Jatco transmission issues
 

Hello everybody,

It’s sad really that my first message here has to be a negative one. Anyways, I have a serious issue with my 75 Jatco box. This saga started a few months back in may, when I acquired a beautiful -99 75 Conny with a blown reverse belt. My mechanic happily changed it for me and for a while the car worked like a dream (unlike my 45 haha). But then, some problems started to appear.

When I start driving with a cold engine, the gearbox is REALLY sloppy, like it needs 3-4k revs just to start slowly crawling ahead. Surprisingly, reverse gear works just fine. After a while, assumingly when the gearbox has warmed up, the box works just as usual. When the box is warm, it’s smooth as butter and it works very well. However, last Friday it REALLY left me down. I went to pick up groceries, the box being sloppy as usual, but this time, it really did not want to move. I gave her like 4-5k revs (I didn’t dare to give it more) and then it triggered limp mode. Triggering limp mode did not make the car move at all. After a while sitting, the box continued it usual sloppiness and drove “fine” for a half-mile, then everything stopped working. Limp mode continuously came on and off, the car did not move forward nor reverse at all. I had to tow it (yes stupid idea) with engine running to just to get it back home. I figured that if I keep the engine running and gear in neutral, maybe it doesn’t do too much damage.

Since then I have tried to read fault codes from the box via Toaf, but there’s none. I even had issues connecting to the box in the first place. Me and my mechanic are astonished. I think that maybe there are connecting issues due to my old nasty coolant leak. The upper coolant hose under the airfilter box had a rupture on it and it was spewing coolant right to the top of the gearbox, maybe that’s causing the connection issues???

But I seriously wonder why the box became so sloppy all of the sudden. When the renovation was complete, me and my wife took the car for a nice long trip without any gearbox issues. After we returned, then the box became all sloppy (that’s when the hose ruptured too). I wonder could it be because of the oil my mechanic used? He said that he used Comma MVATF+ oil and deemed it to be compatible, is it really? He now says that the gearbox issues could be because of gearbox valves or solenoids are getting stuck…?

I work in IT and my mechanic is not really that familiar with the Jatco autobox. He used to take very good care of my 45 and 600 so that’s why I still use him. I hope there’s someone that could give me and my mechanic a nudge to the right direction

Ps: We do not have access to a T4 system, the closest t4 to me being over 500 miles away, and with a broken gearbox I obviously can’t reach it.

Hello Samu and :welcome: to the club.

I'm not quite sure what you mean by a "blown reverse belt" but I take it that your mechanic replaced something in the automatic gearbox.

Here is the data sheet for Comma MVATF+ and it doesn't meet the MG Rover "N402" specification. However I doubt that the fluid could be responsible for such a severe malfunction. I think it's more likely that the gearbox has been underfilled (raising the revs being necessary to achieve the correct fluid pressure).

If you're not able to work on the car yourself my suggestion is that you find an automatic transmission specialist who can drain the fluid and replace it with a type which meets N402 spec. As the JATCO has no dipstick, someone who knows the procedure is essential particularly as the level checking plug is on the bottom of the gearbox casing! :o

Simon

Before a real expert comes along humour me with what I've done for refilling the Jatco box. I know you're supposed to drain the whole box and carefully run it for seconds through each gear to expel the old fluid. I decided some years ago that I would drain and replace with the approved fluid from here till it reached the drain plug hole knowing full well that two or so litres would be left in. Run it for a few miles, up on the ramps, drain and fill as before thus diluting the mixture of old and new. If I was feeling wealthy I would do that again so that the original old fluid would be an insignificant proportion. It may not be strictly kosher but it's worked for me on my 150,000 mile Diesel Tourer. My own local garage - whom I trust - said they would not dare run the Jatco near dry and I got the impression that pretty well all garages would only drain and replace what they could more or less in the same manner.

The correct spec of oil is available at Biltema: https://www.biltema.fi/autoilu---mp/...ljy-2000041561

Do a bit of research and I think you'll find that the Jatco transmission was used in lots of other cars too, so it's not as uncommon as you might think.

Regards

Hello and thank you for the helpful answers. The part that my mechanic replaced was the brake band back in May. I am a little bit sceptical about the biltema oil, as I’ve had bad experience of being their customer in the past. It does list JATCO N402 as supported, but I’d need some sort of clarification on that. Would the oil replacement really be the saving grace for that box?


Sent from my iPhone using Tapatalk

Sure some things from Biltema are poor substitutes like track-rods, but can they really fake the specs of their oil? A Norwegian poster (Beinet1) used the same Biltema oil. Link: https://the75andztclub.co.uk/forum/s...=122171&page=4

Issues connecting to the box with Toaf sound familiar, a Swedish owner very recently had problems too and found a blown fuse. Does Toaf connect, or does it report module not found?

Regards

Toaf’s connection is very very finicky. Sometimes it finds modules, sometimes not. Sometimes it reports an access violation as well. The connection issues however are no way exclusive to the Jatco box either, so maybe that needs more investigation. I’ll look into the fuse and Biltema oil

Sorry couldn't remember earlier but then lightbulb moment. I believe he had no indicator of the selected gear, found fuse 4 had blown and most of his problems vanished (he's just confirmed fuse 4 engine bay fusebox).

I am a big fan of Toaf, use it a lot more than my T4 as I find Toaf 100% reliable. As you're in IT you might find it interesting that I use a RS232 lead rather than USB. I don't understand the difference but I tried a USB one and quickly reverted to the serial lead (WinXP).

Good luck

Being the curious type I discovered that Kuusamo is in Finland.---:D

Likewise :}