Monday 4 April 2016

Glowplugs - again - Overview ...

Glowplug problems can manifest in all sorts of spooky ways, but the system is quite straightforward and there are only two and a half points of failure. 1. The glowplugs themselves, or their controlling relay box. The half reason is the wiring - ie a wire gets rubbed or worn or damaged - very rare.
A glowplug is nothing more than a miniature kettle element, electricity is supplied to the plug, the element glows red hot, which heats the cylinder which helps the engine start when cold. Just like an electric kettle, after 20 years of use they can get a bit temperamental.
When you switch on the ignition, the control relay switches power to the plugs for a pre-determined amount of time, and lights the light on the dash.
All very simple and straightforward, until things get worn out!
Inside the box are relays, and relays don't last forever.
There is also a simple non computerised sensing circuit that does its best to tell you when there is a problem, but it's only output is the light, so it's a bit limited in the information it can tell you. There is no hard and fast way to tell what the problem is, except that whenever the light comes on when it shouldn't, or doesn't go out when it should, then there's a problem.
The good news is that the system is very robust - you won't automatically break down if you have the light on. The worst case scenario is that the engine won't start on a cold morning, and only then if all the plugs are not getting any juice.
The fix is easy - replace the plugs. A garage mechanic can check each plug with a meter and identify the faulty plug, but what's the point of replacing one - it is only the first one to go. Unless you have documented proof or memory to the contrary, then your plugs in a 20+ year old van are likely to be original. So replace the lot.
If you still have a problem then you have to replace the relay. Mercedes will try and charge you a fortune, but they are about £60 on Ebay, and you can DIY.
You can't blame either the relay box or the plugs for not lasting more than 20 years, so replacement will give you peace of mind for another 20.
But like I said - you can last for months or even years with the light coming on and off....but eventually, usually on a cold morning, you will be stuck.

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