The 928 Timing Belt - a Rite of Passage

[dr bob]

The timing belt saga that started almost a year ago was finally fixed in mid-August. I noted the dribble of gear oil from the tensioner after a couple months just sitting on tall hibernation stands. As the car was brought out in the spring, A careful look conformed that there was oil from the tensioner housing. So it all came apart again far enough to replace the gasket where the oil seemed to be seeping.

By mid-July there was oil again, plus I got a timing-belt warning on trailing throttle, after a hard uphill pull. Checked the belt tension when I got home, all was still perfect as far as the belt tension. This time the whole tensioner came out for another rehab. The problems turned out to be a very fine crack in the tensioner boot, a scuffed o-ring on the tensioner piston likely due to careless installation, and possibly some seepage through the threads on the adjuster bolt. A new boot, a new o-ring, and a more comprehensive sealing effort on the adjuster bolt itself has the whole thing functioning correctly again. I've added a few hundred miles to it since then, and as I prep the car for an early-onset winter hibernation period this time, the timing belt and related bits are no longer a concern.

The seepage from the gearbox turned out to be a tired o-ring at the TV (English translation: Throttle Valve) cable that detects/transmits throttle movement from the front end of the car. After sitting for a few months, the fluid level in the sump slowly rises as upper fluid becomes lower fluid. The leaking o-ring is apparently a problem noted by MB owners who enjoy the same gearbox, enough so that there's a better material and ever-so-slightly larger section o-ring available for the duty. The local MB dealer had them on the shelf, so in one went and so far no more drips on the floor. More and regular driving would do the job as well I suspect.

With the timing belt fun and some more serious 2022 health-related issues, I've managed to add less than 500 miles to the 928 odometer this driving season. More wrenching than driving time this year for the first time in my 25 year stewardship period with this car. It's been dusted a few times but sadly not needed any real cleaning or washing. Missed the Treffen event down the street at Sunriver, didn't even enter the car in the Festival of Cars event, haven't been able to attend any of the region tours or even meetings. By next spring I should be able to sit in it for longer periods, and hopefully add some tour events to its log book without adding any greasy fingerprints.

Happy and safe holiday wishes to all who are following along!

[Tom]

First and foremost, I hope your '22 health issues are behind you and that you enjoy a healthy and happy '23! We've had way too many family health issues in '22 as well, so here's to a better '23! Sorry you missed out on those events this year.

Interesting to hear about the MB cross-over parts. My dad's old 450SL has so much in common with the 928. If only they had a 928 on the Rockford Files, my dad might have gotten one of those instead. The 450 is still a fun car to fix up, and I'm getting proficient at the CIS injection, but by no stretch of any imagination is it, nor has it ever been, a performance car. Wallowing 1970's-era luxury at it's finest.

But I digress -- happy to hear your car is all fixed up and ready for the spring. When you said timing belt warning, is that an actual thing on the 928? If so, what condition triggers a warning?

[dr bob]

Thanks Tom!

In the 1970's, Porsche was doing some engineering and assembly work for M-B, IIRC the 500E coupe. This was well before M-B swallowed up the AMG operations, so that car was their low-volume "muscle car". Depending on who you listen to, the automatic box in the 928 was designed by Porsche and manufactured by M-B, .or. Designed and manufactured by M-B and just adapted by Porsche for the 928. Lots of shared engineering talent that migrated between the two neighboring companies as project loads changed. A conversation a couple years ago with the son of Tony Lapine points to the gearbox design coming from Porsche, based on the collection of engineering and design documents that he'd accumulated from his now-deceased dad. Apparently he has a pretty massive collection of that stuff. Tony Lapine led the design effort for the 928 for Porsche, after being poached from GM's Opel operation. His history includes time at GM US during the Bill Mitchell years, and a stint with Zora and the Corvette design team. It's easy to find a lot of design commonality, with features in the 928 that appeared later in the Corvette like the rear-mounted gearbox.

