964 C4 All Wheel Drive Mechanism

CPBMD · 11-06-2008, 05:34 PM

I read an article about the 964 C4 which talks about the complexities of the all wheel drive mechanism which eventually was made simpler. The article goes on to say that this AWD mechanism will fail between 100K and 120K miles. It then is very very expensive to fix if you can find someone to do it. They even talk about converting the car to a rear wheel drive C2 which isn't cheap either.

I am now a little worried. I have 79K miles on the car and it runs well as far as I can tell. I need to come to grips with this so I can enjoy the car without worrying too much about this potential major failure. I am hoping that this problem is uncommon because I was planning to drive the car for at least 150-200K miles. It is the best car I have ever driven. I love the "Raw"feeling of the ride.

Does anyone have any experience with the 964 C4 and the AWD system?

Thank You
constantine

rsrmike · 11-06-2008, 06:46 PM

The only real trouble I've seen consistenly is the hydraulic slave cylinders leaking. There are 2. One control the torque split between front and rear wheels, while another controls the lockup of the rear differential.

The best thing to do is make sure you flush your hydraulic system regularly by someone who has the porsche system tester for your car. There is a procedure when bleeding the system where you operate the control valves that send fluid pressure to the cylinders while the bleeder valves are open. You also need to flush the hydraulic booster pump and accumulator assembly.

The pressure switch that turns the pump on and off, and serves as a warning in the event of a hydraulic pump circuit failure is also prone to go out. They are not hard to change but can be rather expensive for the part. It's best to find a mechanic you feel comfortable and trust that has the tester to be sure he actually does the entire bleeding procedure properly. Many shops simply flush each brake caliper and call it done, where in reality you must go through an elaborate procedure utilizing a pressure bleeder, the "hammer" porsche tester, or newer, and also bleed the clutch slave cylinder. All of which is a very time consuming and irritating, messy process. If they let you watch, that's even better. They should bleed the accumulator in the trunk, both differential actualtors, all 8 bleeder valves on the calipers, and finally the clutch slave cylinder, for a proper hydraulics system flush. Prepare to use ALOT of brake fluid. It's also difficult to clean the old fluid from the oversized brake fluid reservoir on the C-4. I use a vacuum pump, hooked to a reservoir, with a small copper tube squished flat that lets me get into many of the partitioned off compartments in the reservoir, but even still you cant get to all of them.

I think lack of fluid maintanence is the #1 killer of the early hydraulic based AWD systems. Put forth the effort, and you should be ok...

BTW, it's best to do this every 2 years...

CPBMD · 11-07-2008, 05:39 AM

Thank you very much for the information. This is a complex process. How many hours of labor for the job? Is it therefore better done at a Porsche dealership?

Thanks
Constantine

rsrmike · 11-07-2008, 06:41 AM

I don't remember the labor, but I would expect to pay at least 2 hours if done properly. I'm not sure of the dealer(s) in your area, but the guys at my local dealer probably don't know what a 964 is, sadly. Also, with all the pressure put on those guys to turn hours, I wouldn't be surprised if some of the more difficult steps were, well, overlooked.

I would try to find an enthusiast independant who owns at least the "hammer", porsche's early system tester. And explain to him your concerns and why you want a "full hydraulic flush" Mention your knowledge of the differential locks, the accumulator in the trunk, the clutch salve cylinder, etc. And ask if he has the tester to activate the bleeding procedures. If he's not sure what you are talking about, find somewhere else.

I just remembered you have an '89, so you probably only have 2 piston rear brake calipers, meaning you should have 6 brake bleeder valves, not 8, as I mentioned earlier, but that's really not a big deal-just don't go in demanding they flush all 8 brake bleeders!!

Take care

CPBMD · 11-08-2008, 07:55 AM

Thanks very very much for all the information. I printed out your post and took it to an independent Porsche shop with 15 years of experience working on 964's and below. They don't work on 993's and above because of all the expensive computers and software needed to take care of these cars. Anyway he said that he has done this procudeure on 4 of the 964 C4s that he works on. He soes all the fluids in the car at the same time taking 4 hours. He did understand all about the bleeding procedures and was aware of the shortcuts that are taken. I have to ask him about the "hammer". Anyway he said he could do it. So I will get it done in the Spring. BTW he looked my car over and said it was one of the nicest 964's C4 he has seen. In perfect shape well maintained. I was very excited to hear that. I think I got a great deal on the car at $19K with 79K miles 3rd owner. The economic downturn has its upsides.

Thanks alot Cons

Bill Verburg · Bill Verburg's Garage

Is the article you read the 2 parter written by Joel Reiser is the April and May, '07, issues of Pano?

If not that would be a very good place to do some research.

klr10 · 11-09-2008, 02:40 PM

You might want to read this thread. I hope it's as rare as everyone's been telling me.

tbennett017 · 12:58 PM

I am nearing 200K miles on my C4 with no driveline problems. Relax. :-)

BUT the last quote in the above-referenced thread does bring up a concern. I would be leery about rolling diameters on your tires, front to back. That might cause excessive planetary gear motion in the diff, and that car had broken two of them in its lifetime. It's easy to imagine an owner having the wrong tire size and repeating the mistake over and over without understanding the ramifications...

IF a car was running at high speed in a situation like that, I could imagine the stress on that part being quite excessive. Maybe I'm stretching and being a glass-half-full guy, but that speaks to the actual reliability of the part in normal use.. no?

