Macster

Two unrelated topics.

I'll cover the headlight first: It just so happened today I stopped in at the dealer to have a burned out headlight bulb replaced. Some time back I suspected the headlight igniter or ballast or whatever one calls it was going out due to the intermittent behavior of the passenger side headlight to come on when I turned on the headlights.

Experimenting a bit I found that if I turned the lights off then back on the headlight would come on. This felt more like a bulb problem but I ignored it until last night when turning the headlights on and off and on again the passenger side glowed a bit yellow then went dark.

So at the dealer today to get the bulb replaced. I had bought the bulb the other day just to make sure I had one handy. $141 with my PCA discount.

As I let one the techs work on the headlight I walked around to say hi to the techs and one of techs called me over and pointed out a Turbo front diff and cardan shaft on the bench.

Seems what happened is the car needed to be towed and the tow truck operator failed to heed the owner and insisted on towing the car on its rear tires. The result was the viscous coupling was ruined. This is part of the front diff. The tech said the cost to replace the unit with labor runs about $3K.

There's more about this AWD system that those who live in areas where snow is common and continue to drive their AWD car in the winter should be aware of.

If you ever encounter a situation in which the front wheels/tires are on dry pavement, such as might be the case when pulling into a garage with a sloped driveway, and the front tires get on the dry garage pavement while the rear tres remain in the snow and spin as you try to make the last few feet into the garage, do not continue with this. This will cause the front diff/viscous coupling to possibly get damaged.

Another situation -- and I have seen a video clip of someone doing this -- is when the car is stuck and the rear tires are spinning. Do not continue with this. Rock the car without spinning the rear tires until you get the car unstuck or find another way to get the car out of the snow.

"02996ttx50

yeah, you're right as usual macster!

that's why some of us just take em out

paul29

If the vehicle is AWD and the front wheels are on dry pavement the vehicle will move ahead if the accelerator is pressed while in a forward gear, doesn't matter whether the rears spin or not. The bias can change depending on the system but the vehicle still moves. The tow truck scenario is a completely different situation and will cause damage, the vehicle should have been put on a flatbed or rollback tow truck.

SSST

At 3000 clams I would just go ahead and do the RWD conversion. Last year my water pump let go as I was pulling into the driveway. I live about 40 miles from the nearest P repair shop so I called for a flatbed. A regular tow truck showed up and I sent him away.

Macster

No, the car won't move, not if the rear wheels/tires spin. In the video clip I watched the Turbo's rear wheels spun yet the fronts did not and the car didn't show any signs the front tires were making any effort to move the car.

The coupling is viscous not mechanical, and all the spinning rear wheels do is overheat the viscous coupling and ruin it.

This is why these cars have to be dyno'd on a dyno that has the front and rear rollers coupled so the front roller turns at the same speed as the rear roller. Otherwise, if the rollers are uncoupled the rear tires spin the rear roller and the front tires do not spin the front roller and in short order the viscous coupling is toast.

Macster

In the case of the car I mentioned above the tow truck company was paying for the repairs.

But for an owner facing the cost of fixing a hosed front diff/viscous coupling with his own money, AWD to RWD conversion could be an option, provided the conversion didn't introduce any issues with the stability control system or other nanny systems in the car.

"02996ttx50

oh it does, quite thankfully lol. it does

paul29

What AWD Turbo vehicle are you talking about 911, Cayenne?

dprantl

How is it AWD if the rear tires spin and the front tires do not move the car? Are you sure the video you saw wasn't of someone doing a typical burnout while standing on the brakes? The function of the viscous coupling is to transfer engine power to the front wheels if the rear wheels do not have traction, right? Just because it is a viscous coupling and not mechanical does not mean it won't be transferring power to the front wheels, otherwise conventional automatic transmissions would just be generating heat and not moving cars...

Dan

DaveCarrera4

+1. The silicone fluid within the coupling becomes viscous with shear. Shear comes from a difference in the front vs rear rotation. If the rears spin and the fronts don't, the fluid gets thick and wants to transfer power to the fronts. But there is a limit. Only 35% max. IIRC.

paul29

It appears the op does not understand what AWD means ( All Wheel Drive ) nor what it is for nor how it works. Even at 35% bias the Turbo has plenty of torque to send to the front axels in first gear be it manual or pdk. I just changed the oil+filter and did a brake fluid change on my Turbo pdk, it was still on my hoist so I started it put it in drive and took a short video, all wheels are rotating even at idle.

ANJ-911

I think there is some truth to what the OP is saying, pretty informative thread here on Renntech: How does 996 all wheel drive system work?

Sounds to me that the uphill snowy driveway dry garage scenario is possible.

paul29

We will have snow before long, my driveway is sloped up to the garage entrance. I will delay putting the 997.2 Turbo away for the off season to see how it responds in the situation described.

993GT

997TT's utilize a completely different AWD system, 996 uses a V.C. coupler to drive the front wheels, 997 uses a electro-mechanical clutch system..997 actually sends REAL power to the front

DaveCarrera4

I thought the 997 was the same, and that the 991 was changed back to the 964 electro clutch packs and before it, the 959?