The good news on the gearbox front is that there's a large mass of installed 722.x auto gearboxes in M-B cars, enough so that there's a huge amount of support available in the form of parts and upgrades, solutions to problems, great documentation, and thankfully a lot of talent available when it comes to actually working on them. For the 928, the tailshaft housing is replaced by a bolt-on differential case, while the front cover has a bolt-on bearing cover and drive hub where a bellhousing would sit. There are very few other adaptations needed really. I can carry my spare gearbox to virtually any M-B-qualified transmission house, and except for the differential setup, could get a complete refurb with updates for less than an arm and a leg.

I suspect there was a lot of cross-pollenation between the V8 in your dad's Merc and the one that appeared later in the 928. For sure the engine management and CIS fuel system are very similar. M-B had purchased the early Kugelfischer mechanical injection system used in some of their 1960's cars, and the CIS system was a logical successor to that. CIS appeared on a lot of euro cars, from Porsche/Audi/Volkswagen, Saab, Volvo, BMW, plus many more that aren't so common here the US.. It's a pretty impressive system, especially considering it was all mechanical until pretty late in its lifecycle. It's a wonderful and quite reliable system, so long as all the internals are kept clean and wet. Dirt or varnish build-up is its Achilles Heel, matched closely with user tinkering on the scale of most popular causes of problems.

On the timing belt warning: The tensioner is a spring-loaded hydraulically-dampened pisto9n tensioner. On the top where the piston engages the tensioner arm and roller, there's a coil spring that has a plastic sleeve and a a metal contact ring that together form a spring-loaded switch. If the tensioner load reduces to a point where that coil spring can open the switch contact, it sets a persistent warning in the instrument display. Because the aluminum engine dimensions grow a lot between cold and operating temp, a simple position switch setup wouldn't work. The solution that they came up with is quite elegant in it's simplicity really. Of course having that warning come up causes a minor increase in heart rate, perhaps a good thing so you remember to give the belt some attention.

Meanwhile there's an aftermarket adaptation of an Audi tensioner that's a lot better at maintaining a constant belt tension as the engine gets bigger and smaller. The factory tensioner uses a stack of Belleville coned washers as the primary "spring". The stack is very compact, but offers an amazingly non-linear pressure curve. The Audi package has no provision for the factory tension warning system, at least as it's been developed so far. The factory system is very sensitive to correct adjustment, and obviously has some sensitivity to me scuffing an o-ring during regular service with the belt. It's a devil I know better now than I did the first and second and third times I replaced the belt. I suppose I could figure a way to add a low belt tension warning contact to the aftermarket package, but it's also quite likely I have maybe one more belt replacement on my car before I get recycled, so not a whole lot of incentive to re-reinvent the 928 belt tensioner system.

-----

We have deicer and cinders on the roads here already, so the 928 driving season came to an unexpected early end. My favorite local Cascade Lakes Scenic Byway tour route has been closed early for the winter, thanks to some early snowfall there. The car got a new battery this summer, after 10 years of service from the last one. The new tires I'd planned for this year didn't get purchased, probably a good thing in the big picture. The current PS2's are old but still have way too much tread left, meaning I don't drive it enough. Front seats will come out for some leather treatment, likely in spring when it's a little warmer. In the meanwhile, it goes back up on tall storage stands for its hibernation season.

[Rascasse]

I did timing belts on my '83 and '88 and it was pretty straight forward. Fortunately, I had the factory workshop manuals. It was basically step by step. Unlike Ferrari manuals that assume that you know half the steps and are translated to English by someone that only speaks Chinese!

[dr bob]

Welcome to Carpokes!

Like most workshop manuals, the one for the 928 was targeted at a dealer technician who had already survived the dealer apprenticeship and had some other Porsche experience. How to use tools, and some troubleshooting experience. The 928 workshop manual fills in a lot of the specifics about the car, doesn't go over stuff that was already learned in basics. Even after several on my own car, and a couple dozen total including clinic cars, I still keep the manuals and a checklist handy. The first time, more than 25 years ago now, I had three video cameras running to document the process. While assembling, there were a few times when I scrolled through the videos to get some details just right.

