Glow Plug Relays - Circuitry

xmexclusive · Post by **xmexclusive** » 22 Jun 2009, 10:57

Hi All

Anyone got details of the timing circuitry in the typical glow plug relay. I am thinking about trying to increase the preheat time to get better starting with veg oil.

John

Mandrake · Post by **Mandrake** » 22 Jun 2009, 11:25

Are you sure that the glowplugs go off as soon as you think they do ?

I'm under the impression that (at least on a 1.9TD Xantia) although the glow plug light on the dashboard goes off after about 8 seconds, that the glowplugs continued to be powered for a lot longer than that, at least 30 seconds maybe more.

Can anyone else confirm this ? I've certainly found it a lot easier to start sometimes if I give it an extra few seconds after the glow plug light goes out before starting, and that wouldn't be the case if they went off with the light...

Have you tried just waiting a few extra seconds after the light goes out before turning the key ?

Regards,
Simon

Post by **RichardW** » 22 Jun 2009, 12:19

You're right Simon, the plugs do not go out at the same time as the light. You can tell when the plugs switch off if you leave the ignition on without starting it - there is hefty clunk from the relay as it drops out

Xaccers · Post by **Xaccers** » 22 Jun 2009, 12:34

If you're using straight veg oil and having trouble starting, then it may be time to fit new plugs.
Also check for air leaks and the state of your fuel filter (including the mesh one in the tank under the rear seat).
My diesels start first time every time on SVO except when it's actually icy, then it sometimes takes two heating cycles.

xmexclusive · Post by **xmexclusive** » 22 Jun 2009, 14:31

Hi All

Thanks for the replies. Air and filters checked. I was aware of the post running of the glow plugs as well as the pre running. The car was starting fine on mineral diesel but now stutters or stalls on the first start of the day (engine cold) and this is with only 50% veg. Double running the glow plugs solves any problems on the first start but I forget to do so most of the time.
I was just thinking that it would be interesting to know how the timer duration was controlled. From the fact that there is a need for post running of the glow plugs suggests that even mineral diesel needs warming in a cold engine. The fuel that is used for a first start must already be in the injector feed pipes and the injector pump so none of the heating systems you can fit will heat this initial charge so it is all down to the glow plugs to produce a perfect start. If tinkering with a resistor or capacitor to change the timing in the glow plug relay is going to make glow plug condition less critical then I though it was worth considering. Going to 100% veg at next fill up so will see how much difference that makes.

John

Xaccers · Post by **Xaccers** » 22 Jun 2009, 15:37

Jenny had the same problem of poor starting, needing two lots of plug cycles, swapped out two dead glow plugs (the easy to reach ones) and she's much better.
One day I'll get round to replacing the other two!

Take out the two the air filter end, and test each one on the battery using jump leads.

Make sure you've fitted ones that are designed to heat for a long time as there are some that just heat for a short period and they will burn out quickly in a XUD

uncle buck · Post by **uncle buck** » 26 Jun 2009, 00:25

This how the glow plugs worked on my ZX 1.9 TD when i had it...I would imagine the operation will be the same on all of the 1.9 TD engines.

If you switch the ignition on but don't start the engine the plugs will stay on for about 25 seconds (the glow light on the dash goes out after about 10 seconds)..but if you listen carefully you will hear the relay click off a while later.

If you start the car from cold the plugs will stay on for approx 3 minutes after the engine is started or until the coolant temp reaches 60 degrees...which ever comes first.
This 3 minute post heat cycle is to keep emissions down & to stop diesel knock until the engine warms up a little.

If the coolant temp is 60 degrees or more & the engine is started the plugs will only stay on for the 25 second period.

My car (1994) didn't have it but later cars had a micro switch on the pump throttle lever...if this switch is operated for more than a few seconds (5 rings a bell) it also interrupted the 3 minute period.
I believe in order to operate the micro switch the throttle peddle has to be pressed more than half way.

A normally aspirated 1.9 D engine only ever has the glow plugs on for approx the 25 seconds...there is no 3 minute post heat on this engine.

I took the relay unit from my TD apart when it wasn't working correctly & it's quite a complicated unit inside...rather than trying to alter it I would just stick to double heating ...it's a lot simpler.

Also I would advise you (from my own bitter experiance) to keep away from NGK glow plugs on the TD engine....they are rubbish & cause poor starting...don't know if they are ok for the normally aspirated engine.

Cheers.