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Robbie's Guide To Glow Plug Diagnostics
Robbie's Guide To Glow Plug Diagnostics
Introduction
1. A somewhat unloved system but I've had a few requests for a guide on how to diagnose glow plug issues on the D3 / D4 and RRS diesels. The guide will not cover the Jedi skills required to remove the glow plus, as that is covered elsewhere on the forum, but will look to explain the glow plug system, the functions it provides, fault symptoms and effective DIY diagnostic techniques. Whilst glow plug problems are synonymous with cold starting issues (and therefore should be considered in the context of battery, injector and fuel pump health) they do more than just provide pre-start warming.
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Glow Plug System Overview
2. The 6 glow plugs are mounted in the valley of the engine pointing slightly outwards to their respective cylinder and are wired via 2 harnesses that provide power to the 2 banks of the engine. Each wiring harness is powered via a big fuse and a glow plug relay, which is controlled by the glow plug module. On the D3 this module is part of the engine ECU and cannot be serviced separately, but on the D4 there is a separate module under the control of the engine ECU. With a few minor exceptions the D3 and D4 glow plug systems are functionally identical. The glow plug power system for the D3 is shown below:
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Glow Plug Operation
3. Each glow plug receives full system voltage and up to 15 amps. The tip of the plug is designed to rapidly heat to over 1000 deg C in just a few seconds from the high current flow in the fine wire spiral of the heating coil. The spiral itself is compressed in a ceramic powder that prevents shorting whilst providing high thermal conductivity. The plugs are self regulating; as the regulating coil is heated, due to heat transfer from the tip, its resistance increases and the current is reduced to the point where the tip temperature is stable with no risk of overheating (around 3 to 4 amps).
Glow plug sectional view:
Click image to enlarge
4. Unfortunately modern direct injection cylinder heads are already rather crowded with 4 valves and the injector and have sufficient strength to withstand the diesel cycle. This has driven the miniaturisation of glow plugs to the point where those in the D3 & D4 are just 8mm thick. This gives them only a 10 Nm maximum torque value and a removal sheer strength of just 20 Nm. Think broken Bic biro or perhaps dry spaghetti when thinking about the force they can withstand on removal. No really, they are that bad. The removal problems can be further exacerbated by failure modes. Leaking injectors can melt the tip and the tip itself can swell slightly as it ages. As a failed glow plug will lead to additional carbon building up in the cylinder this can cause a wedge of carbon between the swollen tip and the plug bore, making removal without breaking very difficult if it has been inoperable for some time:
Glow Plug Functions
5. On modern low compression direct injection diesels the glow plugs do more than provide a pre-glow function for easier starting. The plugs are effectively used in 3 modes: pre-glow, starting and an extended afterglow mode. As the plugs are self-regulating the glow plug operation is varied by timing as directed by the engine ECU. It is important to note that the sole reference used for the ECU glow plug operation is Engine Coolant Temperature (ECT). Whilst the injectors use a combination of readings from various temperature sensors (intake, oil, coolant and boost temperatures) the glow plug system is dependant on the ECT sensor (the D4 also makes use of barometric pressure for ski-holiday mountain starts, but is still dependant on the ECT sensor). The dash glow plug light does not directly indicate glow plug operation or timing. This has caused confusion and some urban myths regarding cycling the ignition again when the light goes off to provide additional pre-glow heating. If it is seriously cold the glow plug light may go out after a few seconds but the glow plug will remain heated for up to 25 seconds. If you want to extend the pre-glow timing then count to 20 before cranking. The maximum pre-glow timings are:
-30 deg C = 25 seconds
-20 deg C = 25 seconds
-10 deg C= 22 seconds
0 deg C = 19 seconds
+10 deg C= 12 seconds
+20 deg C = 8 seconds
+40 deg C = 4 seconds
+60 deg C = 2 seconds
In addition, the glow plugs are commanded on for the start and immediately afterwards for the timings stated below:
-30 deg C = 60 seconds
-20 deg C = 60 seconds
-10 deg C = 40seconds
0 deg C = 20 seconds
+10 deg C = 10 seconds
+20 deg C = 5 seconds
+40 deg C = 2 seconds
+60 deg C = 0 seconds
6. The final and perhaps least understood mode is afterglow. This required when the engine is not at normal operating temperature. It ensures clean and efficient burning of the cylinder charge, crisper throttle response, reduced hesitation from a trailing throttle, reduced emissions, better mpg and reduced carbon deposits in the cylinder. Put simply, the engine will not thank you for putting up with dead glow plugs.
Glow Plug Testing
7. There is precious little in the workshop manual on glow plug testing and the individual plugs are not monitored by on-board diagnostics. Some of the glow plug manufacturer's suggested methods are almost laughable, recommending removal from the cylinder for bench glow-checks. If you could get them out for testing so easily you would just replace them as they are not expensive. Resistance checking in situ is more credible but can involve digging down through other components to reach the plugs. Resistance checks are useful but some failures only manifest themselves once current is applied, so in my view they are far from definitive.
