Testing Glowplugs

[lazza]

HI

Starting is turning out to be a bit erratic and I suspected dodgy glow plugs. I did a test with everything still connected. Firstly I tested the resistance between each glow plugs tip and its nut (the part you screw to insert it) and I got the same ohm reading as for the multimeter between the positive and negative leads) of 0.8 Ohms. This has confused me as I understand that they should read 0.8 + 0.6 = around 1.4 Ohms. I have some spare ones and they read around 1.4 Ohms (measured at the same points).

I also tested the voltage arriving at the glow plugs and it was only 10.7V whilst the battery was showing 11.8V.

What might be causing these anomolies?

Cheers
Larry

[Hell Razor5543]

The best way (but also one to be careful doing) is to power up each glow plug. What you are after is seeing where the plug glows. If it glows at the tip it is OK, but not if it glows further up (or fails to glow).

The simple test is dangerous. The glow plug will get VERY hot VERY fast. You do NOT want anything flammable anywhere near the glow plug. It will also take time to cool back down, so you cannot handle it for quite a while (I do not know how long). You will need to remove all the glow plugs, and then individually power them up. The safest way to do this is to hold the plug securely in a vice, apply negative to the body of the plug (where the thread is) and positive to the top point of the plug (where the lead connects to when in situ), and watch the hot end of the plug. If the tip heats up it is a good plug, otherwise not.

When I told somebody else how to do this they found 3 plugs were not working. After replacing all the plugs their problem was no more.

Remember, this is a test to be careful doing. If you are not happy doing it, DON'T.

[Old-Guy]

A deep cranked 12mm ring spanner (like this) and a magnetic retriever are both absolutely essential when trying to remove glow-plugs on XUD engines. Nothing else will undo the glow-plug on #4 - even if you undo the injector pipes.

The little 8mm nuts that hold the interconnecting cable on the plugs will make a bid for freedom - vanishing down the front face of the engine block given the least chance. If either from cylinders 3 or 4 (nearest cambelt) go down the back of the injection pump they are beyond recapture by any means know to man (a hose clip has been hiding under the Green Lady's fuel pump for a couple of years now). Stuff all escape routes thoroughly with rag (best) or kitchen roll. My personal technique is to undo the nuts with a 1/4" drive hexagon socket until they are off the thread, screw back on 1 turn only then finally remove with a strong magnetic retriever. The ideal solution I suppose would be to permanently magnetize a cheap 8mm socket.

[Peter.N.]

The readings you got sound fine, 10.7 volts is about right if all the plugs are working and bearing that in mind 0.8 ohms is good, you usually get a higher reading because its very difficult to get a good connection at that low a reading. To give the required heat they should be 1 ohm or less, that equates to 12 amps at 12 volts, but you don't have 12 volts.

Peter

[Xantidote]

From my own tests on TD glowplugs, glowplugs resistances & voltages, I can vouch for all that's previously been said.

@ Old-Guy - can't believe it - you been looking over my shoulder? Your method for the 8mm nuts removal is exactly how I do it.

[Old-Guy]

In my previous post I should have said that when replacing glow-plugs, use exactly the same precautions and the same technique (in reverse).

[Stewart(oily)]

For testing glowplugs I simply hold them in a jump lead (other end connected to a battery terminal) and touch the contact post on the other terminal, a working plug will identify itself in a few seconds. For removing them my flexible ratcheting ring spanner is good for those hiding behind the fuel pump, while my quarter drive deep sockets are ideal for the easier ones (on XUD turbo motors) I also recommend a magnet on a stick and keeping a few spare 8mm nuts to hand DAMHIK copper grease on the threads please too, it might be me removing them next time.