C220 CDI glow plug light comes on but glow plugs are fine

[Lucas Palmer]

Hi everyone,
I bought a 2006 c220 CDI the other day, and I'm doing a couple of jobs on it to get it back into shape (going for a transmission service tomorrow too)
The glow plug light comes on every now and then for a while after starting, so I checked the glow plugs with the multimeter and they seemed fine (all read ~0.5 ohms), so my next thought is the relay. Is there any way to test the relay before cutting it open, and does it make sense that the glow plugs are fine but the relay potentially not?
Thanks in advance
EDIT: Seems like I tested them badly. I tested them again at the relay plug for current flow and one of them seems bad
Should I replace all or just the bad one? I've head mixed opinions

 

[EmilysDad]

Hi everyone,
..... and does it make sense that the glow plugs are fine but the relay potentially not?
Thanks in advance

It makes perfect sense That was exactly the case with my previous car

 

[mioba]

If all the plugs are original, I would replace them all.

 

[Lucas Palmer]

If all the plugs are original, I would replace them all.

I'm not sure if they're original. the car is on 139k and all the 3 other plugs drew around 13A. I'm thinking about doing just the one to have less risk of breaking them

 

[grahamcol]

I think best practice would be to replace them all. However, l can see the argument to replace just the faulty one. Your call.

 

[mattkh]

........ all the 3 other plugs drew around 13A.

I am intrigued to know what instrument you used to measure DC 13a .

 

[Lucas Palmer]

I am intrigued to know what instrument you used to measure DC 13a .

I might have remembered it badly, but they all read roughly the same

 

[Wighty]

I might have remembered it badly, but they all read roughly the same

Aren’t they tested by checking resistance , one probe on nearby metal , one on the glow plug ? Sounds like you just checked they were getting current ?

 

[curious]

Only replace the one that needs replacing, not all of them. (They shear for a hobby when removing, and it would no doubt be a good one that breaks.)

 

[mattkh]

I might have remembered it badly, but they all read roughly the same

Thanks.
I was wondering if there was enough room in there to use a clamp meter to measure the DC current.

 

[Lucas Palmer]

I'll check them properly with the plastic plugs off before I replace them. Just testing the resistance they all read fine but it was a bit fiddly

Thanks.
I was wondering if there was enough room in there to use a clamp meter to measure the DC current.

The relay plug has metal contacts where you can put a tester probe on, and connect the other side to the positive terminal near the battery, its a bit of a second, maybe more accurate test, but I don't know which glow plug it is from the plug, so I'll have to test again to know which one is bad

 

[Lucas Palmer]

Update:
One of the glow plugs was indeed bad, I checked them more thoroughly.
I used penetrating oil for a few days before, and it came out extremely easily with next to no force, and it's all good now