[dr bob]

It is with a heavy heart that I find myself going back in yet again. Third time in in a little more than 2 years. New belt and rebuild the tensioner again, check everything. Scratch head where hair used to grow. I have some video I took of my first 928 TB change effort back when flickering candles were used for lighting. I'm missing some critical step in the tensioner assembly. Or the loose-belt warning part is messing with me. Or I actually have a new belt that's stretching. Whatever the cause, the red "service toothed belt" message appears like clockwork at 3000RPM.

Car is raised a bit to get coolant drained. Pans are off. Everything is lickin' clean. Perfect time to exercise some tools.

Forecast is for 50's this week. Perfect tool exercise weather. I have only one meeting scheduled this week, so plenty of time to spend surmising.

[Tom]

It is with a heavy heart that I find myself going back in yet again. Third time in in a little more than 2 years. New belt and rebuild the tensioner again, check everything. Scratch head where hair used to grow. I have some video I took of my first 928 TB change effort back when flickering candles were used for lighting. I'm missing some critical step in the tensioner assembly. Or the loose-belt warning part is messing with me. Or I actually have a new belt that's stretching. Whatever the cause, the red "service toothed belt" message appears like clockwork at 3000RPM.

Car is raised a bit to get coolant drained. Pans are off. Everything is lickin' clean. Perfect time to exercise some tools.

Forecast is for 50's this week. Perfect tool exercise weather. I have only one meeting scheduled this week, so plenty of time to spend surmising.

Ack, sorry to hear that. Are you using a PCNA belt and 9201? When this happens, do you confirm the belt is actually out of spec before taking it off? Are any fasteners working loose? Gates came out with a Kevlar timing belt for the 944, which I've never tried because I'm too old to change, but I wonder if they make one for the 928? All gear cogs lined up in the same plane, with none backwards? On the 944, the main timing gear on the crank has a lip that looks more natural when on backwards -- and frequently causes issues when backwards. Gear all in good shape -- no breakage or wear eating away at the belt?

How does the loose-belt warning system work. I suddenly want to graft one onto the 944.

[dr bob]

First, the Good News -- Problem solved, and easily attributable to operator error & workmanship. There's a small connecting wire from the tensioner arm through the center belt cover, and a connector for the harness wire from the sensing loop to the dash display controller.

First picture shows the wire from the harness connector on the right, to a female spade connector sitting just below but not actually wrapped around the male tab on the arm.

20240430 Tensioner Wire 1.JPG (2.43 MiB) Viewed 4686 times

Next image shows the little wire connected correctly around the tab on the tensioner arm. Subtle but critical difference.

20240430 Tensioner Wire 2.JPG (1.43 MiB) Viewed 4686 times

The little wire itself is just a bit crispified after about 115k and 35 years plus since the car was built. More good news is that Porsche still offers the wire and connectors, under $25 on order from the local dealer parts counter. It will go into the spares drawer for consideration when I accumulate ~1k on the belt and perform the tension check/adjust on it.

More: I was prepared to perform the whole timing belt service again, thinking that maybe the belt itself (Gates T196, same as current factory supply) if needed. The symptom was so odd that I'd narrowed it down to either an internal cord failure in the new belt, or a break in the sensor wire loop. It was only after I took the picture that I could see the connector only partially engaged with the tab; It felt fine when I reached in to check. So only about 74% percent of the normal labor for belt replacement was needed.

As part of the effort with the top covers off I rechecked cam timing, and corrected the passenger side timing a bit. I can't say enough good things about the Porken 32VR adjusting tool for cam timing. Swaps about 30 mins of work time for the more than a few hours otherwise needed to get the cams lined up perfectly. Works for 32V 928 engines, and would probably work for 16V 944 and 968 engines based on the 928 head design. If there are any owners of those cars in central Oregon we should try it. The test drive after the reassembly suggests that the cam timing correction offers a very noticeable improvement. It also begs the issue of how it ended up slipped. I don't remember having to retorque the front bolt after the recent belt work. Regardless, both heads are correct now.

[dr bob]

Ack, sorry to hear that. Are you using a PCNA belt and 9201? When this happens, do you confirm the belt is actually out of spec before taking it off? Are any fasteners working loose? Gates came out with a Kevlar timing belt for the 944, which I've never tried because I'm too old to change, but I wonder if they make one for the 928? All gear cogs lined up in the same plane, with none backwards? On the 944, the main timing gear on the crank has a lip that looks more natural when on backwards -- and frequently causes issues when backwards. Gear all in good shape -- no breakage or wear eating away at the belt?

