HOW-TO: Pimp your glowplug wiring
Written by Vince Waldon   
Tuesday, 30 September 2008

This HOW-TO describes one way to dramatically improve your glow-plug wiring.


This is also one way to add glowplug wiring if you are convertng a gasser chassis over to a diesel engine and want to do without the official glowplug relay and associated harness.


As always, this is just one way to do things... not necessarily the "right" way, and like most instructions I recommend reading them all they way thru before starting out. 


  • Use these instructions at your own risk
  • Read them through from beginning to end before starting
  • This is how I do things… it is not necessarily the right way nor the best way !
  • Using equipment,tools, and supplies incorrectly could result in serious injury to you or your property or even death

Please note:  You can click on any thumbnail below to see a larger version of the image.  After viewing the large image, you can click on "close" to return to your spot in the text





 IMHO there are several weaknesses in the original VW IDI glowplug harness, including:

  • the full amperage of the circuit (50 amps or so) passes thru the firewall and thru the connectors in the fuseblock
  • the copper buss bar makes removing the glow plugs a pain since you have to completely remove the 8mm nuts
  • the copper buss bar makes troubleshooting a pain since all plugs are connected in parallel
  • the 50A fuse is prone to hairline fractures that takes out all your glowplugs
  • the glowplug relay itself is *extremely* expensive
  • the length and gauge of the factory wiring results in a reasonably significant voltage drop to the glow plugs... I've measured below 9V at the plugs when the battery was delivering 12.5V



Ergo, this design has the following features:

  • el-cheapo garden tractor relay carries all the current and protects the contacts of the expensive glow plug relay
  • all the heavy duty current travels via short wires in the engine compartment to minimize voltage drops
  • all the heavy duty current travels via over-designed wire gauges to minimze voltage drops
  • separate fuses provide individual protection and prevent all plugs from being disabled at once
  • separate and easy-to-remove fuses  make it quick to continuity or current-test each glow plug individually
  • color-coded wires make it easy to trace which glow plug is which
  • modified connectors at the glowplug allow removal without removing the 8mm glow plug terminal nut

Materials needed:

  1. generic starter solenoid:  I used NAPA #SME 701670 since it had the smallest form factor of anything NAPA carries *and* was the cheapest, but any generic starter solenoid that can handle 30 seconds of 50A duty will do
  2. generic fuse block:  mine came from Canadian Tire, but most automotive shops have them.  I picked the long-glass style of fuse because IMHO it's easier to see when the fuse is blown
  3. a length of 4 gauge wire:  I found it cheapest to buy a low-budget set of booster cables
  4. four lengths of 10 gauge wire.. each length a different colour
  5. two lengths of 16 gauge wire.. each length a different colour
  6. four 20A glass fuses
  7. three 4 gauge copper lug connectors
  8. four 10 gauge female spade connectors
  9. four 10 gauge lug connectors
  10. two 16 gauge lug connectors
  11. 4 sheet metal screws to mount the solenoid and fuse block
  12. solder
  13. (optional) a bit of heat-shrink tubing if you like to dress the ends of your cables
  14. (optional) 1/2" plastic loom for bundling the glow plug wires
  15. (oprional) a bottle of liquid electrical tape if you like to minimize the number of bare live wires in your engine compartment












1)  Start by soldering the fuse block to a short length of the 4 ga wire, terminated with a 4 gauge lug as per the above picture.  I solder all crimped connections *and* insulate with red heat-shrink to reduce resistance and keep out the elements.

2) Mount the new relay and fuse block in a convenient location in the engine compartment.  The idea of this system is troubleshooting convenience, so pick a spot where you can get to the individual fuses easily.  Also remember that these are live wires and terminals, so it's a good idea to locate them off to the side where they won't come in contact with wrenches and screwdrivers during routine engine maintenance.  On my A3 the driver-side shock tower is perfect:









3) wire up the soleniod ground (green wire and lug in the above picture), the energize wire (orange wire and lug in the above picture which leads to the original wire that supplied the glow plug buss) and a longer length of 4 ga wire, terminated with soldered and crimped 4 ga lug connectors, which runs between the relay and the battery.  I prefer to go all the way to the 13mm stud on the engine's starter solenoid as the battery feed... I find connections at the battery tend to corrode quickly, get in the way when serviceing the battery, and don't easily adapt to a big 4 gauge connector.

If you're doing this wiring because you're installing a diesel engine into a gasser chassis and don't want to mess with the original glow plug relay and harness you simply run the energze wire (orange in the above picture) thru the firewall to a pushbutton switch on the dash.  The other termnal on the pushbutton goes to a source of power that's hot when the key is in the "on" position.  Push the button for 5-30 seconds (depending on engine temperature) to activate the glowplugs. 


