(NOTE: under construction...comments/suggestions welcome!!!)
On a cold mornings the heat of compression alone is not
enough to get our beloved diesels going; they need some additional heat that is
generated by 4 glowplugs.
Glowplugs are small sparkplug-looking devices mounted in
each cylinder that glow red hot.The
additional heat they generate is critical for reliable starts when the engine
is cold, and when the system is not working properly hard starting results.
System Components:
The glow plug system consists of 4 main parts:
1. a glow
plug relay that controls the flow of power to the glow plugs
On MK2 chassis the relay is a tall
rectangular black box that lives on the left-hand side of the fuse/relay plate
(c)2009 Gord McFarling
On MK3 chassis the relay is a
small square silver box that lives on the right-hand side of the relay
plate.It’s controlled by the Engine
Control Module, a big black relay found on the left hand side of the relay
plate.
2. a glow
plug fuse
On MK2s the glow plug fuse is mounted on the firewall in the engine
compartment, above and to the left of the brake booster.
On MK3s the fuse can be mounted on
the firewall as well:
And it can also sometimes be found on the fuse block, as per the picture of the MK3 fuse block above
3. a
coolant temperature sensor that tells the glow plug system how long to run the glowplugs
MK1s and early MK2s use a brass
temperature sensor mounted on the driver-side head coolant flange:
(c) 2009 Bobby LeClaire
Note that there are two identical coolant sensors, one for the glow plug relay and one for the temperature gauge. The glowplug sensor usually has an XXXX wire.
Later MK2s use a
plastic-encased coolant sensor usually mounted on the central head coolant
flange and has two green wires:
(c)2009 Gord McFarling
4.the
actual glow plugs themselves
Small sparkplug-looking devices
mounted below each injector and connected together with a copper buss bar:
(picture)
Operation:
As mentioned, the glow plugs are
controlled by a relay system.The
system looks at coolant temperature to decide if the glow plugs are needed, and
for how long.On MK3 chassis the system
also notices when the drivers door is opened.
A yellow light on the dashboard
tells you when the glowplugs are needed and how long to wait before starting
the engine.The yellow light is NOT
related to whether the plugs are actually getting power, an unfortunate nuance
that often confuses folks trying to troubleshoot the system..
The relay supplies power to the glowplugs thru heavy duty wiring and a large
fuse (usually 50 amps).
On MK1 and MK2 chassis the relay
shuts off power to the glowplugs as soon as the engine’s starter is
engaged.On MK3s the system continues
to power the glowplugs for several minutes after the engine starts; this “afterglow”
is designed to reduce emissions and improve cold engine driveability.
Troubleshooting:
Troubleshooting the system
consists of systematically tracing the flow of current to the glowplugs and
then checking operation of the glowplugs themselves.
Tools needed:
-test light or multimeter
picture
-(optional) 50A ammeter…a cheap dash-mounted unit from NAPA or
the like works fine
.
I've put mine in a plastic box and added alligator clips since the meter seems to get loaned out often.
Step 1: Check for power at the glowplugs themselves
Using the multimeter or test light, turn the key to “on” and
then check to see if power getting to the glowplug buss.Power should appear for 3-20 seconds,
depending on coolant temperature.
If power is getting to the glowplugs proceed all the way to
step 5
Step 2: check the glowplug fuse
Using the multimeter or testlight, turn the key to “on”
andthen check to see if there is power
on both sides of the glow plug fuse.The fuse is known to often generate hairline fractures that are hard to
see; pulling the fuse out will sometimes reveal a crack.
Step 3: check coolant sensor operation
Find the connector on the end of the coolant sensor and
disconnect it.Turn the key to on and
then check for power at the glowplug fuse… the relay should now stay on for
approximately 20 seconds.If it does
the glowplug relay is probably fine... to confirm the problem is the coolant sensor a multimeter is required:.
(text about checking resistance of coolant sensor here)
Step 4: glow plug relay
If all of your tests have still not identified a fault the
next logical suspect is the glow plug relay itself.Unfortunately it’s a bit tricky to access and has quite a few
connections; the easiest way to check its operation is to install a known good
relay.
If you do want to troubleshoot the actual signals at the MK2 relay itself the pinout looks like this:
Step 5: check operation of the glow plugs themselves
The glowplugs are wired in parallel via the glow plug buss
and have a very small resistance; this makes them difficult to test using a
multimeter. There are a couple of options:
1)connect an ammeter capable of measing at least 60 amps between
the positive terminal of the battery and the glow plug buss.The meter should regester over 60A and then
settle down to somewhere between 40 and 50 amps. Each glow plug will draw 10-12
amps, so if you get a reading of less than 40 amps one or more glow plugs are
defective.
2)Disconnect the glowplugs from the buss bar and then use a
multimeter or test light to check for continuity to ground.Disconnecting the glow plugs from the buss
bar is a pain, particularly the two plugs behind the injection pump, but the
plugs can not be tested when they are all connected in parallel.
3)Remove the injectors and check that the plugs glow red hot by
peering down the injector holes.
Common problems and causes:
Symptom: no power to the glowplugs at all
Probable causes: glow plug fuse, glow plug relay
Symptom: yellow glow plug LED stays on for a long time, regardless of engine temperature
Probable causes: on cars equipped with a water seperator a flashing glow plug light indicates that the water seperator is full or defective
Symptom: glow plug glows brightly at first, and then dimly
Probable causes: water has dripped down into the fuseblock and corroded the glow plug relay contacts or socket. This is particularly common in older MK1s and MK2s with rust around the windshield and/pr radio antenna mount.