8. As discussed earlier, functioning glow plugs draw up to 15 amps when energised with the current dropping rapidly when they self-regulate; it is these values that indicate glow plug health. On the D3 & D4 this current draw is very easy to monitor with a DC current clamp. As the amperages are quite high even a cheap DC capable clamp is more than capable of checking the system.
9. As part of the diagnostics the ECT sensor should be checked for correct operation, as should the glow plug module control of the relays. Testing the glow plug system when the weather is cold is just no fun at all but the ECT sensor system can be easily tricked to read any temperature you like - this can be a sunny day job.
10. The ECT is just a simple NTC sensor dipped in one of the coolant lines. As the temperature gets colder the resistance increases. If you attach a suitable resistor to the disconnected ECT connector (I use backprobes to resistors in a homemade magic box) the glow plug system will react accordingly. The ECT sensor is easy to find on MY07+ D3s as it is located on a coolant pipe next to the air filter box, near the MAF sensor. On earlier D3s it is a bit more hidden but can be found under the engine cover behind the fan shroud quite close to the air intake manifold. It is the grey connector with 2 wires (the EGR pipes have been removed on this MY06 D3):
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Unfortunately JLR got a little more creative with the 3.0L engine and the ECT sensor is buried a bit deeper and mounted upside-down:
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ECT resistance values:
A fun-sized bag of mixed resistors:
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Live data can be useful for determining the actual reported ECT data, as well as double-checking your ability to read resistor colour codes:
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Using A DC Current Clamp
11. The easiest place to attach a current clamp is on the glow plug harnesses on the top of the engine. First remove the oil cap, then the engine cover before immediately replacing the oil cap to availed dropping something down the engine:
Click image to enlarge
12. The glow plug harnesses are colour-coded, with a brown connector for the right hand bank of cylinders and a black connector for the left. On the black connector on the right side of the picture below you can see 3 wires that have been gently separated; each wire serves an individual glowplug:
Click image to enlarge
13. A suitable DC amp clamp should be zero'd and attached to the wire(s) with the ignition off. In this condition you should expect zero current flow. If you are not expecting issues you can clamp all 3 wires at once or check each wire in turn.
Click image to enlarge
14. For the ignition on check you are expecting to see an initial draw of around 45 amps for all 3 wires, or around 15 amps on an individual wire. I typically see a peak of around 14.8 amps before the current is reduced by the regulating coil. In addition to the amperage you should be checking that the timings are appropriate to the sensed ECT. All modes should be checked, including the afterglow function with the engine running, here showing 3.8 amps:
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Summary
15. Checking glow plug operation is not difficult and does not require Jedi powers. Glow plugs are important for correct engine operation and (in my view) should be replaced when a failure is detected. Indeed, delaying glow plug removal will only see more carbon added to the glow plug tip which will not aid in their removal.
16. As ever, there is a mountain of experience on this forum so feel free to add your own top-tips or to ask questions. Taken together with the stuff I have written on power drains, battery health and the fuel pumps this should cover most of the systems that suffer in cold weather or cause poor starting. Glow plugs are often overlooked but can cause symptoms that are easy to misdiagnose. Indeed, a small hesitation when moving from a trailing throttle to a rapid throttle demand could just be down to the glow plugs. If they were easier to remove I am sure we would all be doing so as a matter of routine!
Regards to all,
Robbie
Land Rover - Turning Drivers into Mechanics Since 1948
Member Since: 15 May 2013
Location: Chester
Posts: 7633
Well, if it's a necessity then
It'd be rude not to
Time for some clamp meter studying then
2nd Jun 2015 7:41 pm
derryfrombury
Member Since: 01 Sep 2014
Location: Bury St Edmunds
Posts: 212
Excellent explanation, supporting photos and very clearly written. I'm sure will be a godsend to many in the future.
DerryJoined Aug 2014. Disco 3 SE 54plate, Buckingham Blue, Cream leather.
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2nd Jun 2015 8:34 pm
hugeviking
Member Since: 08 Jun 2010
Location: cotswolds
Posts: 1482
Excellent, thanks Robbie
Considering their importance, its crazy glowplugs are so hard to replace, although I had never considered how crowded the cylinder head was, wonder how much difference a two valve cylinder set up would make, might be worth it if the glowplugs were a doddle to swap.
Hopefully one for the Wiki.
Andi.
2nd Jun 2015 8:44 pm
maplecottage
Member Since: 01 Feb 2011
Location: Norfolk
Posts: 3171
tayaste wrote:
Nope, I seem to own one that appears to have come from a Christmas cracker
This thread is pure gold. I know it's not considered "kind" to say no these days, but no. Just no, ok? And if it's not ok, still no.
2nd Jun 2015 9:52 pm
tayaste
Member Since: 15 May 2013
Location: Chester
Posts: 7633
Maplecottage , how does yours fair up?
2nd Jun 2015 9:54 pm
DG Site Moderator
Member Since: 12 Dec 2005
Location: The Gaff
Posts: 50979
Nice write up Rob I don't know what it does ...don't know what it means ...but it's nice. 21 year LR veteran > D2 GS 2003 > D3 S 2006 > D3 HSE 2009 > D4 HSE 2013 > D4 HSE 2015 > D5 HSE 2018 > DS HSE R-Dynamic P300e 2021
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