The glow plug harness (VW p/n 038971220C) on a 2002-2003 ALH TDI engine uses 4 wires and is referred to as the “4 Wire Harness” (Read more: 2-Wire vs 4-Wire TDI Glow Plug Harnesses). The replacement harness comes with the molded bridge which connects to the glow plugs as well as a very long set of wires that terminate in a plug. What isn’t clear is where the plug goes.

Where is the Glow Plug Harness Plug on an ALH?

The plug for the glow plug harness is hidden inside a plenum cover for the main engine harness which is located right behind the airbox. It is circled in green in the image below.

However, accessing the plug is only the first challenge. Once you uncover the plug you will notice that the wires go from the plug down into the wire loom which is fully taped up and travel underneath the battery. If you follow the path, the wires will travel under the battery, then into the wire loom which crosses the transmission and travels up to the engine. The green wire in the image below shows roughly the path that the wires take underneath all those components. 

In short, you’ll find that removing the old harness completely is impossible. You’ll need to cut the harness at either end and leave the wires in place inside the wiring loom. Additionally, running the new wires from the new harness following the OEM route will require a lot of disassembly.

Can you splice the ALH 4-Wire Harness?

Yes, the 4-wire harness can be spliced. We aren’t going to weigh into the butt-splice vs. solder debate, but we’ll say that we’ve had no issues with good-quality butt-splices. Whichever you choose be sure you purchase splices or shrink wrap that has the weathertight seal inside of them. 

Best Way to Repair the 4-Wire ALH Glow Plug Harness?

Here are the three options:

1. Cut the existing wires and splice in the new harness to the existing wires – it is less of a clean install, and cutting a brand new harness is always hard, but this is by far the fastest way to complete the repair.
   
   
2. Cut the existing wires and run the new wires via the OEM path. This is time-consuming but is certainly the cleanest install. It is also probably the most reliable method as you’re benefiting from the OEM protection.
   
   
3. The third option is a bit of a hybrid – cut the existing wires, plug the new harness in but run it via a different path, like around the outside of the battery, securing it as best you can. This can save time and can work, but you run the risk of the wires being damaged if they aren’t secured properly.