testing glow plugs

[steve6]

can someone tell me how I can test these, my car wont start in the cold anymore :X

 :?

[jtanguay]

can someone tell me how I can test these, my car wont start in the cold anymore :X

 :?

sure fire way is to take them out and energize them.  does it crank fast enough?

[Jettagli16v]

I have been wondering about using an IR thermometer to check each individually.
If you were to keep your IR dot on the end of each plug (Or maybe on the head right where they screw in?)
would you see a temp increase at each plug
(and assume they are working, or not, based on temp rise)?

If the plugs run until the cylinder head coolant temp sensor sees a rise in temp, one would think you could see this rise on the head itself.

Thanks!
-Brad

[steve6]

can someone tell me how I can test these, my car wont start in the cold anymore :X

 :?

sure fire way is to take them out and energize them.  does it crank fast enough?

ok, so they have to come out? no resistance test or anything that can be done?

The car starts if I put the block heater on it... its got a brand new energizer battery.... The glow light plug never turns off when turning the key on.... I assume they arnt heating up enough.

[cyrus #1]

You can check the resistance to see if they are burned out.  Unfortunately, to do so you still need to take the bus bar off and that's most of the work anyhow.

Did this happen all of a sudden or gradually?  If you have a multimeter, you can start by checking the bus bar to see if it's actually getting voltage.  Based on your description of the light never going off, it sounds like you could have a relay problem.

[Vincent Waldon]

Here's an overview of one possible troubleshooting procedure fwiw, assuming all you own is a testlight or cheap multimeter:

1) start by confirming that the glow plugs are getting power.  Sadly the light on the dash has nothing to do with power to the glowplugs. Multimeter or test light to the wide copper buss bar... turn the key on... you should have power for 5-20 seconds depending on engine temp.

2) if you *don't* have power, go upstream to the glowplug fuse... it's in a small plastic box on the firewall next to the brake booster.  It loves to develop hairline cracks, so either test both sides with your multimeter/testlight or remove it and see if it falls apart in your hands.

3) if you still don't have power your next stop is the glowplug relay.... tall relay-looking device mounted on the fuse panel.  Not easy to troubleshoot unless it looks physically damaged... swapping out with a good one is the quickest way


If in (1) you determined you are getting power to the glowplugs then they need to be tested individually.  Unfortunately as they are wired in parallel the buss bar needs to be removed... not a fun job but it can be done. 8mm wrench and a magnet on a stick are your friends here.  With the buss bar removed you can check for continuity at each plug individually... they usually fail open.

If you own a heavy duty current meter you can test the plugs pretty well with the buss bar on... I use a cheap NAPA meter:



Will all 4 plugs in working order you should see 50-60 amps dropping to 35-40 once they warm up a bit (assuming a nice strong battery). Anything less suggests one or more plugs have gone to meet their maker.  Note that I'm giving pretty wide current readings... cheap gauges are not that accurate, different plugs draw different currents, and initial battery voltage has an effect as well.

One other suggestion:  they are not that expensive *and* seem to go in short succession, so I always replace all 4 at once.... such a pain to get the buss bar off I only want to do the job once any given winter. :wink:


I keep meaning to do a HOW-TO on this one... 'course I'd need to track down a stock engine since mine have been pimped and would not make for good pictures. ;-)

[commuter boy]

I've always found the fastest way is to pull the injectors and look down the hole to see if they glow.

The plugs themselves are fiddly to get in and out (especially the one behind the fuel pump) but the injectors are just the lines, four injectors and replace the heat shields for $2.  It probably helps that I've got the curved Hazmet injector line wrench from Metalnerd.

[Vincent Waldon]

 It probably helps that I've got the curved Hazmet injector line wrench from Metalnerd.

Oh sure... thanks for reminding me... Santa did not put that beauty in my stocking this year despite the nice long letter I wrote him. :cry:  :wink:

[commuter boy]

My dad the VW mechanic was out for a visit and got to use one for about 5 minutes.  He looked at me like Old Yeller when I went to put it back in the toolbox so I just handed it to him and bought another one.   Just get it.

[Rabbit TD]

Here's an overview of one possible troubleshooting procedure fwiw, assuming all you own is a testlight or cheap multimeter:

1) start by confirming that the glow plugs are getting power.  Sadly the light on the dash has nothing to do with power to the glowplugs. Multimeter or test light to the wide copper buss bar... turn the key on... you should have power for 5-20 seconds depending on engine temp.

