Modification to make Glowplug Testing Easy

[thorter]

Here is a simple modification that allows glowplugs to be checked on car without any dismantling in a minute or so.

Replace the thick wiring from the preheat relay to the glowplugs with a separate ordinary heavy single wire to each one. Then use a meter to measure the volt drop during preheating across each lead (ie between the relay terminal and the glowplug). Depending on exact wire size, you will get a drop across the wire of about half a volt, for a functioning glowplug, and zero for any that are open circuit.

It is best to use 4 or 5 square mm section wire, though 2.5 square mm might do at a pinch but would give more volt drop. Simply cut four equal lengths, and join all four into a single heavy terminal for the preheat relay connection. The other ends are the separate connections for each glowplug.

The terminals must be properly fitted to avoid poor contact which would give a very high drop and so reduce the heating. If the relay is inaccessible, run a fifth wire to near the glow plugs for the common meter connection. Since this takes no current, a thin wire is fine, but be quite sure it is arranged that it cannot short to ground.

In fact, you will find that the current (and thus the volt drop) starts high, then reduces to about half over the few seconds that the glowplug is heating. It is best to check this happens, because occasionally a faulty plug starts OK, but goes open circuit as it heats up, which you do not find with an ordinary ohm meter resistance check.

I have also seen a case where a plug had apparently correct electrical operation, but the tip had become deformed, and was spoiling the spray pattern. This caused unburned fuel smoke when cold, but was fine once warmed up. Obviously only removal and inspection will find that fault.