HOW-TO: Troubleshoot Your Glow-Plugs
Written by Vince Waldon   
Saturday, 26 September 2009

HOW-TO:  Troubleshoot your glowplugs


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:







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 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


-         (optional) 50A ammeter…a cheap dash-mounted unit from NAPA or the like works fine

. hi_current_meter.jpg















 I've put mine in a plastic box and added alligator clips since the meter seems to get loaned out often.Wink



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” and  then 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:  defective coolant sensor, broken wire to coolant sensor, glow plug relay


Symptom:  glow plug light flashes

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.


Comments (13)Add Comment
written by Kenann, November 01, 2015
Hi Vince,
my '89 mk2 1.6 diesel automatic had a problem with starting on cold weather. I found your post and started troubleshooting. My relay was dead. On my Golf its marked with number "60" but its the same thing. Relay from '84 mk2 that I put in my Golf is super fast 5 sec. and its ready to start.
Thank you for all explanations and tips.
written by john , March 13, 2015
hello from GREECE.i have a caddy 2000model with 1.9 d engine.when the Car is cold the indicator light not flashing and the engine can start.when i disconect the baterie for seconds everything good also sometimes disconect the cooland sensor also good ,it starts.with hot engine everythings Works good.the coolamd sensor is NEW, what to do next i dont know.thanks

Hi John... well, you're replaced the most obvious part...unfortunately it doesn't seem like it helped.

If you can find someone with the VCDS software (also known as Vag-Com) a good next step might be to see what the ECU actually thinks the coolant temperature is... might help you find a wiring issue perhaps.

My other suggestion would be to have a look on for a thread called "Glow Plugs 101 Version 2" (or thereabouts)... it's their master thread on troubleshooting glow plugs on a TDI engine and has lots of great ideas, pictures, etc.

Hope that helps,

written by Dennis Tomlinson, November 25, 2013
When checking voltage at the glow plugs, also check at the battery positive post when glow plugs are on. If you are showing 9.5 volts at the plugs, but 11.0 at the battery post, you are dropping 1.5 volts through the circuit feeding the glow plugs. Easier and faster starts can be achieved by adding a starter relay on the firewall and feeding the relay directly from the battery post with a 8 gauge wire and from the relay to the glow plugs. Add a wire from the glow plugs to the relay to energize it. This sounds wierd but you are making a more direct path for the current to flow to the glow plugs. DO NOT remove the wire feeding the glow plugs and make it energize the relay instead. The glow plug controller in the car needs to sense the voltage drop to make the "on time" for the glow plugs correct.
written by Joe, November 16, 2012
I have been trying to trouble shoot my cold start problems, on my 82 Jetta with a 1.6na. I followed your steps and found that I am only getting 9.6 volts at the buss bar, this gradually increases as the relay stays on but has never gotten above 10.4 before they shut off. Is this normal operation? I cleaned the connections that I could get to.

Hi Joe... the glow plugs are a very large load that starts out at around 50 amps when they are cold and becomes around 30+ amps as they heat up. This is why you see pretty large voltage drop that gets a bit better.

9.x volts in not uncommon on an older car with a fair amount of resistance between the battery and the glowplugs, and it suggests that all of your glow plugs are working (still best to confirm them indivually of course). It's probably worth measuring the voltage directly at the battery as well... if it's around the same voltage you know your connections are good and you simply have a battery with a certain amount of internal resistance. If there's a big difference between the battery voltage and the glow plug voltage there's one or more bad connections between the battery and the plugs.


written by Tyler, September 19, 2012
Hi Vince,
I really enjoyed your information on the glow plugs. I am hoping you have some advice with a cold starting problem I have had with my 1990 VW Jetta GL 1.6L diesel, naturally aspirated engine, IDI engine. When I was driving the car, it had between 147,000 - 170,000 miles on it. It is currently not my daily driver, but still sitting in my garage. The engine was always reluctant to start in temperatures below 35 degrees Fahrenheit. And most of the time it was literally impossible to start when the temperature was below 25 degrees Fahrenheit. My question is this: do you think replacing the injector nozzles would be the best bet to solve this cold starting problem?
I would like to find an "on-car solution" for the cold starting problem because I am not able to plug in the block heater where I live. I bet the previous owners never changed the injector nozzles. I pop-tested the injectores yesterday and all four opened at 1,900 psi, very close to the 130 bar spec stamped on the injector body. However, two of the injectors sprayed an off-center cone pattern; meaning the cone pattern at injection was not pointing straight down. And the other two injectors sprayed almost a straight and solid stream of fuel with no cone pattern.
All four injectors did not pulse/inject fuel several times with one down stroke of the lever of the test stand. I noticed the video of the injector you had on the pop-tester and that injector pulsed fuel several times for one time you pushed down on the lever when pop-testing the injector. Is this a sign of bad valve to nozzle body clearance inside the nozzle? Or do you think this is a nuance with the pop-tester stands we use?

I will greatly appreciate your opinion and advice.

Tyler Wright

Hi Tyler.. pop tests don't lie... and it sounds like at least a couple of your current injectors are in pretty poor shape... so replacing 'em all sounds like a good idea. Don't forget that you'll need new heat shields to go with your new nozzles.

In terms of hard starting, white smoke, etc in general:

A diesel needs good compression, good glow plugs, and good timing... so I'd start with a compression test, a run-thru on the glow plug system, and a timing check.

Finally... re your pop-tester question...the injector in my Youtube video tended to pulsate but in general they simply issue a steady stream for as long as the breaking pressure is exceeded... and should snap closed just like they snap open.


written by steve, August 05, 2012
Hey Vince love your site,I was a Mk1 Renault 5 man untill I got myself a Japanese Mk2 Golf 1.6td after moving from europe to New Zealand. I love a car that "does what it says on the tin" However,I find that my coolant warms up too quickly.Not overheating as such but I can start from cold no problem and do about 1-2 miles and stop,but when I go to re-start the car is too cold to do without glow plugs but too warm to activate them.Can I do away with the temp sensor so that my pre heat circuit comes on every time I start the car hot or cold?



Hi Steve... yup you could just pull the plug on the temp sensor (and tape it)... the relay will then think it's -30 at all times and run the glow plugs full cycle on every start.

On the other hand, at least in the VW world the temp sensor itself is a very cheap part... might make sense to replace that first and see if it helps... might potentially save wear and tear on the much more expensive glow plugs.

just a thought,

written by Doug, February 12, 2011
Hi Vince, I have a question. I just installed a KAE #321911261C relay and four new Bosch #80010 plugs and I still have a starting problem. I think the relay is a slow relay and my plugs are fast ones, would this cause a starting problem? Do I have the wrong plugs or relay? My car is a 84 Rabbit L 1.6. Thanks for your help.

Hi Doug:

"Fast" plugs and a "slow" relay really shouldn't cause any major starting issues IMHO... the plugs will simply heat up a bit faster than the relay is expecting and run a bit longer.

Ideally the relay and plugs should match, but the truth is "slow" plugs are getting harder to find... most people run Bosch Duraterms which are compatible with either relay.

My guess: something else is causing your starting issues. Have you confirmed you're getting power to the buss bar itself?
written by Jason, September 29, 2010
Thanks Vince! My relay seems to be ok it was just a bad connection in the socket like you said. Its fine now. But still 60 seconds of relay action though. My coolant sensor is bad, I check the resistance and its way out of the acceptable zone.

Big help! Thanks for the site!
written by Jason, August 09, 2010
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.


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.


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 !

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,

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!!

thanks for the comment,


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