Syncro Speedo diff drive gear replacement

HarryMann/Ray: When your Syncro's speedo packs up, many look toward the cable breaking or the speedo head - which is rare.

I've known the speedo head to start clicking and the needle to jump around before packing in completely, but not the cable to break.

But by far the most common cause seems to be the plastic drive gear within the front-diff, stripping a couple of teeth 

- easy to check... if you can get the large alloy nut undone that locates the cable into the diff. Don't crush this (its ally) unless you have a new cable anyway, so use plenty of brushing and plussing, and as thick a spanner as possible ( a hydraulic split-ring spanner might be ideal) but I'm not sure of the size. As the front wheels won't turn together with propshaft on, then simply ensure top-end at speedo is NOT spinning when underway + PLUS + twiddling that end rotates the cable at the diff-end - bingo, say YES to both these and it's got to be the diff-drive thats broken.

The nylon gear inside the diff can be changed without removing or dis-assembling the diff. The plastic gear now only comes from VW as a complete assembly with the shaft, bush and bearings - ready to insert. A 5mm roll-pin, along the top of the cable attachment boss, aligned fore-aft, secures the gear/shaft/bush assembly into the diff housing. Remove roll-pin; remove old gear assembly (noting that couple of teeth at least are damaged, if not think hard!); replace gear assembly (in exact alignment for pin; replace roll-pin; replace cable (lube inner while have chance); connect back up at speedo, test drive!

There's two problems drifting the pin out. Access - to get a few good strong blows to get it moving; a drift 12~13" long allows a decent swing and strike. My parallel drift set had a 4.8mm drift that just didn't seem beefy enough to get it moving, and from what Ray says below, don't try anything much smaller than this else you could be swaging the hollow roll-pin out rather than drifting it out.

I found 15 minutes on a bench grinder was time well spent, whittling a suitably sized tapered punch down to just under 5mm over the first inch and being as careful as possible to keep it nicely round and concentric. Steel punches and drifts are often made from hex bar, so to get some extra length (bringing the striking face well aft of the diff support cradle), I had just to grind a shade off two opposite angles and then a bit of masking tape to pad it nicely in a 1/2 drive extension gave a tool that transferred the blow well enough to shift the pin on the second strike of a club-hammer. Must be held as near to dead-in-line as possible - some hoses neeed forcing back out of the way. If someone offers to do this for you on a proper cylindrical grinder or in a lathe with a grinder attachment, specify 4.95mm +0.0/-0.05 as the finish size.

Ray and Harry exchange some banter on doing this job

Could be heaven (1 hour) or could be hell (a week or more?) - pick through this to make sure its somewhere in between.

Harry: Ray, I stripped two other diffs I have, both 5.43s and one was appalling, been right mullered around the boss and roll-pin area, so expected a stripped gear to come out... It did.

Second one, helluva job driving out roll-pin but gear assembly as good as perfict, so thats the one I put in today. Think they're about £80, and you can't buy just the gear anymore, whole assembly only. (over £100 now, check ETKA for the right one!!!, there are many and depends on both diff gearing and number of teeth on pinion).

Ray: I had a nightmare driving the pin out. I didn't realise it was a roll-pin and used a small drift which just drove the dirt out. The assembly would not come out so the bloke next door made up a puller for me which of course just broke the housing. I repaired that with araldite which is still OK after nearly a year and even after having a new VC fitted! The assembly cost £110 from Wayside Parts! Now I know what these roll-pins are like, todays one in the diff-lock came out easily with the correct size drift

Harry: Ah! Sorry to dredge up a troublesome saga then Ray.

I mole-gripped it, got a good purchase, made sure it would rotate both ways a bit, squirted a bit of plus-gas around and then used same grip to lever it outwards... mullered the thread though, but knew the unit was knackered and they come as whole these days.

Yes, 5mm for that one and 3mm for the difflocks. Too small and just tries to expand the roll-pin creating more bedlam. Even a genuine 4.8mm parallel drift wouldn't budge it, so made one up (above), and then I could finally get a good blow from a club hammer on it... moved immediately then.

Now that seems to be the trick... dead in line as poss (though not really possible on front diff) and a really hearty blow, anything less leaves it cold.

SyncroMonVie: I also unbolted and removed the mounting bar that supports the rear of the diff. This allowed me to get a better angle onto the roll pin.

NB. Might be worth changing diff oil afterwards, though the molten/sheared/ground up plastic teeth seem to disappear.

PS. As with all these jobs, good light and visual scrutiny of the point of impact (at the pin end) helps a lot. As does cleaning up the whole area beforehand... Strike that ally boss instead of the centre of the roll-pin one time too many and you may regret it. If the drift is bouncing around a lot upon impact -that's a warning sign - maybe not heavyweight enough blow or not directly in line. So a headlight is becoming an important part of the mechanics toolkit for working underneath. Vehicle needs just a gentle front lift for access to do this job, though get it on a lift and it'd transform the job completely!