CPBMD · 11-16-2008, 08:09 PM

Thanks Tom
That is great to hear that you have the C4 at 200K miles. I aspire to that! Have you changed all the hydraulic fluids every 2 years like "rsrmike" recommends in his post? I am planning to change the fluids in the Spring.
What are the "rolling diameters on the tires". As long as I use the correct tires on the car I should avoid that problem, Right? I am running Sport Contact 2 tires from Continental. Front-205/55 R16 91V and Rear-225/50 R16 92V Are these the correct tires for my C4? I hope they are because I just bought them.
Thanks for the information.
Constantine

klr10 · 11-17-2008, 05:34 AM

Hi Guys! I'm the 'lucky' owner of the C4 that's currently having its gearbox replaced. The first time the planet gear smashed (33K miles ago), it was replaced at a Porsche dealer and I have the bill from the previous owner for £8.5K! There is no mention of tyres being changed or non standard sizes, the invoice lists the work as 'transaxle exploded'! The current problem is the same planet gear. The mechanic who identified the problem and is working on my car at the moment is ex Hongkong race team, and has worked on these things a lot. I'm on vacation in Canada until the weekend but will be driving up to see him on Monday, as I've got a new foam pad for the engine bay. I'll look at the tyres and wheels and chat to him about it, let you kinow what he has to say on the subject. I'm pretty sure they're bog standard tho... As others have said these trans are normally very reliable, but when I tried to get a used planet gear set, a few companies I spoke to weren't surprised and said it wasn't unusual. It worked out cheaper to buy a used box.

unclebilly · unclebilly's Garage

I am running 18's on my C4 and it has 160k miles on it. The tranny was rebuilt before i bought the car.

tbennett017 · 11-17-2008, 09:00 AM

Quote:

Originally Posted by CPBMD

Thanks Tom
Have you changed all the hydraulic fluids every 2 years like "rsrmike" recommends in his post? I am planning to change the fluids in the Spring.
What are the "rolling diameters on the tires". As long as I use the correct tires on the car I should avoid that problem, Right? I am running Sport Contact 2 tires from Continental. Front-205/55 R16 91V and Rear-225/50 R16 92V Are these the correct tires for my C4?

Fluids: Yes, change them if you can when you bleed the brakes. The C4 braking system can be bled with some simple tools (Pressure bleeder) and Adrian Streather's book that outlines the procedure - It is NOT the same as the C2 - you will need a "Hammer" diagnostic tool to bleed the PDAS locks. Streather's book explains this very well.

Tires: Those sound right. I am not an expert at this, but I would be careful about going to a larger tire in front, as some people have recommended... as that might create the improper diameters. You can imagine if the front diameters are not the same as the rear, then there will be a mis-match.

Benski · Benski's Garage

Quote:

Originally Posted by klr10

Hi Guys! I'm the 'lucky' owner of the C4 that's currently having its gearbox replaced. The first time the planet gear smashed (33K miles ago), it was replaced at a Porsche dealer and I have the bill from the previous owner for £8.5K! There is no mention of tyres being changed or non standard sizes, the invoice lists the work as 'transaxle exploded'! The current problem is the same planet gear. The mechanic who identified the problem and is working on my car at the moment is ex Hongkong race team, and has worked on these things a lot. I'm on vacation in Canada until the weekend but will be driving up to see him on Monday, as I've got a new foam pad for the engine bay. I'll look at the tyres and wheels and chat to him about it, let you kinow what he has to say on the subject. I'm pretty sure they're bog standard tho... As others have said these trans are normally very reliable, but when I tried to get a used planet gear set, a few companies I spoke to weren't surprised and said it wasn't unusual. It worked out cheaper to buy a used box.

I know its old but I would love to know the end of this story since I'm looking for a tranny for the second time!

Doug&Julie · 09-22-2010, 04:59 AM

Constantine, how is the C4 running? Mine has 156k miles and no issues. ...well, I bought it that way and dumped a lot of money into fixing leaks all over the place, including fluid changes in the front diff, etc. But still...no problems with the AWD system.

tgage · 09-25-2010, 12:33 AM

You can also use duremetric 's version of the hammer to bleed the differentials. It is much cheaper!

You really should be able to bleed it yourself, but... Motive bleeder ... A couple of lines to extract fluid into a bottle. 7, 10, 11 mm wrenches, jacks, hydraulic Jack. And then you are all set. That is a few hundred bucks

Tom

tokoro · tokoro's Garage

I have a 1994 c4 I'm in the change of the clutch and flywheel, in the shop they told me the two slaves that go in the transmission are without pressure like if you push them they don't go back, are they different from the clutch one? is this normal? In the console the Differential lock selector the yellow light is always blinking when getting a curve.
if someone can help me i will appreciate it THANKS

sml · 01-27-2011, 12:26 AM

not necessarily linked to your question, but there was a few comments in here about bleeding the brakes/diff system .. if you want to do it without the hammer ....

KTS 300/Bosch Hammer - Rennlist Discussion Forums

In summary ...
"Could always get them to do it by using the C4 switch on the lower dash - that's what I did last weekend when I did mine. Turn it to the left, crack bleed valve, when fluid stops close valve. Cycle the switch to the right then back left again and repeat. Takes about 20 turns to do each of the locks, but works."