Glow plug tests

[BugCo22]

Ok so I'm having trouble with white smoke on cold startup. Initially thought of glow plugs, so checked relay, fuse etc and finally changed the plugs. No difference and found old plugs to be ok. Been testing and playing today and I've got some odd readings with an ohm meter. 

 

Firstly I assume the engine block is neg charged and the glow plug cable is positive, when active.

 

So testing today and testing from the glow plugs that I can reach, I am getting 0 ohms from the back of the plug (pos) to the engine and neg and then also to the fuse ?? So this indicatates a dead short. So I disconnected the glow plug loom at back of engine and did same test from engine side to neg and fuse and got 0 ohms on both. Then tested glow plug loom and got 0 ohms to neg on battery and fuse ?? 

 

Can anyone tell me what's going on or if I'm doing something wrong ?? Cos in my eyes, I have a dead short on glow plug harness and rest of engine harness ?? Yet the fuse has not blown ??

[stef123]

It's late and I've had a curry and some beer, I'm trying to work out what you are doing but here's a few tips, if you can call it that.. lol

1) To test the resistance of the glow plugs you need to disconnect each of them, to do this on your engine means removing the inlet manifold. Each plug should measure around 1 ohm roughly. By testing them all connected together you are measuring resistors in parallel which is probably why you see a near dead short.

For argument sake, 4 glow plugs @1.2 ohms each.
1/RT = 1/R1 + 1/R2 + 1/R3 + 1/R4

1/RT = 0.83 + 0.83 + 0.83 + 0.83

1/RT = 3.32

Therefor, RT = 1/3.32 = 0.301 ohms, which on a low end multimeter may appear like a dead short. 0.301 ohms at 12v is 40 amps. 

2) By testing like how you are, you are very likely getting a reading through loads of other circuits.

3) Continuity testing circuits is a far from ideal test! In fact, its a pretty crap means of testing lol. Voltage drop testing and substituting load is by far a more accurate way. You might have a low resistance reading but it doesn't mean its capable of carrying current. Even measuring voltage is very inaccurate, until you draw a load, voltage means nothing.

Have I said enough crap for a saturday night! lol @iantt Sad act is at it again on a Saturday night  

 

Hats off to you for having a go

[BugCo22]

Cheers dude, I was there myself last night so no worries lol. Ok what you said about the plugs being in circuit makes sense and I may of over looked that and explains a few things. However, my rubbish drawing shows my test points. A and B are a plug and socket at the back of the engine bay. A has 4 wires for each plug individually, B has 4 wires going into engine loom. 

From B, I would expect that to come from fuse/relay being a positive leg. Im guessing the wires are joined together somewhere ?? However when I test continuity back to the fuse I get 0 ohms, As expected. But also when I test the same pin on B to the neg on battery  I also get 0 ohms ??

That can't be right surely ??

[stef123]

Across your glow plugs under the inlet manifold, it will be one wire going to all 4 plugs? 

[iantt]

What vehicle do you have ? Your profile is Mondeo MK3 2.2 and your in s max section. Have you changed vehicles? 

[BugCo22]

Yes I have changed cars. I do have an s max, 2.0 tdci. Not changed pic yet as car needs a clean for a photo lol.

[BugCo22]

That's what I thought with the single wire, it must spit into 4 somewhere in the loom as each glow plug is supplied individually at the plug/Glow plug harness.