4) prepare the 4 different-coloured lengths of 10 gauge wire with lug connectors on the end:









As you can see I've taken a lug connector and cut off one side.  When installed correctly this circular shape will stay in one piece as the glow plug nut is tightened, but will allow the wire to be removed without having to fully remove the glow plug's nut.  Why is this important ??!!  Well, those 8mm nuts are not only an enormous pain to thread on and off in cramped quarters but also have a life of their own... they love to leap off and drop down into the most inconvenient hidy-holes, requiring you to spend hours with a magnet fishing them out and cursing the day you were born.


5) Attach the glowplug wires to each glowplug, orrenting the connector so that the opening wants to close as you tighten the glow plug nut:









6) Route the glowplug wires neatly thru the engine compatment (I'm a big fan of plastic loom) and terminate them on the fuse block with soldered-and-crimped female spade connectors:









You'll notice I've cleverly arranged the wires in order... number 1 glow plug at the top, etc.  I've also added the fuses in this picture, and a layer of  liquid electrical tape (the black gooey stuff) to the buss side to help prevent an electrical short


You're done !!  It might end up looking something like this:









You'll notice a few extra wires in this picture... the wiring at the top of the picture is a feed to a 30A breaker for accessories including a fog light relay (the blue  and yellow wires at the top). 

Not shown is a wire from the fuse side of the new relay which to an LED on the dash... a HIGHLY recommended addition:










This yellow LED shows me that the glowplug relay is actually engaging the glowplugs (unlike the glowplug light in the cluster).  The one drawback of this LED is that it won't tell you if a fuse is blown... you'd need to run 4 LEDs to do that, since there are four separate feeds. However, unlike the stock system where a blown fuse takes out all your glowplugs, if a fuse does go you will still be able to start pretty easily on the three remaining glowplugs.  Someday I'll design a fancy monitoring circuit that traps blown fuses... I used to be an electrical engineer after all.


Questions / comments / suggestions for improvement welcome !! 


(c) 2008 Vince Waldon 

Comments (27)Add Comment
written by M.C Dreidel, October 07, 2016
Alternative to the fuse box, it has LEDs and if the fuse goes out the LED light will turn off for easy diagnosing!
written by Terry, October 02, 2016
Anyone try this on later model PDs, Clean Diesels, or Jeep CRD? I imagine a "Tune" to the ECM.

Hi Terry... later glow plug systems use pulse-width modulation to control the glowplugs in a much more detailed way than the bang-bang way the earlier systems did.. and so are rated at 7V or 5V as a result. Feeding them a solid 12V from a starter relay would probably burn 'em out... so not recommended IMHO. smilies/smiley.gif

written by Hugo, May 29, 2016
Hi there.

Nooby here. I have a Golf MK3 1.9 Gtd, anda wanted to do the "pimp". Just wanted to know, the starter solenoid, what's his role on the circuit?

Hi Hugo... the starter solenoid is just a really beefy relay. Its role is to supply a large amount of current (50-60 amps) to the glow plugs thru the shortest possible path, when commanded to do so by the original glow plug timing circuitry.

It does what the OEM glowplug relay does, but with less resistance and, because it's bolted into the engine compartment, along a much shorter path. Less resistance and a shorter path equals more voltage delivered directly to the glow plugs.

At least that's the idea. smilies/smiley.gif


written by Jeremy, February 24, 2015
Great write up! I am going to the parts store today to start this conversion on my '84 Rabbit. One question though: If the initiating feed is coming from the original feed to the buss of glow plugs then am I to presume that I leave the original glow plug relay in the harness to function as normal? Like to determine cycle time, on/off, etc? My relay seems to not function correctly...IE the LED on dash remains on forever.


Hi Jeremy... yes, you've got it... the system I've described can either be triggered with a manual pushbutton or the original glow plug relay.

One thing to check: the glowplug LED is actually unrelated to the glow plugs having power... and it has two functions:

1) "wait till I go out and then start"
2) "your water separator needs draining"

If you're getting a steady glow plug LED that never goes out it could be a malfunctioning relay but it could also be that your's is model year that got the water separator... and it needs draining. I've also seen solid LEDs when the wiring to the water separator is damaged... or even more common where someone deleted the water separator and just taped over the wires. The water separator is a rectangular plastic box up under the rear axle beam, passenger side.. and so the wiring gets mud and road salt and can cause the LED to go on.

hope that helps,


written by Ray, December 17, 2012
Great diagram let me add the old 6.9 international diesel is glow plugs are 6 volts and should only be engaged 5 seconds with 12 volts the wait 5 seconds the engage again.
written by Steve, December 05, 2012
Great ideas and simple, thanks, Steve
written by mr man guy, June 19, 2012
just FYI some glowplug set ups are 9V and making them 12v will shorten the life of your plugs immensely

(they do also come in 7v and 12v)

Hi there...yes it's true that later VW (TDI) diesels use glow plugs that run at lower voltages.. the PD engines are a good example.

