Voltage To Glow Plugs

[rg1642]

Hi,

While trying to sort the starting problem with my wifes 1.8 TDCI Focus (It's the much recorded fires up and then stops, firesup, runs lumpy with throttle, clears, then runs perfectly for the rest of the day problem) I noticed that while the battery is putting out 12.75v only 10.5v is getting to the glow plugs. The glow plugs are all operational but could the low voltage have any bearing on the starting problem and what could be causing the reduced voltage?

Thanks

Russ

[jeebowhite]

sounds more like faulty plugs than a faulty voltage?

[rg1642]

Plugs were new 6 months ago and I checked the resistance & continuity on them anyway when I was looking at it Saturday.

[jeebowhite]

Alternatively the harness might be an issue. I think it can be affected by moisture and salt from the winter roads and can result in an ineffective connection?

[rg1642]

Thats interesting. Is it a plug in to the main loom or a direct spur off of it?

[stef123]

It could be voltage drop... Given the load and size of the cable

[jeebowhite]

Unfortunately I am not too familiar with the setup of it, but I recall reading about the ingress somewhere recently looking into it for the CMAX.

[rg1642]

It could be voltage drop... Given the load and size of the cable

Whilst I'm not an electrical idiot, I have only just progressed from that level, lol. I know on my focus 1.8 TDCI I get plus 12v on the feed to the plugs, just my wifes is lower. Is voltage drop a fault or does it just happen? Ta

[stef123]

Are those voltages with the engine running?

[rg1642]

No, thats as I turn the key to warm up the plugs.

[stef123]

What are you getting at the plugs with the engine running?

[rg1642]

Dunno? Not tried that. Will have a look at weekend.

[FOCA]

It could be voltage drop... Given the load and size of the cable

Exactly what i was going to say - as the load increases (due to the resistance of the glow pugs) the voltage will drop, 10.5v going to the glow plugs may be quite normal, under load, the battery may not be 100%, with a newer, or more powerful battery, there may be less voltage drop

To give you an idea i have a digital vottmeter gauge in my car - after an overnight lay-up, the voltage starts at 12.5v,(if the car is left longer, it will drop lower, after 2 weeks laid up, it can drop below 12v, if it ever does this, i recharge the battery) i run a smaller battery than normal to save weight and a solar panel to help keep the battery charged

When i turn the key to put the glow plugs on, the voltage drops to 10.5v, then, when i turn the engine over, it drops even lower, like 9v or zero on the voltmeter, after the engine is started, the voltage reads 14.5v or more (smart charge system) (these are typical figures, i will check them later)

[rg1642]

Cheers bud. :) I can get my head round that. I asume therefore if the voltage drops the ability to draw the amps to heat the plugs would reduce and therefore take longer to heat up? The glow plug light on the dash only lights up for a second, the exact opposite. Thats if that light is illuminated until the plugs are up to temperature which is what I thought it was supposed to do?

[stef123]

The glow plug light doesn't tell you when the plugs are up to temperature but the colder it is the longer it will stay on - this is to give the plugs the chance to heat up.

[rg1642]

This one has only ever stayed on for a second since we got it 40k miles ago. Whatever the temperature. Mine varies from 1 to 6 odd seconds. What I am thinking is are the plugs only getting a seconds worth of ampage and therefore not getting hot enough to keep the engine ticking over after start up. I know that common rails are not supposed to 'need' the glow plugs to run but this is an 11 year old with 140k miles. The engine is still strong as an Ox and out performs my 2006 model. It is just a pig to start, in all weathers and temperatures. Get it going and brilliant car.

[stef123]

You are correct about common rails, they only need a bit help when the temps get very low.

If the car is a pig to start in all weathers and temperatures then I would say the glow plugs are probably not the problem.

If you can measure the voltage going to the plugs when the engine is running and how long it stays on for.

[FOCA]

This one has only ever stayed on for a second since we got it 40k miles ago. Whatever the temperature. Mine varies from 1 to 6 odd seconds. What I am thinking is are the plugs only getting a seconds worth of ampage and therefore not getting hot enough to keep the engine ticking over after start up. I know that common rails are not supposed to 'need' the glow plugs to run but this is an 11 year old with 140k miles. The engine is still strong as an Ox and out performs my 2006 model. It is just a pig to start, in all weathers and temperatures. Get it going and brilliant car.

