The autobox in my dad's old XJ40 (that's a Jag XJ6, probably a ZF box?) keeps switching into limp home mode, but occasionally doesn't and works fine. Gearbox fluid level is apparently OK. Is there anything that it's likely to be (electrics maybe?, or is it just a gearbox that's had it?
What's the battery voltage like? A low battery voltage can cause *all* sorts of autobox fun and games. Any fault codes?
JB
Hi
Take it to a gearbox specialist, and don't mess, it's not worth it. It could be a multitude of things, gearbox ECU, poor voltage as mentioned, a cooling problem, filter blockage or just in need of a service.
Nigel
Its either the gear position selector sensor, or the 'overload' relay in the boot.
Tim..
Not a clue. Are the fault codes readable off the dash with the trip computer (ISTR they are)?
On an XJ40 that's worth under grand?
just a gearbox that's had it?
Scrap it then!
Nigel
Or just find out what's up, it's probably something trivial.
If the volts getting to the gearbox ECU are low it will go into limp home mode - I've a feeling this is because the power solenoids need a full '12 volts' to work properly, so to prevent any damage to the clutches it goes to the single gear state of limp home.
So it could be the alternator or more likely a poor connection within the gearbox loom or earthing. It is after all a Jag. ;-)
If you've got a DVM that logs high and low volts, couple it up to a cigar lighter plug (Halfords, etc) and check if the voltage is going under 11.5 or so with the engine running. That would show an intermittent charging fault. Wiring the DVM to the ECU loom is more tricky. ;-)
Got a multimeter but don't think it does that fancy stuff. It's got a hold button and that's it. There have been alternator problems in the past though.
Multimeter seems to think it's pretty healthy. 14.4V or so after a second or two of running, 13V or so before...
Hi
You mention fluid level is ok, what is the condition of the fluid? Brown? Smells of burning or is it still a healthy red?
How many miles has the box done since it's last service?
Sometimes a simple oil and filter change can cure many faults on an auto box, your fault could be a sticking valve within the box, or low pump pressure.
The fluid degrades with age, and if it's been over worked at any stage, then it's effectiveness has been compromised, allowing slipping on the brake-bands, and the oil to overheat, and then the box will go into limp home mode.
It's the same with a voltage drop the box will go into a pre-programmed mode.
Volvo had a similar fault on their autobox's in the late 90's, it turned out to be a faulty pressure sensor switch, which bolts on to the side of the box, next to the inhibitor switch. This presented the same symptoms, as your Jag.
But you say it's intermittent, which doesn't sound electrical, therefore I would look at something which is variable, and the only thing that will change is the oil temp, and pump pressure.
Nigel
Intermittent electrical faults are depressingly common on XJ40s!
Don't think any of these will make the box go into limp home mode - although I'm open to being persuaded I'm wrong.
On limp home mode with an electronically controlled box, you only get one forward gear - usually third on a four speed box - neutral, park and reverse.
Not looked at the oil yet, but you can start the engine and have the limp home mode come on, then restart it a few times and have it not come on. Start it again, and it'll come on again... Might give an oil change a go if it looks especially manky.
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