Having said that, these instructions generally won't work for that style of enginesmilies/sad.gifa) there are no threads on the glowplugs and (b) the glow plug harness is already individually wired so that the car's ECU can detect defective glowplugs.

Good point, though.


written by budoushi, May 05, 2012
I am now in the process of getting a VW diesel Engine ready to drop down into a Suzuki Sidekick, with the help of Cheap Jerseys a ACME Adapter Kit.. I think I will use your idea , not only is it very practical and easy to maintain, but I think it looks Dame Cool.. ha! ha!
written by Justin, April 27, 2012
This is very creative. I have a 1997 chevy truck w/a 6.5L diesel. I can just run a bigger fuse block to power an 8 cyl diesel correct? Also, does this mod just use the standard cycle time for the glow plugs if it's wired as you instructed (w/o going through the firewall)?

Hi Justin...yup, no reason this shouldn't expand from 4 to 8 plugs... obviously using very very heavy wire for the main feed.

And yup, this simply adds a high-current relay as a driver but if connected to the original drive wire, as described, will preserve the original glow plug timing.

written by george, January 15, 2012
just finished my version of your plan. got a chance to test it out Saturday am when it was 15 degrees out in Indiana. started like a summer day, no smoke, no sputtering. about 45 seconds of power to plugs and it started great. have pics if you would like to see them and details. thanks again
written by george, December 11, 2011
just found your site and it fits what i was looking for about glow plugs! was thinking of going in that direction, now it is confirmed with great details. thank you. my problem is that i have to run my glowplugs for several minutes before car will start. i have juice to plugs and have bypassed relay. now i will try your idea. thank you!
written by Steve-O, December 01, 2011
Hey man, just did this mod! works great! forgot to plug my jetta in last night, and she fired right up with the glowplugs. never started better! and now i can see when the glowplugs actually come on... that stock glowplug light is pretty useless haha!
written by JAMESMOR, December 04, 2010
My friend your idea was excellent and my 1978 mercedes benz 240D is running thanks for you.

thank you so much.

jaime chavez
written by Jim Russell, May 25, 2010
Vince, this is really the bomb. I am sprucing up my BMW marine diesel engine in my boat. It uses the VW relays and chews them a few times a year. I am removing the entire glow plug relay circuitry and simplifying it. IT currently uses a common rail series system as well (6 Cylinders). I will use a push button actuation from the helm. Question, The stock system used a bukh solenoid rated at 45 amps for this system? I am going to go heavier duty. The old circuit had very corrroded connections. and two 40 amp breakers as protection to the relays. I am using a Blue Sea fuse block with 6 20 amp AGC fuses, should I place an 80 or 100amp fuse up stream of the new fuse block?

Thanks for the help and the great idea.


Hi Jim... thanks for your comments.

Yes, it sounds like there's some room for beefing up... I like your idea of a beefy solenoid and 20A fuses in-line.

In terms of a master fuse...adding a master fuse is certainly OK and an extra level of safety... always a good thing on a boat. My personal opinion is that you're probably actually OK without it. Assuming you take care to prevent the wires from abrading with vibration etc the vast vast vast majority of shorting issues are going to come from the glow plugs... and like the founder of GM used to say "parts left out cost nothing and cause no service issues!"

Just my perspective!

written by tummyacid, January 23, 2010
Very nice description!

I have just completed this on my 81 Rabbit and she doesn't smoke anymore when starting!

Just one caution: perhaps it is a problem with another part of my car but when I was testing this design after installation, I drained my battery. Lucky a friend was around to give me a boost but I think it should be worth noting that the additional efficiency can lead to a depleted battery if you aren't careful.

Thank you again for a wonderful howto
written by R Jones, December 21, 2009
Hey, Great write up.
I've owned several IDI's in the past and if I ever get another this may be the first mod. However I would add an additional part that works well in the marine world, a Tupperware type of snap top flat container.
Drill holes on one side to insert the wiring through. Then attach the container in the chosen location (use the same screws as the fuse block) so that the wiring is on the lower side. Run a little sealer on the wires but don't completely seal to give a little ventilation. After all the connections are made the cover is snapped on and provides protection from the elements and misdirected wrenches. (If the container is large enough the solenoid can also go there).
written by Jeremy (8v-of-fury), December 01, 2009
Dear Vince, your write-up is the "bomb-diggidy". I have recommended it to well over 50 people on many diesel sites. Even some Ford 6.9 IDI guys when the ford system took a dump on them.

