determine if the mechanical fuel pump is pumping fuel when the engine is cranked. If it’s pumping fuel when cranked, then the fuel pump should be good. If it’s not pumping fuel when the engine is cranked, check for rotation of the engine timing gears and the fuel pump drive gear.
Be sure to follow all safety guidelines in the appropriate TM.

A less labor-intensive way to check the gears' rotation is to remove the front inspection cover (peanut-shaped cover) and the mechanical fuel pump.

Image of peanut-shaped cover over cluster gear
Removed front inspection cover (peanut-shaped cover) allows for gear inspection

Once the cover is removed, you’ll be able to confirm rotation of the camshaft gear and fuel pump drive gear visually as the engine is cranked. If either of the gears doesn’t rotate as the engine is cranked, you’ve got a damaged front cluster gear and the vehicle needs a new engine.
The unit will have to remove and replace the engine. Fixing a damaged front cluster gear is depot-level maintenance because the engine must be rebuilt. That’ll cost units about $89,000.
Click the forward arrow on the following image to play a video of the non-rotating camshaft and fuel pump drive gears as the engine is cranked.
VIDEO | 00:05 | Cluster Gear
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