Trying to understand how the hdi glow plug circuit works

[bigknickers]

My 53plate 90s hdi estate has trouble starting ONLY on a cold morning. The glowplug light never illuminates so that can't be right.

BUT I would like to understand how the whole magical glow plug process should work. It seems there is a water temperature sensor (is that the same sender unit that supplies the temperature gauge) that may tell the ecu to tell the glowplug relay to switch on the glowplugs...............but how do they switch off n why is there a supply coming from the ignition switch?

I'm ready to learn from all you clever lot!

[lozz]

Plenty of info and tech info on the www.

Just type in..for eg Citroen psa Hdi training manuals or mechanics manuals.. Add PDF on the end and 'plenty will pop up,

A quick 2 secound search.. and igot these for eg..

http://www.amtgarageforum.nl/public/top ... iemens.pdf

https://www.c5club.cz/files/navody/2005-C5_C8.pdf

the net is swamped with them..you can be there all night reading them,

[bigknickers]

Thanks Lozz, I had a look at your links....................I couldn't understand any of it (I'm not much of a mechanic).

[steve_earwig]

So maybe we should just leave it as "magic" and move on to see if it's actually working, yes?

[Welly]

Common Rail diesels don't really need glow plugs for starting, not unless it's about -10deg C outside. They 'assist' starting and remain on for a bit after starting to help with emissions.

Poor-starting HDi's are normally a problem with a fuel leak, blocked fuel filter or even a worn fuel pump/injector problem.

Has the car had petrol through it before?

[bigknickers]

Thanks welly, here's the thing, if what you say is right, the car would be difficult to start every morning, not just when it's cold, or am I wrong?

Steve E, you're absolutely right, we need to fix the problem, but it's a sensible man that wants to find out how something works too don't you think?

[jasper5]

From a mechanic's point of view you should be checking and eliminating possible problems!

Check those glow plugs before you do anything, they do fail even though the light rarely comes on and they are not needed until the temperature gets very low.

[bigknickers]

I've tried to find a place to measure the voltage to the glowplugs but there is no bare wires to put the meter tips onto............am I supposed to strip the wires?

Secondly, the car is really slow to pick up, so, I'm thinking it may be the MAF sensor, so I looked at it (super clean) so I thought maybe it's a dodgy Ebay type ripoff item......I'll pull the plug off n see if the performance improves BUT I couldn't get the plug off, does it just need a stronger yank?, or am I just a hopeless gibbon ?

[jasper5]

I've tried to find a place to measure the voltage to the glowplugs but there is no bare wires to put the meter tips onto............am I supposed to strip the wires?

Take off the connection that goes to each glowplug and use a test lamp to test each plug....fit the test lamp to battery positive and touch the tip of the plug...if the light lights up the plug is working.

If you want to see the plugs working take them out and put them across a battery with a jump lead....positive to the electrical connection and negative to earth on the plug.

[trufflehunt]

BUT I couldn't get the plug off, does it just need a stronger yank?, or am I just a hopeless gibbon ?

If I recall correctly, on the underside of the MAF sensor plug, there's a lug holding it in place. Think I just unbolted the
2 bolts on the whole MAF unit, so I could lift it up, which gave me access to the underside of the plug.

[bigknickers]

Good man Truffle, even pulling off a connector requires specialist knowledge!!!

[steve_earwig]

Should be a wire clip on the underside, push it into the connector and it should release. Hold on, I think I have a pic...

I've tried to find a place to measure the voltage to the glowplugs but there is no bare wires to put the meter tips onto............am I supposed to strip the wires?

You should be able to get to the nuts holding the connectors to the glow plugs. The ones above the injector feeds here:

I reckon you should check for voltage first before disturbing anything, as it was the lack of warning lamp you were concerned about. After that we can move on to seeing if they're still alive...

[Doggy]

Just to add my 2p worth, the glow plug light does NOT tell you when they're energised. It indicates when you should wait before turning the starter. Don't be misled by the warning light.

[bigknickers]

Thanks for all the info lads, would I be right in thinking that I can only measure the voltage on a COLD day when the glowplugs will actually be switched on?

[GingerMagic]

I'm no electrician - but I should imagine putting power directly to the glow plugs would bypass the temperature sensors that would turn them on, and the plugs should, er well, glow..