$80 Glow Plug Harness?! Not on My TDI!

VW Glow Plug Harness

This is the original, poorly-built, overpriced piece-o-crap that VW sells.

It’s been a bit since I’ve last posted on this blog.  I’ve got some things going on that I’ve been hoping to share, but things haven’t panned out, yet.  I hope to make an announcement about some changes I’m making in life soon.  Soon!  Meanwhile, I’ll just say I’ve been quite busy on a project.

Additionally, my kids and I have also been working on a quadcopter, now that building a 3-axis stabilized platform is really pretty simple and inexpensive.  When we get it flying I’ll post a video and blog on that project.

We’ve had an unusually harsh winter here in Indiana, so I’ve been driving the diesel-converted Suburban [aka the Zombie Apocalypse Command Center (ZACC)] quite a bit.  When road conditions allow it, though, I save fuel by driving my 2001 Jetta TDI (ALH).  Sure, the Suburban gets reliable economy in the 20s, even in four wheel drive, but the VW moves me around with economy in the 40s.  Paying half as much to commute is a good thing, even if its in a beat-up little car with 260,000 miles on the odometer.

Because temperatures have been low, glow plug function has been important, and I recently noticed some issues with cold starts.  Making a cloud in a parking lot with a rough-sounding engine is embarrassing!  I often cycle the key on-off-on in order to increase glow plug heating before cranking the engine.  Recently it didn’t help, so I pulled the VW glow plug harness off and checked continuity for all four plugs.  Last year one of them died (no continuity) and replacing it made a huge difference.  All four plugs checked out OK, this time, so I figured my problem was another bad glow plug harness.

The glow plug harness on these vehicles is a poorly made plastic molding with only two conductors inside.  One conductor feeds the positive side of glow plugs #1 and #2, while the other takes care of #3 and #4.  Engine grounding completes the circuit for these plugs.  I’ve been reading a number of complaints about them going bad repeatedly because the actual connections get corroded at the glow plug terminal.  Bad design!  I’ve already replaced this part once.

Being a bad design is one thing.  Paying $80 per copy is another!  I hunted down the part number and searched all over the Web for a better deal.  Prices varied a bit, but they never went south of $50, so I decided that they were all horribly overpriced for a simple component that I could build myself.

I measured the connection on the dead glow plug from last year (not sure why I kept it, but it came in handy).  It came in at exactly 4mm and my recent multicopter work got me thinking about 4mm female bullet connectors.  I went onto eBay and bought a dozen sets of gold-plated male and female connectors for $5 delivered.

My Homemade Glow Plug Harness

This harness was made from 4mm female bullet connectors soldered onto 12ga wire with heat shrink (and some electrical tape–needed some larger heat shrink).

When they came in, I pulled off my VW plug harness to judge the needed wire lengths, soldered the 4mm female connectors onto the business ends of some wires spliced into a pair of Ys, covered everything in heat shrink (OK, yeah, some of it is actually electrical tape…for now), and spliced these assemblies into my wiring harness.  The bullet connectors were just a bit loose on the plugs, but I solved that by squeezing them ever so slightly with a crimper.  Now they fit onto the plug electrodes more tightly than the original harness does.

And, of course, my homemade harness works beautifully!

–No more embarrassing smoky, rough startups!

Comments (43)

  1. Fred

    The harness from the Ford diesel pick up trucks works like a charm. It is solid, 4 high quality wires and connectors, with waterproof booties. it sells for 20-25 dollars all day long.

    • Mark-A-Billy

      Fred, can you share what year/model pickups and possibly a part number? This is very helpful!

      • Fred

        Glow Plug Wiring Harness Ford Powerstroke Turbo Diesel 6.0L 2004-10. Sorry I don’t have a part number. The ebay listing I bought it from has since vanished. I just copied the title straight from my purchase history. It worked like a charm on a 2001 Beetle TDI with the ALH engine.

        • Mark-A-Billy

          Awesome, Fred. I’ll have to look up this part next time I need one!

          • Fred

            When you look up the part number, please post it for the next guy or gal in need of a harness. By the way, it is a 4 wire harness, so it can be used in both the older and newer TDI’s. For the older ones, it has to be spliced (and soldered) in pairs following the firing order, so that the cylinders that are up will always get heat at the proper timing. The 2 glow plugs at the center of the engine are pretty close to each other, so the harness will kink there. Make sure the kink points away from the head, to keep the harness as cool as possible.

    • Mg01

      The original post is talking about a 2 wire harness, got any good info for that? I’m unable to find anything short of $100 and its pretty ridiculous.

      • Mark-A-Billy

        Mg01, we all certainly agree about those prices! Look around the site some more, and you’ll find that we’ve been adapting Powerstroke wiring harnesses for the four-wire systems: http://www.theartofdiesel.com/glow-plug-harness-powerstroke-parts-on-a-vw-tdi/

        It didn’t work very well on my Passat, though. I had to trim lots of plastic to clear a coolant line. Also, the cheap Powerstroke harness I used was too cheap and the connectors wouldn’t hold their crimp and I wound up with open circuits. I wound up using some wire and some of those same 4mm bullet connectors, just like the original post. You just need to ohm out the connections to make sure you connect the right one to the right plug — otherwise your ECU will misidentify if you have a bad plug or one comes unplugged.

        • John Culley

          I turned mine into a 2 wire harness. I still have a CEL though. I think my relay may be bad. I put a meter on the plugs and cycled the ignition. I got no readings on any of the plugs.

          • Mark-A-Billy

            John, did you connect the remaining two wires in some way? The ECU will be looking for a specific resistance value or current flow. If, for instance, you ran a single wire to two plugs, while choosing not to use one of the wires in the harness, the connected wire will see lower-than-expected resistance, while the unused wire will show infinite resistance.

            I’d probably redo it, sending an individual wire to each of the four. This would likely fix your CEL.

          • John Culley

            I may have misunderstood. I bought the Ford harness and it was 2 4 wire harnesses. I took one of the 4 wire harnesses and spliced 2 of the wires that go to the relay together (spliced according to the wiring diagram).

          • Mark-A-Billy

            Sorry, John, I misunderstood you. What you did is completely correct, if you have an earlier model (such as an ALH) where there are only two circuits for the glow plugs.

            If I’m understanding you correctly, now, then it really does sound like a relay.

          • John Culley

            No worries. this hasn’t been a huge priority since it is very warm down here in South Georgia.

  2. Todd

    Thanks guys.
    Fred, the glow plugs aren’t timed, like spark plugs, so it doesn’t matter what order they are in. They only facilitate starting in the cold.

    • Fred

      I know they are not timed, but they work in pairs and I figured it is best to pair them as per the firing sequence. This way at least 2 cylinders will glow at the best possible time if one of the two harnesses fails. Best of luck with your project.

      • Todd

        The ECU powers all four at once.

        • Fred

          Yes, but if one of the two wires fails, it is best to have the two cylinders that come up to TDC with glowing plugs. This is why is good to pair them as per firing sequence. In very cold temps, a split second is a long time. This is how I do it and how I shared it. Everyone peels the cat his or her own way.

          • Mark-A-Billy

            Thanks to both of you gentlemen for your contributions to the blog!

            Fred, your recommendation for this harness is awesome. Todd is correct in observing that the TDI glow plugs all heat together. There isn’t really any timing involved, other than ensuring that the cylinder temperature is high enough for the air/fuel mixture to ignite when compressed. I think the main reason that VW even bothers to split the harness is so that a current drop can be observed and when the check engine light comes on, troubleshooting is narrowed to one of two glow plugs. Actually, they might be on separate relays, too, to split up the current.

          • Fred

            They are on one relay, which splits the current into the two wires. It is the relay 180 under the driver’s side of the dash. This is a very strong (and expensive) relay. On the 2004 and up models the wiring was split in 4, with connections that made possible to identify the faulty plug via Vag-Com diagnostic software. However, those cars came with the 2.0 Pompe-Duse engine, which had electro-mechanical injectors placed inside the cylinder head. The injectors were actuated by 4 extra lobes on the single camshaft and the rest of the lobes were thinned down to make space for the extra 4 lobes. This made those lobes so thin that they wear out before the 200K kilometer mark, also wearing out the tappets. At this point, the cam and valve tappets must be replaced. This affair can get expensive, especially for those that don’t do their own work. There are stronger aftermarket cams made with better metal alloy, but they retail for upwards of 500 dollars. The Pompe-Duse engines are somewhat quieter, but wear faster and only get 35 MPG. They were replaced in 2009 with the engines that became famous for the “smart” emissions software. They have a CIS injection system similar to those that came in the old golfs and cabriolets that really rocks, as these heavier vehicles still get 50-55 MPG. I hope the scandal fizzles out, as I’d love to put my hands in one of these.

          • Mark-A-Billy

            Fred, I’d like to get my hands on one of those, too, but I think they are in the process of paying off the owners and collecting the cars. I believe they’ll either retrofit with urea injection or crush them. I’d rather see if I could get a bargain-basement price for one, but no such free-market solution will be offered.

            I have a 2005 Passat wagon with the PD engine. I bought it with a dead automatic and I turned it into the car that SHOULD have been sold here in the states by installing a 5-speed manual purchased from Dutch Auto Parts. It was shipped to me across the ocean. My cam is actually fine at 190,000 miles. What’s worse than the cam in these is the balance shaft assembly whose tensioner will fail, taking all the teeth off the sprockets and leaving the engine without a functioning oil pump. So, when I got the car I promptly deleted it. I used a kit that allows the use of an ALH oil pump, chain, and tensioner. Vibrations are still hardly noticeable.

  3. Tim Bryant

    I’ve just bought some bullet connectors and tried this…no engine light yet.

    • Mark-A-Billy

      Cool! I’m still using this approach on my Jetta TDI a couple years later. The only thing I’ve had to do is check the female connectors to ensure that they don’t loosen. If/when they do, I pull the connector off and give it the tiniest squeeze with a pair of pliers. My Passat TDI has the original harness, but I’ll try the Ford harness if/when that one goes.

      • Fred

        Glad it worked for you. I tried the method, but my light would not turn off. This is when I tried the Ford harness. I got a used one on Ebay for 10 bucks, just to try it without spending much. It worked like a charm. Once I took out my original harness I noticed that it was bad on a couple spots other than the ends and this is why my light would not turn off, even after soldering the tips. By the way the harness I tried was for a Ford Powerstroke Turbo Diesel 6.0L 2004-10. Credit goes to the original poster for opening my eyes to realize that I did not have to fork out 80 dollars for the VW harness.

  4. Greg George

    I went in for a compression test today, and they declined the test on the grounds that my 3rd glow plug harness would fall apart when it was disconnnected. They offered to replace it for me (battery and tray removal plus part) for $250.

    Thanks for the advice about avoiding the purchase of a 3rd low-quality glow plug harness for my 2001 TDI New Beetle. I went to the local Ford parts department and bought the harness for a 2004 Ford 6.0 Liter turbodiesel Power-whatever truck, cut off the connector and soldered (did not use a crimp connector to avoid corrosion issues) for $70. I did not bother to re-run the wiring up to the plug, just re-used existing wiring (this had already been done twice with VW parts). Probably paid too much, but the quality is much higher than that of another crappy VW glow plug harness.

    Wired yellow & red to wire for plugs 1 & 2, blue and white for plugs 3 & 4.

  5. John Culley

    Just found your article and was leery about wiring a harness up from scratch until I saw the part about the Ford harness. I just found a right and left harness for the Ford 6.0 on eBay for $25 shipped. It comes with both harnesses so I see it as a 2 for 1 bargain. My only concern is the length of the harness. Also, it looks like they are 4 wire harnesses. I assume I’ll need to solder the 4 wires together. Any tips on the best way forward?

    • Mark-A-Billy

      What year and model TDI do you have? It makes a difference.

      For my 2001 Jetta TDI (ALH engine), two wires come from the relay and split into four. I’d ohm out which ones go where, so that if you get a check engine light it tells you which pair of plugs to check. Then, you’d wire two of the Ford wires to each of the appropriate outputs. Actually, I’m not sure if it’s even that sophisticated, but that’s how I did it when I used the RC bullet connectors. The same would be true of the Ford harness.

      For my 2005 Passat TDI (BHW), there are four wires and the ECU is smart enough to tell you which individual plug isn’t working. Again, I used my multimeter to determine which wire went to which plug on the original and the replacement (Ford) harness. Then, I wired up the Ford harness appropriately. I found the Powerstroke harness to be long enough for my application. When in doubt, hold one harness alongside the other before cutting and be sure to give yourself some extra length for splicing.

      I had to trim the plastic plugs on the Powerstroke harness where it came close to a coolant pipe on the BHW engine. Recently I went back in and trimmed the bottoms from all four plastic plugs, exposing the connectors. I wanted to crimp the connectors a bit for a tighter connection. After crimping, I put some heat-shrink tubing around them to minimize the opportunity for something to ground-out. I should have taken some snapshots to share.


      • John Culley

        Mark I have a 2000 Jetta with manual transmission. The harness currently on the car is a homemade harness that was done quite poorly. I bought it that way and want to make sure it’s done properly this time. I will trace the wires from the relay and see what goes where. I also have a schematic if I need it.

  6. Fred

    I saw a set of 2 glow plug harnesses for a 2004-2009 Ford diesel listed for $24 bucks on Ebay today. Ebay item number is 301536086505

  7. John Culley

    So I got the new Ford harness installed. It wasn’t very difficult. However, I still have a CEL. Do any of you know if I need to actually reset the light, or does the car do that every time on start up? One item of note on the Ford harness, the length between glow plugs is longer than needed. I may end up cutting some length out and soldering the wires back together with some heat shrink to remedy the problem.

    • Fred

      Your CEL may be due to the additional splicing you did on the harness, as it increases resistance on the circuit. This resistance is seen as a bad glow plug by the ECM. The less splicing the better. Also check your glow plug relay, it may be triggering the CEL light if it is bad or if it has a bad connection. If you bought a set of 2 wire harnesses, try the second one without any splicing other than the necessary to make it work. I did mine without even soldering (I did the “cold soldering technique) and that was enough to turn off my CEL. The wire in the very center was a bit too long, but instead of cutting it and splicing it, I just routed it upwards and away from the heat of the head. That did the trick for me.

      • John Culley

        Thanks Fred. The Ford harness is a 4 wire harness so I had to splice it down to 2 wires. I then spliced them to the 2 wires coming from the relay. I don’t know how I could splice it any less. I will check the relay tomorrow.

        • Fred

          You did it right, it should have turned your CEL light off. Make sure every connector fits snug on every glow plug. Check the resistance of the glow plugs, it should be 1.5 Ohms or less, if I remember correctly (google this to be sure). You may have a bad plug and is a simple thing to resolve, certainly easier than checking every inch of the two wires coming from the relay and checking the relay itself (unless you have access to a known good relay). I hope you can resolve the glow plug blues, it sucks when they fail in the winter.

          • Fred

            I forgot to mention that a weak battery may give you a glow plug CEL and trouble code. Keep in mind that the cranking amperage of the battery decreases as the ambient temperature plunges. If the issue is your battery, make sure to replace it with the best AGM battery you can afford, the higher the CCA the better. AGM batteries have higher cold cranking amps in the cold winter temps. Our TDI’s are very hard to crank due to their high compression and require a lot of juice when the temps are cold.

          • John Culley

            When I first got the light I checked the plugs and they were good. I pulled one out today and it didn’t look great. I’ll probably change them just to positively rule out a bad plug. Fortunately I live in South Georgia so the coldest it usually gets is low to mid 40’s. so I have that going for me.

  8. Fred

    The trouble code will be stored until you erase it, but the engine light goes off when the correct conditions are met. Proper battery amperage plus proper glow plug wiring resistance will turn the CEL light off, even if the trouble code is still stored. By the way, dirty battery terminals will cause similar effects as a weak battery.

    • John Culley

      The battery is probably 6-7 months old and the posts are clean as are the cable clamps. I tested the harness for continuity prior to installing so I think the problem could be between the harness connection and the relay, or as you said, the relay itself. More troubleshooting. I’ll keep you updated.

      • Fred

        Check the connections on the fuse block that sits on top of the battery, make sure the positive wire is clean and tight. Another thing you can do is to run a ground wire from the negative pole of the battery to the engine block, use a good, heavy gauge wire for that. I’ve gone through this and even replaced a good starter without needing to. I replaced that fuse block with a clean one from the boneyard and that improved things a lot. I think the limited space restricts the size of the battery that can be fitted and the stock 650 Cold Cranking Amps is not enough to crank that engine in cold temps. I fitted a slightly taller battery with 1000 CCA and even that was questionable. Perhaps the car should have had a stronger starter, like the big rigs have. I wander if someone has run a dual battery setup on a TDI. The TDI’s really lack starting juice and/or a better starting motor when they are used in low temps.

    • Mark-A-Billy

      Guys, I believe that I had to clear codes on both of my cars when I’ve run into this in the past. Perhaps I assumed that I needed to, but didn’t really. I’d recommend giving it a try, at least. I have the full-up VCDS, but basic OBD2 codes like these can be cleared with a $15 device found on eBay — which is a lot better than making repeated trips to the parts store to use their free equipment.

      • John Culley

        Thanks Mark. I live pretty close to O’Reilly so it will be an easy trip. Even if that’s not the issue, it’s worth a try.

  9. Fred

    I’m up in Ontario and it gets really cold. I don’t run my TDI in the winter, as starting it in freezing temps is quite an issue. I think the MK4 TDI’s have too small batteries for winter use and the tray space is not enough to fit a larger battery. In my experience, when the temps are below -10’s, batteries will drain useless if the car does not start in the third or fourth attempt. In contrast, during the summer months, the car starts within 1/2 turn of the crankshaft, even with a weak battery. I have owned 4 MK4 TDI’s and they all behave the same in the winter. Besides, they suck in the snow, too low to the ground and very poor traction.

  10. Lew in Denver

    You, sir, are a genius. I didn’t even need to remove the harness – just chopped off the misbegotten end connector thing, soldered on the RC connectors – left all conductors the same length and used plenty of flux on them – soldered them into the gold RC connectors and finished with head-shrink. Reconnected and voila, no more #2 glow plug code. Thanks!

    • Mark-A-Billy

      Awesome! I had a later post about using some Ford harness, but I’ve since gone back to the RC connectors. I think this works much better and I’ve used this approach on both of my TDIs. Glad it was helpful for you!

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