Tim S wrote:
cynic wrote:

My '03 Discovery has begun a sporadic fault. Symptom being the engine
management fault light comes on and the accelerator pedal has no effect
although the engine continues to run at tick over. This has happened
twice now at about two weeks apart. First time just before Christmas on
a busy dual carriageway during peak period. I managed to coast up onto
the grass right out of the way of the traffic fortunately or some loon
would no doubt have come to a sudden stop in my rear end. I called the
RAC out and on arrival the fault had disappeared. the engine started
and ran ok so we travelled to a nearby car park where the RAC guy
plugged his "universal" comms laptop into the link but the Land Rover
system would not talk to his set.
He theorised that the fault was transient but would be in the engine
management memory and a Land Rover agency could interrogate it.
Having tried and failed to make an appointment at a reasonable time
before Christmas the problem recurred on boxing day but I found simply
turning off then on again was sufficient to reset the unit again. I am
now due to take the vehicle into an agency next week but they were not
confident the memory would still hold the fault.
Have we anyone in our numbers who have knowledge of the engine
management system on a Discovery 2.5 Diesel and is it feasible to get
hold of the commes interrogation software to run on my own laptop?

A reasonable starting point would be the throttle potentiometer, assuming
the disco uses such a thing as its throttle position sensor.


Agreed. It *has* to use such..since the ECU fully controls the
injectors. And therefire the throttle pedal must talk to the ECU and not
anything in the engine direct.


Might be an intermittent wiring fault or a dodgey pot (or a million other
things, but you have to start somewhere).


I think not a million other things.

Consider. The engine went to idle (default safety setting) but otherwise
ran perfectly. A light came on. Now that pretty much tells you that the
ECU said 'I have an out of spec condition, I don't know what to do, and
this is my best response'.

Since the engine didn't misfire, go smokey or in fact do anything
untoward, one may conclude that by and large the fault wasn't with any
other sensors like temperature and so in, or with an injector or the
injector system. I.e. it wasn't with the feedback part of the engine
control at all, it was with the primary input to the ECU.

Couple that with my experience that the worts feature of that era and
type of landrover is crappy electrical connections, and I feel there is
very strong evidence that its a corroded connector to the pot.

Now it gets speculative, because I am not intimately aware of the
software in that ECU.

It is pretty standard practice to monitor all the resistances of the
sensors (and they are all by and large variable resistors/thermistors
etc) and to generate a fault condition when they go outside range.
However whether this will result in a single instance of the fault, that
then goes away causing the system to go into failsafe, or whether it
will note the fault, light the lamp and *carry on* is a moot point.

I know that my Jaguar used to go into 'limp home' mode (50mph max and no
bloody power) on a variety of faults - high gearbox temp was one. But
stopping and restarting cleared the fault.

It may be that ANY instance of dodgy throttle pot on the Disco results
in 'go to idle and stay there' it being deemed that to drive with a
throttle sensor problem is potentially too dangerous.

Again, I am not sure whether the pot is wired AS a pot, or just as a
variable resistor, but I suspect, motor manufacturers being what they
are, that its as a resistor. Saves a wire dunnit?. Inspection would show
whether its two wire or three, anyway. I would ALSO ssdupect that 'high
resitsance=low throttle' is the way any sane person would do it, so that
high resistance in contacts results in low throttle..we don't want the
thing to kick into full chat if a wire falls off - even momentarily.
Shorts are far less likely..

So, logic suggests its a two wire system, that is monitored for
resistance that is too low, or too high. Mostly the latter.

And it went too high,. That MIGHT be a faulty/worn pot track, but 03 is
a young car, and it seems fairly unlikely to have worn in that time.
Alasos a worn track is FAR more repeatble..go to that throttle posuitin
and it WILL show a problem in a large number ofcases. Its not doing that.

OTOH MY experience of the Defender 2.5 tdi, which shares most of the
engine parts with the Disco is that dodgy connections are mandatory on
these vehicles, It's their weakest link frankly.

I suspect that ALL that plugging in a diagnostic will do, is tell you
that a high out of range throttle pot resistance was detected. That
takes you nowhere. It just means that the garage can do what I have done
and intuit a problem in that circuit. And charge you £80 for the
privilege. All they will then do is simply replace the throttle pot
anyway, because that is a guarantee that it's not that as a problem,
clean up the connectors and stuff it all back together and hope that the
£400 quid cheque doesn't bounce.

If it is an intermittent connection, and if as I suspect it takes but
one instance to blue screen the ECU till its rebooted, as it were, then
my pragmatic advice is to find the connections, do a thorough job of
cleaning and putting water repellent stuff on them, and then run the car
a bit .. it will always get you home, if you simply switch off and
restart, until the time it doesn't, at which point the fault will be a
lot easier to find.

Now, anyone know why my defender suddenly has got a long soft brake
pedal? Just been serviced (always a mistake) by the very expensive
dealers..who have in the long past under previous management managed to
introduce expensive faults..its got plenty of fluid in it..so can't see
how air got in..














Try reseating the connectors. If
you can ascertain that it is a simple pot, disconnect it and use a
multimeter to see if it's giving sane and stable readings as you move the
throttle over the operating range.

As you mention, the ECU *should* record its last error at least and Land
Rover should be able to read this out. However, they might try to charge a
full hour's fee (like 80 pounds or whatever) so another approach is to try
a small garage or tuning specialist - such folk have offered to do a diag
dump for me for as little as 15 pounds, until they found out their kit
wasn't compatible with my mouldy old '98 Daewoo.

Don't know if that's of any help or not...

Cheers

Tim