KEEP UP THE GOOD WORK MR. Electrical Engineer smilies/wink.gif
written by steve, October 24, 2009
do you know of a way to use this system with a 98 tdi the glow plugs just do not stay on long enough on mine i'm using wvo at about 50/50

Hi Steve:

You could certainly use this system if you wanted to make your glow plugs fully manual (ie pushbutton switch on the dash) but there would be a couple of issues to worth thru... the connector on TDI glow plugs is a press-fit and the ECU would constantly throw the Check Engine light if the stock system is disconnected.

A different approach might be to use the stock system but lengthen how long the glow plugs run. This post on
explains how to do that.

just a thought,

written by Gord McFarling, October 23, 2009

Instead of cutting the connectors at the glow plug end, I used spade connectors. They work just as well and save a few moments of time.

written by drew, April 02, 2009
Just a quick question.
Do you suggest we use the same fuse (above the master cylinder) since they frequently deteriorate or go with a glass tube type (ie. car audio amp fuse/holder)? Is a 50amp fuse still neccessary since after this mod, it's only a signal wire to the solenoid?
Thanks for a great mod!!!

Hi Drew:

My opinion is that the stock glow plug fuse likes to fracture because it's a bit under-sized... it's rated for 50A nominally and the glow plugs can surge to 60A and beyond... over time this causes the fuse to eventually say "enough already!", particularly given some good old diesel vibrations.

You're right that in the new configuration it's simply now a signal wire to the relay so you could certainly replace it with a different fuse style but my opinion would be "meh, why bother?!". It's not being stressed anymore and will likely now last forever... and I've got other ways to spend my time then swapping out fuse blocks. ;-)

When I do this mod I *do* refresh the stock glow plug fuse, in case the one that's there is nearing the breaking point (and vibration tips it over the edge), but that's the extent of the attention I give the old part of the glow plug system.

I also like leaving the old system in place in case I ever need to temporarily revert for some reason... maybe the garden tractor relay dies one cold morning and I need to rig a bypass.

Just my thoughts... thanks for your comments !

written by Martin, March 09, 2009
How much voltage should be coming in from the original glow plug harness?

Hi Martin... the answer is a bit subtle!!

Ideally you'd like to see the full battery voltage (12.6ish) at the glow plugs, but they draw 40-50 amps and there is *always* some resistance in the system, so a bit of voltage drop is normal. I've personally seen 10.5V (with a fully-charged battery at 12.6V) and has heard of people being in the 9V range.

Actually, ff you see *no* voltage drop, or only a little, it's probably a clue that you have one or more burnt-out glow plugs. Fewer working plugs = less current draw = less voltage dropped thru the various internal resistances.

I believe the plugs are rated with this in mind (ie they function OK at 9V) but I like 12V better, hence rewiring them. ;-) On the other hand, if you're seeing pretty much the same voltage at the plugs that you see at the battery there's probably an issue.



written by Chris Wilkes, January 01, 2009
Vince this is a great upgrade, I just wanted to add that you could monitor the 4 fused feeds with one LED if you used 4 diodes, one from each fused lead to the LED

Hey Chris.... that's a fantastic idea. I'll noodle up a schematic and add it to the page... a great addition.

Now that I think of it... perhaps I'll rig up a 555 so that it flashes the existing light for glow plug buss power rather than run another LED. Image a flashing glow plug LED meaning "there's a problem with the glow plug system" as opposed to "your water separator needs draining" (IDIs) or "check your brake lights" (TDIs). ;-)

thanks again.. great thought !!

written by mtnbob, September 02, 2008
Thanks Vince!!!
I finally got around to doing this mod, and it works like a champ!!!
I still am using the glow plug circuit to activate the solenoid, but as soon as I figure out how to wire it to a switch in the car, I'll do that. Thanks again.
written by bob villemaire, August 23, 2008
vince your awsome....can,t exspress how much i appriciate your time youv,e commited to fellow vw drivers,owners,lovers,workers,whatever
absolutely amazing!!!!!!!your site has s much info [understandable and very clear]
thanks so very much i just wish there was more helpfull people like you in this world.
what would a mechanic charge [hour wise] to change 4 glow plugs?any idia?
written by Bert Carrier, August 22, 2008
Vince, you rule. I just did another set of injectors using your other procedure, works like a champ. I don't even bother calibrating, just sand them a touch on the plate glass and go!
This is my next project!
written by Paul, May 10, 2008
Hey There..

Wow this is a very smart idea..I actually though of doing this many moons ago when I drove a diesel VW Jetta. I like you totally agree about those freaking little nuts that go onto the glow plugs...It's Crazy!!

I am now in the process of getting a VW diesel Engine ready to drop down into a Suzuki Sidekick, with the help of a ACME Adapter Kit.. I think I will use your idea , not only is it very practical and easy to maintain, but I think it looks Dame Cool.. ha! ha!

Cheers my friend..

written by edan, April 15, 2008
Sweet!! I may be trying this sortly...

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Last Updated ( Friday, 27 November 2009 )