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80-Series - No forward drive

BobMurphy

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scotland
Hi, I'm a stranger to this Forum as I have a 90 and a 100 . . . However . . .

A neighbour just phoned, he has a friend with an 80, petrol Auto.

The 80 was taken to a garage for an MOT test; they put it on a brake tester (rolling road !) and after that it would only move in reverse.

It seems the garage has now had the truck for seven months while they scratch their heads and wonder what to do next.

Did I have any ideas ??

My initial thought was an Auto Box problem. If it was Transfer Box it wouldn't move in either direction (Unless there is something in the 80 Transfer box I don't know about).

I then wondered about the 'lock-up' mechanism in the torque converter, could this have a 'one way' mechanism that is faulty ?

I suspect the problem is in the Auto Box, but I'm no expert on those things.

Any thoughts you experts ??

Bob.
 
I would suspect that the VC didn't like having the rear wheels stood while the front ones were being driven by the rollers. Just a guess.
 
They shouldnt put it on the rollers as its permanent four wheel drive. However, not sure why that would cause the issue.
 
I've read of the VC failing and giving both full lock up and no drive at all but never in just one direction, ditto the transfer box. My moneys on the auto box as it's the only component with asymmetric directional operation but it's a guess.
 
I have had a VC fail and it locked up solid, front prop shaft removed and I drove it as a rear wheel drive for a bit.
If it’s got reverse the transfer box should be ok.
I have had the front axle drive flanges just about making contact and they do drive until some force is applied then they slip.
Is the gearbox oil level correct?
Torque converter lock up has its own solenoid, not sure it’s used for reverse?
 
Any unexpected noises when attempting to drive forwards? Does the engine rev freely in D, or do the revs stall? I'm wondering if 'will only move in reverse' means the VC is seized forwards but not backwards. More information needed really but main gearbox failure seems the least likely cause to me, unless it's pure coincidence it happened when it did.
 
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If the VC can fail open, just engage the centre diff lock.
I removed a VC from the transfer box and the vehicle drove fine.
I think the most common VC failure is to lock solid.
 
Hmmmm interesting.

I am familiar with '90' Transfer boxes but a 'VC' (Viscous Coupling ?) is a new one to me. Is this specific to the '80-Series' ?

I'll need to do some research unless someone can provide an explanation of where it is, what it does and how it works.

Thanks for the input, I don't know the 80's owner and as my neighbour is heavily involved in the "Scottish Off-Road Club" he could be anywhere in Scotland. I'll find out more when my neighbour is back home.

Bob.
 
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The VC is just a fluid filled clutch stuck in the back of the transfer box which basically acts like a limited slip diff. It's just a set of closely fitting plates in an oil bath that offers some resistance to rotation but will slip enough to prevent transmission windup. The diff will never be fully open as the VC will still transfer some power to the rear axle if the front wheels lose traction. It's a device aimed at improving on road handling and performance in permanent 4wd setups. The 4wd Sierra I had years ago had one.
 
This thing I think Screenshot_20231108_133523_Photos.jpgScreenshot_20231108_133503_Photos.jpg
 
That's the puppy. I'm curious as to how just removing it when it fails (usually locking solid) makes the centre diff function like a standard, non VC equipped item or does another part need to be added in it's place or another mod done at the same time?
 
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Hi Towpack,
I had a manual 80 that the VC failed on.
It was like driving with the centre diff lock in all the time.
I removed the VC from the transfer case and the vehicle drove fine etc.
I think it would be the same as the earlier transfer box with no VC fitted?
 
Any luck on your fix as a year on I'm in the same boat. Also where best to get a new gearbox from.
 
The centre dif prevents wind up re front and rear axles. The VC runs in parallel with the centre dif so if it detects front axle going faster than rear it locks up so drive is restored to the front until a normal patch of road is reached and the VC disconnects. Works the other way round as well. No difference in road holding detected when I took mine out.
 
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