The 993/964, G50/G64 Variant Transmissions, servicing and repair.

 

Back around 1998, I was at a meeting at a Dana Spicer training center, and in the lobby was a display case with a G50 Variant transmission in it. Turns out that at the time every part in these transmissions was made in Michigan and then shipped off to Getrag Germany for assembly, go figure?

Over time the manual transmissions in the 993 & 964  have proven to be extremely durable and with regular gear oil changes can provide trouble-free service well over 100,000 miles. 

Do you have a limited-slip differential internal to the transmission case? A quick test is to put the back wheels in the air with the transmission in gear. If you turn one of the rear wheels and the other rotates in the opposite direction your car is equipped with a standard differential. If the other wheel rotates in the same direction your car is equipped with a limited slip differential.

The other way is to look at the option code sticker under the frunk lid or in the owner's manual.  

*Here are the related option codes:

1. 219 Regular differential (without limited slip) 
2. 220 LSD Limited Slip Differential with 40% lock 
3. 224 ABD Automatic Braking Differential (Traction control), An Automatic Brake Differential (ABD) system works by using the Anti-lock Braking System (ABS) to apply braking power to any individual wheel that may be slipping during acceleration.

*In cases where this feature is standard on a vehicle, it may not be listed on the option code sticker.

All G50 Varient transmissions are a constant mesh design, meaning all the gear pair sets are fully engaged to their mating gear and spin freely until by shifting into a specific gear their attendant synchronizer ring locks the specific gear set in as an active part of the driveline. In this design, the actual gear sets are not subject to wear from engaging or disengaging only the synchro rings that facilitate their locking in wear over time. 

Because the lower gears are subject to a greater number of gear changes while driving and because of the greater speed differential the lower gear synchros need to deal with they are the first ones to usually exhibit wear out in a street-driven car. In fact, when disassembling the gearbox for service the first and second-gear synchros can exhibit extensive wear whereas the synchros for the upper gears show little or no apparent wear. This is why unacceptable shifting experience tends to show up shifting between the lower gears first.

On rare occasions, a street-driven car with as little as 80K miles on the odometer may need servicing if it exhibits shifting problems that can not be attributed to problems with: 

• The need to have the clutch actuation hydraulics bled to remove air,
• The hydraulic clutch actuation or the lever in the peddle cluster, 
• A worn clutch, 
• Out-of-adjustment or worn shift linkage components or
• Failed engine mounts; 

Engine mounts are easily tested by slightly lifting the engine from the engine case using a hydraulic floor jack placed on the reinforced center of the engine case a bit forward of the  oil drain plug outlet. If one sees more that just a smidgen of vertical movement of the exaust pipe tips the engine mounts probably need to be replaced. Never use the engine case to lift the rear of the vehicle off the ground as it is specifically verboten in the owners manual and discussed here.

Also if you are new to the 993/964 it is important to push the clutch pedal all the way to the floor and that any carpeting added doesn't interfere with this. If not pushed all the way in some cases one will experience grinding while shifting. I have found sliding the driver seat a notch or two forward eliminates the tendency to not push the clutch pedal all the way to the floor.

Metal grit at the drain plug -- Shiney, usually gold-colored, metal grit found at the gear oil drain plug is normal in these transmissions as the synchro friction rings are sacrificed and with use over time. The synchro teeth even if heavily worn will generally outlast the friction face on the synchronizer's rings by a long shot so the grit found at the drain plug really is not a sign of the need for transmission servicing in the absence of other gross operating symptoms. 

1st & 2nd gear synchros & grinding – Grinding is the sound and feel of the system slipping over multiple synchro teeth with every shift into a specific gear. This is not a common occurrence, just an experience I had. If the gears exhibit a single ding or two when cold that is mitigated by slightly slower shifting and it disappears completely once the tranny warms up, this seems to be normal for many of these cars as the transmission ages and the synchro friction rings lose some of their effectiveness. It does not necessarily require servicing. However, if the gears grind with every shift into a specific gear and the problem only gets worse with a gear lubricant refresh it is a dead giveaway you have this issue that should be investigated. This was an 80K+ odometer item for me.

I have read endless circular discussions concerning shifting feel and various brands and viscosities of gear lubricants. My view is, as the synchronizer friction rings become worn their ability to grip and synchronize the gears is diminished very gradually over many miles of use.

At some point, the ability of the rings to grip is so tenuous that very small changes in lube viscosity or temperature can result in significant differences in shifting acceptability. At this point, I feel looking for a gear lubricant one likes better may not be the answer. The transmission may simply need to be serviced and have any synchro rings exhibiting wear replaced. Usually, this is only the 1st & 2nd gear rings. On the upside, unlike earlier 911s the 993 does not require the engine to be removed to service the transmission or clutch.

 More information concerning gear lubricants can be found near the bottom of this page under the section labeled "Manual Transmission Gear Lubricants."

Metal chunks at the drain plug & repairing the differential  – When the transmission is open for synchro replacement on a base model Carrera the ring gear on the stock differential may exhibit some steel chips missing from its face along with these metal chips showing up at the drain plug.

Any shop that does a reasonable amount of Porsche transmission work will have a used or repaired standard differential on hand. This is because many times when a previous customer upgraded to a limited-slip differential the shop retained the original. These parts may be available at a very low cost to you. 

I have an unsubstantiated theory that when a base Carrera, which doesn't have a limited-slip differential, is pushed to the point of wheel spin, at the point where the tire reestablishes grip it jackhammers the standard differential gears. This may be the cause of rather large steel gouged chips from the ring gear face showing up at the transmission lubricant drain plug. 

Although seeing these chips can be disturbing they are not an indication of a serious issue with the transmission requiring immediate repair as many folks seem to drive their cars subsequently for thousands of trouble-free miles and only become aware of the damage at a gear lubricant refresh.

Engine vibration and body boom after replacing the transmission mount:
The original transmission mount for the standard G50.20/21 transmissions was 

Transmission
Mount

964 375 055 02. It has now been superseded and consolidated with 964 375 055 81 originally specified for G50.31/31/33 series used in a Carrera RS. The new part has both 964 375 055 81 and 964 375 055 82 printed on it. The rubber in the new part appears to be much stiffer than the original which transfers greater vibration into the car which has caused dissatisfaction for car owners. There are examples of owners spending large amounts of time and money looking for the root cause of the vibration in the clutch or internal to the transmission. 

Others feel the new mounts can be reasonably quiet if installed correctly. Claiming "if one deviates in the rotation of the mount [from the correct position] at installation by more than 10 degrees or so you will have horrendous NVH (noise, vibration, harshness) problems." They reference a picture in the shop manual for guidance on properly installing the mount.

Until Porsche sorts out this issue or an aftermarket manufacturer

Assembly Mark "A"
must point upwards for
 the correct positioning
of the mount.

sorts it out for them it may be a good idea to hold off on refreshing the transmission mount as part of servicing a clutch or transmission on a 964 or 993.

Here is a page that discusses the clutch and its role in shifting issues.

Here is a page that discusses engine mounts and their influence on shifting.

Here is a page that discusses related flywheel issues.

Another anecdotal solution to a transmission noise problem:

Recently I came across a posting on social media where the author has his car set up with an RS Light Weight Single Mass Flywheel, The RS transmission, and the stiffer RS motor mounts. His experience is that changing brands of transmission gear oil contributed significantly to quieting down transmission noise even with his setup which is inherently noisy.

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