Glow plugs

Goglio704 · **Posted:** Sat Jan 05, 2008 8:52 pm

I've been doing a little research on the CRD glowplug system. I've had somewhat harder starting since the weather turned colder. I don't think it is glowplug related, but it gave me some incentive to go poking around.

Things I've found so far:

1. With the engine at about 30F, the glowplugs were showing about 1 ohm of resistance each. This was measured at the connector for the Bosch glowplug control just behind the battery.

2. At this temperature, the glowplugs initially are powered at 7 volts or just under. They drop to about 4 or 4.5 volts at about the same time the glow plug light goes out and float around at this voltage for 15 or 20 seconds before turning off if the engine is not started. If the engine is started, they float around between 3 and 5 volts for a long time. I'll try to define this time more closely, but I know it was at least 30 to 45 seconds. It may be dependent on either coolant or charge air temperature.

Things I want to find out:

1. Do the glowplugs change resistance based on temperature?

2. Is there a clearly discernible point at which the glowplug system shuts off?

Uffe · **Posted:** Sun Jan 06, 2008 8:50 am

For starters I believe you should always use google and wikipedia for research. I just spent ~10 minutes to find out more. I'll post what I found. I do not believe the glow plugs themselves change resistance noticeable. I know the voltage across the glow plugs are controlled by the BOSCH controller to perform the pre-heating and after-heating function. For pre-heating you need high power (higher voltage) and for after-heating you need just to maintain high temperature and thus less power (lower voltage).

I'll give you a bit of information which is general to most glow plug applications. This covers the very basics of glow plugs.

Tomorrows technician wrote:

Inside each glow plug is a high resistance heating coil. The plugs are designed to handle high current loads (up to 300 amps in some applications) so they will heat up quickly. Within four seconds or less, most glow plugs on newer vehicles will reach a surface temperature of 850 C (1,562 F), though on some older applications the wait period can be 30 to 60 seconds or more. Glow plugs in a fast-start system take only about four seconds to reach 1,000 C (1,830 F). The higher operating temperature combined with a faster heating rate means less waiting and less cranking to start the engine.

Normally, current to the glow plugs is cut off when the ignition is turned to the start position. Since the late 1990s, glow plugs have been designed to remain operating while the engine is cranking and for a short period of time afterward to keep the fire lit while the engine warms up. The after glow period following startup may range from 20 seconds to a couple of minutes depending on the application, ambient air temperature and engine temperature. When the glow plugs are no longer needed, the system control module or thermo time switch de-energizes the glow plug relay, cutting off power to the glow plugs so they can cool down.

Go here for the full story:

http://www.tomorrowstechnician.com/Arti ... esels.aspx

More information:
http://en.wikipedia.org/wiki/Glow_plugs

Take a look under problems and maybe the glow plugs are "worn" because you live a cold place. Perhaps replacing them will give you the boost you need to get back to good cold starts again.

More here with waveforms: http://www.picotech.com/auto/glowplug.htm

Goglio704 · **Posted:** Sun Jan 06, 2008 10:56 am

Thanks for the links, but that wasn't really what I was looking for. The two questions I posed are items that I intend to find through hands on research on the CRD. I'm not looking for internet generalizations here. I've owned and worked on diesels (mostly my own) for over twenty years so I am not looking for diesel newby info. I want to gather and post real world, application specific data. If a couple of other people do the same, we'll have some useful data. Some of this may or may not be in the service manual. I recently ran across a place in the diesel section of the manual where it mentioned how the fuel primer pump would turn on when the key is turned on. :roll:

This is either a carryover from the 2.5 service manual, or a very prophetic statement from someone who knew a lift pump would be needed. :wink:

FYI, many newer glow plug designs incorporate some form of thermistor to change the glowplug's resistance in response to heat and make it somewhat self regulating. My Nissans have this type of plug, and the plug's resistance changes dramatically in response to heat.

http://www.ngk.de/NGK_glow_plugs_viewed ... 689.0.html

I'm sure Bosch has a similar product.

I found the Wiki recomendation of replacing glow plugs annually laughable - especially for a KJ.

Uffe · **Posted:** Sun Jan 06, 2008 11:47 am

On boschs site I found their plugs change resistance with temperature indeed. My statement regarding resistance vs. temperature is thus most likely false for newer GP designs.

http://www.boschautoparts.co.uk/pcGlow5.asp?c=2&d=1

I am sorry I didn't know what type of information you were after, to me at least this wasn't clear. I have about 2 years of experience owning a diesel car, obviously I cannot supply you with much new information. Did you read the link I posted with waveform analysis of a certain type GP? I found that interesting as they have used a better way of representing a true time series of measurement points.

If you truly want information regarding the GPs in our CRDs you should be able to gain much information from attaching a HALL sensor to the CRD and an oscilloscope to capture the time series of a cold start. Channel 1 should record voltage, channel 2 should record current. That way you can exactly determine the amount of energy put into the GPs. And you can also determine the resistance of them as well. Temperature will be tricky, but can be done using extensive research on the material used in them and the mass of them.

gmctd · **Posted:** Sun Jan 06, 2008 12:28 pm

Standard glowplug elements are of nichrome, same stuff used in your electric toaster and ovens and stove tops - the heating element does change resistance as temperature changes, which is why glowplugs in a 12v system are designed to operate at some voltage between 6-9v - usually the glowplug harness is designed to drop system voltage such that the 'plugs glow to orange\yellow repeatedly without burning out

The ceramic glowplugs are similar to those quartz radiant heaters that debut'ed in the '80's, the quartz headlamp bulbs, zerox\ibm printer\copier elements, etc, where touching the element with bare fingers ruins the element because the natural skin oils\salts etch the quartz, which will then crack due to the temperature extremes.

The quartz elements also change resistance with current\heat, so must be designed at lower voltage to attain the correct heat range - the lighter elements in Diesel fuel can ignite ~500deg at atmospheric pressure, cranking cylinder presures are ~350-400psi, cylinder temps are ~1200deg, bright orange\yellow glow is ~1450deg.

The glow plugs could be used for measuring cylinder operating temperature by use of a specific range low-ohm meter with high resolution, which could also monitor real useful life of each 'plug - the front module\ECM monitors voltage on each plug, which must be within specific range indicating a functional glowplug, or will DTC.

Goglio704 · **Posted:** Sun Jan 06, 2008 1:15 pm

Uffe,
I could borrow a Fluke scopemeter from work. I don't think it has a Hall Effect CT or a shunt for measuring DC current, but I could probably come up with something. That would provide the definitive data, but it wouldn't help the average CRD owner much since they couldn't run the same tests themselves. I am looking to gather info on a presumed healthy glowplug system and post the info for future reference and comparison. Info gathered with a standard multimeter will be much more helpful to the average owner since anyone with a multimeter can reproduce the tests themselves.

Glow plugs

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