How does the loose-belt warning system work. I suddenly want to graft one onto the 944.

Tom, I generated an extensive response yesterday, but apparently I type slowly so the system dumped me when I pressed the post button. It had logged me out, and I couldn't seem to get back to recover all the text again. Oh well...
The synopsis:

-- Belt is a Gates T196, the same belt that Porsche currently supplies private-labelled as the Factory Replacement.

-- I have the 9201, but regularly use a simpler twist tester supplied by enthusiast-owner Jay Kempf. It's a recreation of the tool Porsche originally supplied for the duty on the earlier 928 cars. I've verified it's accuracy and repeatability against the 9201 for the timing belt. It's faster and easier to use, requires none of the fiddly calibration bar stuff the 9201 demands, and is frankly a lot less fragile than the now-$$$ 9201 tool. The biggest use the 9201 sees these days is while tensioning the flat-ribbed alternator belt. Otherwise it's stored safely in oil paper, in a specialty-tools bin. Gold has risen in price only slightly faster than the market for intact 9201 tools. The Kempf tool has merely doubled in cost since I bought mine way back when.

-- The was a very limited run of blue 'racing' belts commissioned by a 928 parts vendor about a decade ago now. His website has a 'not in stock' flag on it now, and has for quite a while. Meanwhile, the not-blue versions have Kevlar aramid reinforcing cords these days. The blue belts were slightly thicker, which limits the way tension can be accurately measured with either the 9201 or the twist tool.

-- The gears were all new at the previous belt replacement. I'd purchased the gears when a very good opportunity presented, and decided to store them on the motor even though the originals were still quite serviceable. Those live in a bin of originals pieces cycled off the car, most still quite good actually. The next owner will have them with the car, per my estate plan.

-- The crank gear has a pair of umbrella washers to help guide the belt should it drift. They are installed correctly. There's some very immediate damage on the belt edge should they be mis-installed. Thankfully no evidence of that damage so far.

-- Belt us running where it should on all the gears so far. The pivot bolt for the tensioner arm, plastic bushing for same, and the actual belt routing are the most common causes of poor belt tracking. Those pieces were replaced with the belt in the first episode of this spell-binding saga now a little over two years ago.

-- No loose bolts or fasteners. The tensioner did get a couple rebuilds/reseals as I chased the erratic warning indication. I have a healthy and loving relationship with torque wrenches and drivers for all the fasteners though, and follow the factory's protocol of marking each one after proper torque is confirmed. So OK so far.

-- The belt tension warning is simple yet elegant. The tensioner arm is isolated from engine ground by a plastic bushing. In service, there's a small coil spring also isolated from engine ground and an intermediate piece between the tensioner pin and the arm. When pressure from the tensioner is sufficient, the little coil spring is compressed and the intermediate piece is in contact with the pin, completing the connection to engine ground from the tensioner arm. If pressure from the tensioner falls off for any reason, the little coil spring is able to separate the intermediate piece and open the ground loop.

In the pictures, the sensor cable connects to the tensioner arm, and also the sensor circuit from the display driver in the gauge cluster. The loop is ignored for the first few minutes after engine start, then sets the red 'toothed belt service' message if the loop opens again. The spring and insulators and intermediate piece add about 20mm to the end of the actual tensioner assembly. Part of the reason this works is the extremely non-linear action from the stacked Belleville washers used to provide pressure in the tensioner itself. There's a very sharp 'knee' in the pressure vs. travel curve.

Contrast with a tensioner that uses engine oil pressure on a tensioner piston, with very consistent pressure applied throughout the tensioner's travel. Without that 'knee' from the spring washers, pressure remains constant until the belt has actually failed or the tensioner runs out of available travel. Better in that case to monitor the actual position of the piston, maybe with microswitches, looking for a piston that has not extended (piston or hydraulic failure), and also for one that's over-extended due to belt stretch or bearing wear.