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LT77 gearbox: MTF vs ATF

NeilA

In Fourth Gear
Thought I would share my experiences of gearbox oils in case it could save someone grief/expense. Just for the record i'm not recommending/trashing any brand and don't have associations with any oil company or retailer. These are just my observations with my gearbox.

I have a 1991 200tdi Defender with a LT77 gearbox. Its done 185k so its a bit tired and doesn't like downshifting from 3rd to 2nd. When i bought the car the gearbox was only half full and the fluid was a nasty colour. After a bit of research I decided to try a synthetic oil to see if it could improve the shift quality.

First of all i tried a synthetic MTF marketed by Difflock, Evolution 1/MT75D. Had read a lot of positive forum comments on its use in the LT77. It certainly shifted more smoothly, still a bit stiff in the cold, but I found that it would bounce out of gear when shifting into 5th, accompanied by an awful graunching sound like I'd missed the gear. I ended up having to hold the gearstick in 5th and gently feed in the clutch hoping that it would stay in. I though that perhaps it was a bit too 'slippy' and there was not enough friction?

Next I tried Redline MTL. A synthetic ester based MTF which a lot of people think is the best out there for this box. It is a bit thicker than the Difflock stuff, has a GL4 rating, and very good low temperature viscosity. It is marketed for use in manual gearboxes that specify an ATF. They also make a big deal out of using the correct friction modifiers for manual gearboxes. This stuff made the box feel very tight. Each gear slotted in very precisely, there was no way it was going to jump out of 5th. It was definitely thicker and there was a loss of 'feel' in the gearchange. It was great for about 1000 miles but then the downshift from 3rd to 2nd deteriorated to the point where it often would not go in at all. The box was stiff in all gears, but particularly in the cold. I concluded that it was basically too thick and had too much friction for my box. Interestingly, when I drained it out it had changed from red to a brown/orange. Its unlikely to have degraded, so i think it may have done a really good job of cleaning the gearbox. I think esters are supposed to have this effect and perhaps the gearchange deteriorated because the box became squeaky clean?

Next I decide to go back to an ATF. I decided on a synthetic ATF and didn't want to pay through the nose anymore. After a bit of research I went for Motul dexron III. Motul have an excellent reputation for synthetic racing oils but the prices are not as stupid as some. I've just done the oil change with this and so far it is excellent. Much thinner than the MTL, you can feel what the gearbox is doing, slots nicely into 5th without bouncing out, and will downshift from 3rd to 2nd with only a bit of baulking. I've only done a few miles so far but I'm pretty pleased. So perhaps thin and slippy enough for downshifting but with sufficient friction to hold the gears. Another thing I noted is that the car feels less sluggish than with the MTL. Originally I thought this might be down to the fact that I had put heavy grade gear oils in the transfer box and diffs at the same time, but actually I think the viscosity in the main gearbox has much more of an effect.

Some people obviously have great results with MTFs but I have not found them to be better than ATF in my box. If anyone wants more info this site has some useful data: http://members.shaw.ca/jbarge/MTF.htm

I purchased my Motul from a company that sells it on eBay. They provided a really good service.

Neil
 
Good info Neil, thanks.

I wsa having endless greif with my gear changes, but discovered it was all down to things being loose in the remote shift mechanism. While I was messing though, I decided to change the oil and went for Castrol ATF Dexron III. It has certainly quietened the box down and smoothed things out a bit, but after looking at what came out, and the amount of metal filings in it, that's no surprise. I was going to track down some MTF but couldn't find any when I wanted it, so just cracke don with what was to hand. Your finding suggest that was no bad thing, although maybe for the next change I'll track down some Motul.
 
Hope you dont mind me bumping this thread to ask a related question;

Has anyone used gearbox additives in the boxes and diffs etc?

Im thinking of trying some Slik or Molyslip in mine.
Is this a bad idea?

My box is fine at present, apart from a bit of a 'notch' changing into 2nd (worse when cold).
 
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