2) if you *don't* have power, go upstream to the glowplug fuse... it's in a small plastic box on the firewall next to the brake booster.  It loves to develop hairline cracks, so either test both sides with your multimeter/testlight or remove it and see if it falls apart in your hands.

3) if you still don't have power your next stop is the glowplug relay.... tall relay-looking device mounted on the fuse panel.  Not easy to troubleshoot unless it looks physically damaged... swapping out with a good one is the quickest way


If in (1) you determined you are getting power to the glowplugs then they need to be tested individually.  Unfortunately as they are wired in parallel the buss bar needs to be removed... not a fun job but it can be done. 8mm wrench and a magnet on a stick are your friends here.  With the buss bar removed you can check for continuity at each plug individually... they usually fail open.

If you own a heavy duty current meter you can test the plugs pretty well with the buss bar on... I use a cheap NAPA meter:



Will all 4 plugs in working order you should see 50-60 amps dropping to 35-40 once they warm up a bit (assuming a nice strong battery). Anything less suggests one or more plugs have gone to meet their maker.  Note that I'm giving pretty wide current readings... cheap gauges are not that accurate, different plugs draw different currents, and initial battery voltage has an effect as well.

One other suggestion:  they are not that expensive *and* seem to go in short succession, so I always replace all 4 at once.... such a pain to get the buss bar off I only want to do the job once any given winter. :wink:


I keep meaning to do a HOW-TO on this one... 'course I'd need to track down a stock engine since mine have been pimped and would not make for good pictures. ;-)

I've often read that you can test them all in the car with an OHM meter.  Unhook the bus-bar and check the reading on the easiest one you can get to and if it is good, hook the bar back up and take a reading of them all in series and if they are all good the number will be in the neighorhood of 4 times higher than the individual one read, or in multiples of the known good one telling you how many are bad.  If any are bad then you have to test them individualy.  If they are all the same brand plug and type they should all read equal or pretty close.

[Vincent Waldon]

Well, they are about an ohm each, so the 4 in parallel is 1/4 of an ohm...  a sensitive ohmmeter could probably pick that up, although at that end of the scale the leads and fuse of the ohmmeter probably enter into it as well.  Mine reads about 8/10 of an ohm with the leads shorted.  Temperature of the plugs would also enter into it... there's a whole school of diesel tuning based on that.

Always many ways to skin the proverbial feline, heh?! :wink:

[steve6]

Did this happen all of a sudden or gradually?  If you have a multimeter, you can start by checking the bus bar to see if it's actually getting voltage.  Based on your description of the light never going off, it sounds like you could have a relay problem.

I was always parking it in my garage with the block heater since the beginning of winter, and at work it was underground parking, one day I parked it outside and wouldn't start, I had to get it towed home, block heater on it the next day and it started.  I have a feeling a couple glow plugs are working.  The glowplug light.. it used to shut off after about 60 seconds, but in this extreme cold snap we've been having, I havn't seen it turn off yet..

I knew about this long ago, just addressing now... aint no worries the other jetta starts in -30c without issue. but it has a ***ty transmission  :cry:

[steve6]

stupid double post

and I have a brand new Energizer battery from walmart.. less than 2 months old.  

I was getting lots of cranking.. lots of smoke, just no fire  :lol:

[turborabbit]

All that stuff about ohms and such is making my head hurt. I "want" that stuff, but you "need" a battery charger and a compression tester. Always know your  compression values and the battery charger will test the glowplugs. They will get red hot if they are good, toss them if they don't.
With an inexpensive product warantee, Pull-a-part gives me an endless supply of glowplugs. It sounds like you need at least 2 if not 3. Goodluck-warren :mrgreen:

[steve6]

All that stuff about ohms and such is making my head hurt. I "want" that stuff, but you "need" a battery charger and a compression tester. Always know your  compression values and the battery charger will test the glowplugs. They will get red hot if they are good, toss them if they don't.
With an inexpensive product warantee, Pull-a-part gives me an endless supply of glowplugs. It sounds like you need at least 2 if not 3. Goodluck-warren :mrgreen:

Diesel blocks from the vw's never make it to the wreckings yards around here... engines always get pulled first.   :x

Ill just order a few and change atleast the easy 3.. lol.. $20 a piece  :shock: