VW’s 1.9 TD Problem and Solution

Added NOTE : This post was written with the ‘flicker’ electrical problem we see in the sine wave of a diesel powered generator, how to mitigate that peak power pulse?

When diesels are used as prime movers,  the fewer the cylinders the larger effect, the lower the flywheel mass, the more noticable as well.  Any errors or omissions in fact.. as it relates to the VW design, please scroll down and use comment. This may be a basic physics study about mechanical and electrical energy, and how to manage that peak power stroke.

I have been studying an interesting problem  that VW had in their Turbo Diesel, and what they did to fix it.  It’s best to tell enough of the story to give you a good picture, and you can follow up with some Google searches to learn more…If you have better information, more precise info, please scroll down to comments and let us know… please!

The first failures noted by enthusiasts probably started getting some attention in the early 1.9 TD (IDI) engines. The 1.9 was a lot like the 1.6TD that was pretty much trouble free, BUT the stroke was made longer and the engine produces a lot of torque across the range. Peak Horsepower is similar to the 1.6 TD

Problems were soon noticed, crank mounted pulleys >SPROCKETS< began to wobble, the cam belt sprocket got loose and literally ate away the key integrated into the sprocket. To bring the picture into focus, the sprocket anchors the Serpentine pulley with an integrated harmonic balancer. IF this rather small key gives way, we have a train wreck because this is an interference engine, when the valves hit the piston tops, it may be cheaper to order a good used engine than to try and fix it..

Some believe the majority of the trouble was caused by taking the sprocket off the crank, and NOT torquing it to spec, and not using the new bolt, others suggest the new TDI used a better system for keying the crank and sprocket because of these problems, regardless of the root cause of the trouble.

Now  there’s some interesting study here. There were few problems in the 1.6 TD and now a lot of problems in the 1.9? We’ve noted the longer stroke, but there’s another thing VW did, and I’m not so sure they caught the significance of making this change and accounting for the rather brutal peak torque easily communicated via a far more efficient drive system, yes, they dropped the VEE belts, and moved to the serpentine system to join many other manufacturers!

There are several additional factors that appear to contribute to the failure, some pointed to the reuse of the bolt that held the sprocket to the crank designed for a single use (torque to yield).  Apparently both professional mechanics and  home mechanics ignored this fact, and reused the bolt. Others pointed out that the torque setting is about 111 ft pounds, and many who changed out these cam belts probably did not have a proper torque wrench to apply this amount of force, AND unlike the better mechanics, didn’t understand that you don’t need take this sprocket off to change the belt, in many if not all cases.

We know there’s less penalty if we over engineer something VS leave a system like this in place for Owners to risk the loss of an engine, and since the recommended belt change is at 60K miles, it’s possible that the dealer or VW itself could >EAT< the loss.  Add to this the bad press VW gets for half Assed engineering, especially for a company where many ASSUME the typical GERMAN engineering has been applied.

But the real story is what VW did next, and how we might apply this knowledge to our hobby of slow speed diesel prime movers and home power generation.

When this problem became all too apparent, VW and Associates were likely in the middle of  developing the new TDI engine, this used the same block and crank as the 1.9 TD, but different heads and pistons. The engineers looked at this marginal integrated key, and the quarter moon spline on the crank nose and decided to move on to a new sprocket design, this used a ‘D’ shaped hole on the ID of the sprocket, and the same crank  was milled with a flat instead of a spline to receive this new sprocket. If I got the story right, some of the latest TDs also had this improvement applied.

But there’s more! What was the root cause of this failure? If we study Harmonic balancers, they are there to mitigate torsionals seen along the crank, flywheel and main trans shaft, whatever is causing this additional stress (remember the 1.6 TD, and the 1.6 NA applications did not have these problems, so we might assume there was less stress there) might help create some early failures in the crank shaft, or other elements tied to it.

So here’s what I find interesting, they added an over run clutch to the alternator, it’s integrated into the pulley!

What’s fascinating is the use of this Sprag clutch has found it’s way into other German cars, including some of the Porsche 911s!

So what’s happening here? maybe we allow one of Canada’s diesel motor heads to explain it in his terms..

Hi George… happy to clarify further if I can… some thoughts below:

>The one way, or sprag clutch alternator pulley is the very first I’ve

>ever heard deployed anywhere to solve a problem like this.

On the early days of their serpentine belt implementation on the AAZ engine (MK3 chassis 1993+) VW used a standard (solid) alternator pulley, meaning that the alternator is directly connected to the crankshaft drive pulley via a serpentine belt.

This engine (and its gasser cousins the ABA and VR6) unfortunately became a bit infamous for wearing out the crankshaft sprocket, key, and keyway.  It’s generally accepted that a large contributing factor to this problem was that the alternator has a small pulley and a lot of angular momentum; during deceleration it tends to want to overdrive the engine, putting large stresses on the crankshaft drive pulley (and the keyway) therein in a direction opposite to normal. Over time acceleration and deceleration results in the crankshaft sprocket rocking loose… on a diesel the result can be catastrophic.  The small pulley on the alternator exacerbates the problem (over previous designs) by increasing the alternator RPM and giving it a mechanical advantage when overdriving the larger crankshaft pulley.

At some point just before they moved to the TDI engine VW switched to a 1-way ratcheting alternator pulley (which they continued with on the TDI engine).  This pulley allows the alternator to automatically de-clutch from the drivebelt and freewheel on its own… preventing the alternator from applying reverse torque to the crankshaft pulley when the engine decelerates.

Here’s a thread with some discussion and pictures I took doing a retrofit that may help.

> “I hear it unload” I just don’t understand

> what’s unloading, can you help me understand?

With the hood up if I rev the engine and then allow it to decelerate abruptly I can hear the alternator pulley clutch disengage and the alternator spin down independent of the engine… it’s a noticeable “winding down” sound distinct from the engine, which has already nearly returned to idle.

While driving the effect is notifiable by watching the tach… the engine slows down rapidly but the tach (which is driven by the alternator) drops more slowly.

> Where can I get this pulley?

It’s a fairly standard dealer part and is also available thru some of the web-based suppliers… the part number I used is in the thread above.

> thanks for any help..

No worries… always interested in sharing information back and forth between fellow VWers !!!

Happy to discuss further if that’s helpful,


Here’s a link to give you some idea as to where this one way pulley is being deployed..

not only is this an interesting problem and fix, but it is an interesting part that we can migh use got the picture yet??

many thanks to Vince, another excellent motor head in Canada where they have a lot of fun cars we can’t have!

Now on with the story….

Among us DIYers is Brad, a mechanical engineer who’s small company of about 60 employees at it’s peak developed a lot of equipment for manufacturing. We were discussing methods to ‘clip’ the peak off our AC wave form right where the power stroke delivers it’s rather brutal peak torque. (See Bill Rogers Book for a well documented picture and explanation of this).

Brad said he had a similar problem in some industrial equipment, and he considered two solutions, one being a mechanical accumulator, the other being an over run clutch, the clutch was tried first and worked beautifully, and breakage was eliminated in the equipment completely.

At that time I look for inexpensive over run clutches I could build into the Serpentine pulley and found none cheap enough to be of interest to our ‘all too frugal’ DIYer group, but here it is!

Now for applications..

What will the application of this sprag clutch do for us to mitigate the typical ST flicker? As we think about this, we will likely consider the mass of the gen rotor, the potential to add to that mass, and also the load on the generator, all variables that might have an effect on the actual percentage above the preceding pulses we find our ‘spike’ to be. We know the human eye can recognize a rather small change in this peak voltage in typical lighting. Furthermore, the sprag clutch would not mitigate or help to normalize this pulse thru additional losses as a VEE belt and extra mass on the gen head did in the old startOmatic designed long ago by Lister At Dursley plant in UK. We might note that this was a satisfactory solution at the time because diesel fuel was far cheaper for the average Brit to buy at the time.

But there’s another application this part might lend itself to…….

While visiting Jack Belk’s place, I noted he had an old two wheel tractor that made use of a sprag clutch between starter and crank shaft. Once the engine started, the clutch free wheeled on the crank shaft.

Consider this, we take a typical cheap starter like the GM unit, we fit the sprag clutch pulley to the nose, and we then add a second pulley onto the shaft of the typical ST gen head. Certainly, there are about 300 other ways to do it that will come to mind, and if we can fit the sprag to the gen shaft, the belt or chain is no longer a parasitic load. I have used the electric clutch off a common AC automotive unit and grafted it onto the mopar starter and it spins the 6/1 ST gen set up like it was running at full speed! This sprag clutch is likely a better part for the Diyer starter.

If you look at the price the Indians want for ring gear conventional starter options, and consider the problems you have if the starter itself develops problems, the junk yard starter looks mighty attractive, especially after we DIY it with the sprag clutch pulley.

?A question?  will the sprag clutch add to the overall efficiency of a small diesel power plant? I think there’s a chance we can measure the difference in fuel/KWH figures with it’s addition >when< we deploy slow speed prime movers in generator applications.

Now all we need do is find a person who will stock a pile these sprag clutch serpentine pulleys, and ship them to us DIYers for a 10% or less mark up 🙂 Maybe we look for a Socialist or commie who’s certain that any mark up higher than 10% is hiway robbery? Got any ideas as to who we might recruit? (inside joke?).

A special thanks to Vince in Canada for his excellent ‘hands on knowledge’ and explanation, and to VW for creating both a problem and solution we could study and learn from.

This entry was posted in Engines. Bookmark the permalink.

6 Responses to VW’s 1.9 TD Problem and Solution

  1. James Beahm says:

    I have a TDi and my Alt clutch failed. Course there are two ways they can fail. One by free wheeling in both directions, in which case your battery goes dead and your idiot light comes on. The other way is it freezes solid and no longer free wheels in either direction.. That’s what happen to mine. How did I know? because the idler pulley started making a racket at idle. The belt would really jump around and cause the idler pulley to hit it’s limit stop.

    A lot of cars use them now days, it just makes the wholeaccessory drive run so much smoother.

  2. frank says:

    1.9 diesel engine light came on lost power, smoking 160,000 miles oil film in intake. suspect turbo failure?

  3. George B. says:

    sorry, maybe another reader will post same..

  4. Arthur Waring says:

    I have rebuilt and rebuilt again many 1.6 td engines. The problem I often have afterwards is a leaking rear main seal. When rebuilding these engines I often note that it has a three notch or three hole head gasket. When I measure the piston to deck height clearence it invariably calls for a single notch or hole. My wife’s 91 Jetta wich we bought new had main seal leaks and the dealer changed the seal and clutch plate under warranty in year 8. Now when I disaaembled the engine I noticed a three hole head gaskett. Maybe the dealer put it in I don’t know. Other 92 engine I disassembled was the same. Is that the solution to the leaking real main seal? An aftermarker dealer in San Deego California told me not to retorque the head bolts for the second time after 1000 kms. Maqybe that is weakening the bolts? Do you have any ideas?

Leave a Reply