Volkswagen used two different styles of glow plug harness between 1997 and 2003. 1007-2001 models used a “2-Wire” aka “Early” harness that had, yes, 2 wires. 2002-2003 models used a “4-wire” aka “Late” harness – again, 4 wires.

TDI 2-Wire Harness Information

The 2-wire harness was the earlier design used in many VW ALH TDI engines. In this setup, two wires run to the glow plug harness, each splitting into two ends, which then connect to the four glow plugs in the engine. The 2-wire harness essentially connects two glow plugs in parallel to a single wire.

While this is a durable design there are a few clear problems:

Disadvantages of the 2-Wire Harness:

1. Less precise monitoring: Since two glow plugs share one wire, the ECU (engine control unit) cannot monitor each glow plug individually. This makes it harder to pinpoint which glow plug might be faulty if an issue arises.
2. Potential for uneven power distribution: Over time, the power distribution to the glow plugs may become uneven, potentially leading to varying glow plug performance and uneven heating in the combustion chamber.
3. Hiding Failed Glow Plugs: It is possible for this harness design to hide failed plugs from the ECU. The ECU compares the resistance across the two wires – if the resistance is different it is an indication that there is a failed plug. However, if two plugs fail, one on each wire, the ECU won’t see a difference and won’t set a CEL. We’ve seen countless cars with two failed plugs that never set a check engine light or any codes.

Note: the 2-wire harness is sold with wire pins at the end, but they cannot be used on US-market TDIs.

TDI 4 Wire Harness Information

The 4 wire harness introduced in 2002 provided each glow plug its own wire. Moving to a 4-wire harness also required a more complicated glow plug controller. Together the upgraded controller and individual wires allowed each plug to be monitored individually, improving the reliability of the ECU to detect a problem.

Every modern diesel uses individual wire-to-plug harnesses.