Parker 7 has commented in a recent post -

“The ECU is electrically not powered until two relays closed, so not hot at all times”

Not hot of course referring to “no power supplied and not active”



My understanding of this is that the ECU has 3 B+ inputs (12v +)



: P1104-24 Wire colour BLG from the ECU control relay to ECU high power black connector

: P1105-17 Wire colour BLG from the ECU control relay to the low power black connector



: P1105-33 Wire colour WK from fuse 12 in the rh engine bay fuse box. This fuse is

ignition controlled. ie. 12v only available with ignition ON. This fuse also

powers the ignition coils.





Thus when the ignition is turned on, initial 12v is provided to the ECU at P1105-33 this in turn allows the ECU to provide a ground output from P1105-18, wire colour NP to the ECU controlled relay. This ECU relay is then able to supply further 12v power to the ECU input P1105-17 and input P1104-24, both with wire colours BLG.

The 12v to the switch side of this relay comes from fuse 6 in the rh engine bay fuse box, ECU relay supply current from fuse 11 in the same fuse box. Both fuses are live all times and do not require ignition ON, however the relay does require the ground signal from the ECU mentioned above before the relay energizes.

This relay also provides 12v power to -

:Fuel canister close valve

:Injectors 1 to 6



The fuel pump relay receives its ground signal from output P1104-19 at the ECU.



Am I correct in assuming that once ignition is OFF there is no power supplied to the ECU.

I find no evidence of power supply to the ECU once ignition is OFF.

It does state in the ECU details in Jaguar publications that the software is programmed into two eproms and one eeprom. The ECU has nonvolatile memory so that on-board diagnosis and adaptive information is maintained if the vehicle battery is disconnected.

To me this completely debunks the often quoted stated comment of disconnecting the battery to reset the ECU.





John Herbert

1996 XJR

 

Preach, Brother! I have been trying to debunk this claim for a long time. Crossing the disconnected positive and negatives will also do nothing.

 

The one common method around of ignition key sequence as you sequence the grounding of the positive battery cable made me think back then , but doesn't the positive cable removal instantaneously drop out / open the king relays ( on the fuse boxes ) required to power the devices on the positive wires positive electron flow direction ?

Unless some draining does accure in the instant , but your positive cable is still in the air moving toward ground either at the battery negative or car frame ground acting as an electron well ( does the same thing )

My experience in tinkering with the SLCM connections wiring at the specific large BT - 4 connector above the fuel tank has made my security system fail on my part of tinkering and was able to recover after a long overnight time hard reset without the key sequencing

Other cars do have a relearning time after ECM removal or simple battery disconnect to settle back down to good engine regulation ( usually high idle ) , but this simple disconnect does not drain the electrons from a positively charged / tripped / stored device in the X300 ECU , but then it could be my inaccurate detailed observations at the time and something else was at play not aware of

Thinking through against reading first and wiring guide today during pre - snow errands on your findings John

Contributor's contributions help us all exercise our minds to be able the crack the code toward resolutions

 

Reviewed your earlier post / observations and same as what I saw a few years back

Expect # 1 fuel pump relay closing ground provided by ECU at pin 18 , somewhere along the line 19 got in my head and referenced that as 19 the past

electrical fuel cannister close valve not on most production years / regions ( California emissions meeting improvement ? ) models and originally a vacuum controlled Rochester valve

My understanding then and still is the X300 engine ECU can not be reset as it has " isolated " power sources

 

I've been an active member of this forum for nearly 13 years.

I have often seen recommendations for doing a "hard reset" for various ailments to reset the ECU, Security module, BCM etc. I can't remember anyone ever reporting back that that they touched the battery leads together and suddenly their issue was fixed. Maybe I missed something?

 

I can recall some members reporting success with a hard reset on some models, but I can't recall if this has ever worked on an X300.

My understanding is that the capacitors that provide persistent power to keep-alive-memories in some electronic control units can have their charges drained by connecting the battery cables together for some period of time, which shunts the capacitors to ground. Depending on the vehicle, the ignition switch may need to be turned to the ON position (position II) during the reset procedure to complete certain circuits necessary to drain the capacitors. Draining the capacitors' charges presumably de-powers the KAMs, causing stored data such as fueling adaptations to be lost.

I also remember that not all capacitors in all ECUs can be drained by the hard reset. A specific example I seem to recall is the TCMs in transmissions with combined Mechatronic TCM/valve bodies, but in the X300 the TCM is outside of the transmission, so I don't know if shift adaptations can be cleared by a hard reset.

Cheers,

Don