The ECU (engine control unit - they may call it different names to confuse us/ make it sound more "technical" but its still the ECU) controls the duration of the glow plugs and if/when they come on

The ECU reads the ambient air temprature, and the head temprature,(the actual metal of the head, not the coolant temp)

So depending on the head temprature the glow plugs are on for different lengths of time - if it is a cold winter morning, and the car has not been started, the glow plugs may be on for a lot longer than if the ambient temp is warmer or if the engine has been started previosly - it is all controlled by the ECU, if the engine is warm enough, the glow plugs may not come on at all - there should be know need to put the glow plugs on twice before starting - that may be counter-productive and may cause you to flatten the battery before the engine starts, you should not need to touch the throttle - (some owners do these things to help start the car but it can make matters worse - its a question of trial and error, but ultimately, the underlying problem should really be fixed)

you should crank the engine when the glow plug lights go out - the light may or may not correspond do the glow plugs actually going on

There was some discussion about whether the glow plugs stay on after the engine is started - well on older cars they don't, but on more modern cars - ie- on the Mondeo mk3 the glow plugs can stay on for upto 30 seconds after the engine is started, if the revs stay under 2k (apparently) this is mainly for emmisions/ to help the engine warm up quicker

Electrical load -

if you connect a load to an electrical source the voltage may drop the greater the load the greater the drop

if we compare it to a bicycle - you are turning the wheels with a constant power output in a fixed gear- if your back (driven) wheel is off the ground the wheel/ pedals will spin freely, if the wheel/ bike is dropped on the ground the wheel/ pedals revolutions will drop - the wheel/ pedal revolutions will drop further when cycling uphill than on the flat

In the same way the battery voltage will be high if its not connected to anything (like the bikes pedals/ back wheel spin fast with the back wheel of the ground)

Once a (realatively) light load is applied to the battery (like the interior lights or the central locking circuit) the battery voltage will drop a little (like when the bicycle is cycling on level ground)

Under a heavy load (like the glow plugs or the starter) the battery voltage will drop further (like the bike is cycling uphill)

Of couse, its a lot more complicated than that, the battery is not a constant source and progressively loses power

So the battery must power the glow plugs, then crank the engine with its own electrical power, only once the engine is started does the alternator power the car electrics and start charging the battery

A diesel usually needs a (more) powerfu battery than an equivelant petrol-engined car because lt has to power the glow plugs, then crank the engine, the complession on a diesel engine is much higher and it is more difficult to turn over

At colder temps a lead-acid (including silver-calcium) battery puts out less power, a diesel engine also is harder to start at lower temps too - so a diesel car may be fine all summer then be difficult to start in the winter

[rg1642]

I might just get a plug out and connect it up and SEE how long it glows for. I changed the head temperature sensor so that should be ok. So if it know the engine is stone cold they should come on for a few seconds at least this time of year.

We shall see tomorrow, or Sunday. :)

Thanks for the replies as well. :)

[rg1642]

Ok todays findings.

Took all the plugs out and checked them. All ok. Conected one up, linke in a multimeter and got 10.5v for 4 seconds then it diminished to next to nothing. However there was no heat whatsoever at the plug tip. Just to make sure I connected up the battery from my car and got 12.9v for 4 seconds but still no heat. Checked relay with the known good one from my car and just the same a good healthy click.

The only unusual thing was when I took the plugs out the first one, (cambelt end) looked like new and was wet? Couldnt tell if it was oil or diesel. All the others had grey deposits on them and looked used. I have done a compression check between the first and second cylinders and got 24 & 25 Bar. (It is a cheapy Silverline tester). I couldnt get to the other two cylinders without disconnecting the rail and at this stage I am not worried about compression as the car runs so well when going and has plenty of grunt.

I look on Youtube and all the 'test your glow plug' videos show the plugs glowing. These were stone cold.

Any ideas?

Ta

[mick85]

Did u test them off a battery an see if they light up

[FOCA]

Ok todays findings.

Took all the plugs out and checked them. All ok. Conected one up, linke in a multimeter and got 10.5v for 4 seconds then it diminished to next to nothing. However there was no heat whatsoever at the plug tip. Just to make sure I connected up the battery from my car and got 12.9v for 4 seconds but still no heat. Checked relay with the known good one from my car and just the same a good healthy click.

The only unusual thing was when I took the plugs out the first one, (cambelt end) looked like new and was wet? Couldnt tell if it was oil or diesel. All the others had grey deposits on them and looked used. I have done a compression check between the first and second cylinders and got 24 & 25 Bar. (It is a cheapy Silverline tester). I couldnt get to the other two cylinders without disconnecting the rail and at this stage I am not worried about compression as the car runs so well when going and has plenty of grunt.

I look on Youtube and all the 'test your glow plug' videos show the plugs glowing. These were stone cold.

Any ideas?

Ta

Check the resistance (in ohms) of each of the glow plugs if each is a low value (a few ohms) its ok, if it is open circuit or a high resistamce(several kilo/ mega ohms) - its a dud ,

Did you earth the glow plugs to test them? (they are earthed through the head)

it is possible to check the glow plugs without removing them, just disconnect the leads and chack the resistance of each glow plug individually

[rg1642]

I checked the voltage through the plugs and the resistance. All the plugs were ok. I assumed the plugs would earth through the multimeter or I would not get a reading.

[rg1642]

Did u test them off a battery an see if they light up

Didnt think of that. Wouldnt I need some big wire to do that with the amps it must draw to heat them up?

[mick85]

No a pliers piece wire wit two crocodile clips remove ur car battery connect live to tip of glow plug an hold glow plug wit plier touch side glow plug off earth an it should glow red hot