Sunday, February 18, 2007

Locking front differential

I started out with the original front differential that had no diff lock, but looking at it a bit closer, I could see that the casting was prepared for a diff lock, all that had to be done was to bore out the housing on a milling machine, drill and tap the hole that holds the electrical switch and tap the two holes that hold the actuator on.
The diff carrier, fork, push rod and guide from a Syncro transmission will fit in there without modification, so I put the original ring gear onto a locking diff carrier that I still had from a core tranny, put new bearings in and set bearing pre-load and gear backlash.

I also took the Viscous Coupling out, there was quite a bit of wear on the splines and I had noticed that the RH side front tire had a lot of cupping.
When it gets older, the VC gets hard, heats up and eventually fails, so I took the VC apart, cut the splines out of it and pressed these into a sleeve. Drilled 4 set screw in each end to prevent it from slipping and voila, a coupler.
Now without a VC you need a decoupler in the tranny, so I will cover this in a next post.


TDIWesty said...

I also sent you a version of this question via the samba forum:

I have an on-going TDI Syncro Westy project. I finished the basic project a few years ago, and am now in the refinement/maintenance/repair mode. I am currently rebuilding & upgrading my transaxle including taller 4th & 5th gears and a Peloquin LSD.

I noticed in your blog concerning the locking front diff that you mentioned setting the backlash. I am now ready to set my pinion/ring backlash, but I don't have access to the VW 381/7/8/9 tool.

I am considering making a tool to measure the backlash (utilizing my magnetic base and indicator). Do you know the radius at which the VW backlash specs apply, or can you measure it for me if you have the VW381/7 tool?

Thank you for any help you can offer.

Eric Poulson
Bellingham, WA

Herman said...

Hi Eric, I don't have this special tool, I use a small magnet that I stick onto the differential carrier and position the tip of the dial indicator onto the magnet about where the end of the ring gear tooth is.

If you look at the drawing on page 39.28 of your Bentley manual, you can see that they measure close to the top of the ring gear tooth.

Cheers Herman

TDIWesty said...

Many thanks Herman!
I will use your magnet method.


Adrian said...

Hi Herman,

SLightly off-topic question related to diff lock. Are you aware of any electrically actuated diff lock valve. I'd like to replace the pull knobs on my '89 Syncro 2.6i.


Herman said...

Hi Adrian, I would keep it simple with a mechanical knob.
One option would be the pull knob from busschmiede in Germany:
look for : Unterdruck Ventil, but it is about 30 Euro's.
Another option would be a 4 way control valve from Clippard:

You see them for sale on Ebay and are very cheap, it is just not a pull knob but a toggle switch.

Karl said...

Hi Herman,

I love this blog! I'm wondering if you considered replacing the viscous coupling with a Torsen T-3 center differential?

Thanks for posting information about your Syncro!!


Herman said...

Hi Karl, thanks.

It would be nice if you could install a Torsen differential instead of viscous coupling but this will not work with this transmission. The problem is that the gearbox out put shaft should go to the Torsen first and then to the front diff and rear diff, like in an Audi Quattro that has hollow shafts, but in this transmission it was designed to go to the rear diff only, the front drive was kind of an after thought.

Cheers Herman