Other random thoughts:
-Because the glow plugs draw a lot of current good ground
connections are important, so it’s a good idea to check the various ground
cables, particularly the battery ground connections and the connection between
the chassis and the transmission
-The glowplugs are also very demanding of the battery,
particularly during cold weather.A
good battery and charging system is an important part of cold-weather starting.
-Bosch Duraterms are generally thought to be the most reliable
glowplugs available on the market.They
incorporate many 3rd generation technology features and are compatible
with all glowplug systems.On the other
hand, Autolite glowplugs have a
terrible reputation.
-Whenever I find a defective glowplug I tend to replace all
four.It’s such a pain to remove the
buss bar to find the problem that I try to do it as little as possible, and it
seems that when one plug goes others are generally not far behind… it kinda
makes sense that they have a similar lifespan.
-I’m a big fan of a light on the dashboard that shows actual power
is being delivered to the glow plug buss.Unlike the stock yellow dashboard light it will show you when a fuse is
blown or the relay is misbehaving.
My 83 Turbo Diesel Rabbit just started with this problem. For a while the glow plug light stays on for 60 seconds before shutting off. The light comes on for 60 seconds no matter what the temperature of the motor. There is 12v power going to the glow plugs when the light is on. Today the light began coming on while driving and the relay clicks on and off rapidly as the light flickers. I disconnected my glow plugs at the fuse on the firewall to be safe while driving around.
Bad Relay?
Hi Jason... yes, if you simply had the relay staying on for 60 seconds regardless of temperature I'd suspect the coolant temp sensor and/or the wiring between it and the relay, but if the relay is intermittently coming on while you're driving it's more likely to be the relay or corrosion in the relay socket. As our cars get older water leaks down the antenna opening or around the windshield and into the relay panel... causing all kinds of strange behavior. Might be worth a look before you spring for a new relay.
cheers,
Vince
... written by armando,
March 26, 2010
hey there vince, your site is great and very useful. i have a question, i have a mk3 jetta td, im getting power to the glow plug and to the dash, but i cant seem to get the car started. iv been using ether sad to say, i live in canada so it still cold over here lol. im going to trouble shoot somemore. and another question, my motor was re built and its making a hard knocking noise, sound like a jackhammer, i changed the piston rod barrings and when i started the car the noise was gone for like 10sec then it came back. any ideas, my brother inlaw said it neeeds over sized barring. thanks hope to hear from u soon.
Hi Armando... thanks for your comments.
In terms of your glow plugs, it sounds like you've confirmed that they are getting power. I'd recommend continuing with the diagnostic... the next step is to remove the buss bar and check each individual glow plug for operation... as detailed in the how-to.
Unfortunately I can't comment on your bearing noise...engine sounds are real tricky over the internet and I'd hate to lead you astray.
cheers,
Vince
... written by Chuck,
November 21, 2009
Vince: As a new owner of a 1981 Rabbit diesel pickup, I find your articles extremely interesting ... and useful for this 'rookie'.
I have noticed one thing I'd like to pass along to you; and hope you don't think I am interfering. In trying to print the articles out, the pictures don't seem to want to print.
Then I noticed you have the pdf symbol, and thought that's the answer. But, again, in looking at it, wherever the picture is from the article, in the pdf file is nothing but a blank.
Just a 'heads up'. Hopefully you can track down why the pdf is missing the pictures ... which are of great value.
Looking forward to other articles from you in the future ! Chuck
Hi Chuck... thanks for your comments.
In terms of the pdfs, yeah, there's a bug in the library that comes with this version of Joomla that causes the pictures to be missing from the pdfs.. my web hoster is keeping their eyes open for an upgrade that will resolve this one.
Simply using the browser to print, or clicking on the printer icon next to the pdf icon, seems to work for me... if you send me your operating system and browser version we can investigate further.
thanks again for your comments,
Vince
... written by Nils,
November 19, 2009
Just wanted to say thanks for all the useful, clear-written information that you provide! It really stands out as an example.
... written by wes,
November 09, 2009
Hi Vince, I have to admit I am amazed at the clarity that you put forth in you how to postings. It makes it seems silly for me to suggest something to you or others. I have a simple suggestion- if the electrical circuit tests OK and that is if someone is trying to diagnose if they have failing/failed glow plugs I have found that if you have the fuel injectors out, turn the key on and look down into the injector holes. If the injectors are cold before you do this then its possible to identify which ones are not glowing with the intensity of the others and/or slower to reach "peak" glow (compared to the others in the series). Dimming the shop lights or doing it while dark outside helps to that effect. I know you should change them in sets but for a out and out failure I dont have a problem going cheap. ( Save The Planet ! )
Hi Wes... heck no... not a silly suggestion at all... the best ideas are often the simplest!!
In fact... I have a spare head on my workbench with 4 glow plugs and no injectors in it as we speak, and was thinking about taking a picture of the plugs powered up... your suggestion tears it!!
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Bad Relay?
Hi Jason... yes, if you simply had the relay staying on for 60 seconds regardless of temperature I'd suspect the coolant temp sensor and/or the wiring between it and the relay, but if the relay is intermittently coming on while you're driving it's more likely to be the relay or corrosion in the relay socket. As our cars get older water leaks down the antenna opening or around the windshield and into the relay panel... causing all kinds of strange behavior. Might be worth a look before you spring for a new relay.
cheers,
Vince