2.5 ECU Modes, Super limp

[xmexclusive]

Hi All

Forgot to post this when it happened but think it is worth noting.
Was taking one of the cars for a holiday in Petersfield.
The reason for the trip was that it was going out of regular use.
The clutch was beginning to slip occasionally.
I need it stored until I have an engine ready and unit space.
Anyway tripping gently west on the A303 using cruise to keep at the speed limit.
Worked fine until a fairly steep incline somewhere beyond Wincanton.
The clutch started to slip a little and the revs went up a bit.
Then it seemed as if the ECU got control of things and the revs started to drop.
Revs increased a little and it tried to reset speed to 70 and failed.
ECU then took drastic action and shut everything down.
No power, total limp mode, dash lights out.
Then the ECU rebooted through the start up routine and we were running again.
The reboot had also locked cruise out.
Must read the ECU faults sometime.

John

[noz]

Hi John,


That is an interesting one. Most of the symptoms sound like general power failure. On my first XM 2.5TD I found after a lot of investigation that someone had hollowed out the inside of the stop solenoid. It looked whole from the outside and even had the wire still attached. However, there was nothing on the inside. The engine started and stopped OK. So I had no option but to assume that the stop solenoid was simply a safety device and that the normal start stop was governed by one of only two other possible signals to the pump. One being the advance retard signal and the other being the quantity of injection signal. I assume it was the latter which stopped the engine when the ignition key was turned off.

The injection system on the 2.5TD is a half-way house between a purely mechanical system (like the 2.1TD) and the next generation of injection systems i.e. Common Rail. The pump used on the 2.5TD would seem to be a normal mechanical pump but with the centrifugal advance and the fuel quantity governor both replaced by stepper motors. Limp home mode on the surface would seem to consist of both stepper motors remaining at their minimum or default positions. That gives enough fuel for a tickover amount of power which is enough to get you to a safe place.

The above rambling probably won't help but I thought it was worth throwing it into the mix. The faults will be interesting when you get around to reading them. Particularly if one of the faults turns out to be low ECU voltage !

cheers

noz