oh well you buy a cheap car on spec and get what you get......

I fixed the coolant loss from a blown pump to thermostat bypass hose......that was the easy part

then it was time to drive her......but not for long

all started well.....drove ok except for a suspension warning from the get go.....but it drove fine.....changed up and down automatically.....shifter is not stiff....when the car was at temp I toggled the traction control and sport mode switches just to see if lights went on and off.....at which point the gearbox fault warning kicked in and yellow light came on....not sure if related but that's how it happened

it was still auto changing but I noticed not so well.......then it got worse.......got to the point when it wouldn't change out of first......managed to get home a few miles away all in first......shifter still not stiff.....and using it wasn't making any difference so was still in first.....went into neutral and park ok and went into drive without issue.....but on take off wouldn't get out of first

now im guessing that after a cool down it might restart and gearchange with no problems for a short while before doing this again

Engine running ok , cooling fan coming on, no coolant warning, no obvious noises or smells.......don't know if my coolant prob had anything to do with this latest glitch......

Any ideas gents

UPDATE......

As I expected..... I took car out after it had properly cooled down......and sure enough the gearbox fault dash indicator did not light up.....drove for a few mins only and the car changed through all gears... I didn't run it long enough to get to full temp..so im thinking this is trans oil temperature related.....but still hoping someone can chip in with some focus

 

Hi there


Bad oil could cause shifting problems, and so could a dodgy TCU I suppose. Also a possibility is the valve body with either mechanical or electrical problems.

No doubt someone else has better ideas but what I'd be doing now as a minimum is changing the oil, filter, and O rings. It's easy done on a lifter, possible but awkward while on stands lying on your back on the floor.
You could also have worn clutch rings, which will surely modify their behaviour based on oil and temperature. It's impossible to know that without dismantling the box though, unless you have a very good and reliable service history.

With the right space, time, and tools, I would drop the box and service it with a rebuild kit, paying attention to the A drum for damage, and be ready to buy one of those if required.

When I did mine (myself, without help) it took me a weekend. I had bought all the parts required - rebuild kit, 20L oil, A drum. It cost about £450 for parts including the oil.

Yours might not actually need a full rebuild, but not knowing the history it may well be the safest option. Certainly though, oil first, as having that go wrong will cause failures elsewhere if not renewed in time.

Where in the UK are you?

 

Im in Essex .....southend


found this on the possibility of it being only electrical....


https://mercedessource.com/problems/...fting-problems


very useful series of vids on the electrical issues which can cause a limp home mode on the 722.650 MB transmission


but still suspicious of having just fixed a coolant blowout and perhaps having this problem as a related issue....coolant air lock?....is trans cooling linked to engine cooling


going to take it to a transmission firm tomo.....but expectant of a sharp intake of breath and 'you will need a new gearbox sir' response

 

Southend - too far for me to help unfortunately. I'm near Sheffield.

So you have the Merc transmission, which means not the ZF I was talking about. I don't know whether anything I said still applies to those as much as it does the ZF but I'm sure someone who does can help.

You're right about the 'new gearbox' response though. Reading articles about everyone else's gearbox troubles it's a common report from garages.

The oil in the ZF is cooled via the radiator. If it gets too hot then I'm sure it will have consequences.

 

What year and model?

Assume from your post that no codes were thrown?

Also sounds like when it is functioning, the shifts are smooth & at appprox the correct RPM's?

That it happened when you toggled sport mode might indicate an electrical issue as sport mode tells the tranny to change the shift points & it seems that when operated, something went pear shaped.

From the symptoms, I'd look in 3 areas:
* Wiring and connectors to be sure all is good & there are no chafed wires, esp in the Sport mode circuitry.
* It's possible that there is an issue in the valve body. blocked orifice/bad solenoid/stuck piston, etc.
* the fluid level is low &/or it is dirty causing the issue to only manifest occasionally.

However, as it seemed initially to come on with the sport mode selected, I'd poke in that area first. If it repeats, try to verify that only 1st is available (ie try to start off in 2nd or 3rd & see what happens). I would normally expect a gearbox fault to cause the tranny to go in to limp-home - which should mean 3rd in the only gear that can be selected. If it stays in first then the issue seems more likely to relate to shifting - either the command isn't getting through or the VB is failing to respond.

If, when working, it shifts smoothly and there are no slips, then I'd not think of a bad drum / clutch pack issue as being the highest probability.

I'm thinking that the fluid level might be the first point of check - at least it is the least intrusive. At the same time check the smell. If it got hot & burned the frictions, it will smell burnt - but I doubt that is the issue from what you describe.

After that verify all electrical connections are good.

Next level gets more involved as the valve body check requires the pan to be dropped & electrical gremlins may point to a bad wiring harness/connector or fault with the TCU.

 

1999 XJR


went to an auto trans expert.....ran a full diagnostic on the gearbox with 2 grands worth of snap on diag tool and came up with 'input turbine speed' fault which he put down to the electrical board atop the valve body having gone bad......that and/or plug connector/harness...


he cleared the code and it ran ok for an afternoon......then I drove it today .....ran ok for about an hour and a half before throwing the gearbox fault again and car going into 3rd gear default because it hadn't been stopped....when stopped and restarted it ran in 2nd gear default....then when stopped and cooled right down it all started normally and ran ok all the way home being a journey of 30 mins....no doubt if longer then it would have thrown a gearbox fault again....he reckoned putting it in sport mode just triggered the board situation but want the root cause


so its looking like a bad valvo body circuit board and/or plug.....the auto guy checked my fluids and seemed happy.....test drove it and was happy with shifts


bottom line is £565 for a new board/plug. oil and filter change.......to be honest if that fixes it its a good deal


thanks for input people.....it was valuable......


could do the job myself but with 2/3 rds of the price in parts its not worth crawling about under the car


now all I have to cure is the suspension warning and the old girl is perfect.....the fault doesn't seem to have any ride or reduced performance associated issues so hoping that is nothing more than a bad connection

 

I was able to buy the board and plug for about $135 on ebay, but on the XKR it was a pain to replace because it was necessary to disconnect the exhaust pipes at the cats and unbolt the right side cat to gain access to the plug. I expect the same is true for the XJR.

 

I think even if I bought all the stuff its not really worth my while to crawl around and do the job.....plus the cost of the trans fluid .....new oil pan/filter and connector


plus I hear that the electronics board may need special tools for setting up for shift speed changes....so its making sense to have an auto pro do it


oh well......car cost 1200 uk pounds about $1800....so factoring in $800 for the auto work and $700 for tires and shes good to go for a few more miles.....body is really good condition with no arch rust tho seats will need re colouring and headlining replacing....all in all....good deal


have been driving today with no gearbox fault but it seems its time temperature related as to when it comes on.....if I only drove a few mile a day I would never know it had a problem


one other question tho.....the suspension fault I have with gearbox fault only gives me an amber dash light warning....is this restricting performance or is it just warning me

 

odd isn't it....but probably not too odd.....have been running it back and forward on normal traffic 8 mile runs with no sign of gearbox fault.....im still pretty sure if I left it in traffic long enough it would cook something and the fault would reappear but for a few days nothing....don't want to try and fault it and crawl home in second or third but it perhaps that's the only way to know ive got to do the gearbox elec panel repair to really fix the issue.....suspension fault warning still on but no symptoms so hopefully wiring issue