I could use some insight on a no start issue.
My 2004 Discovery II failed to start. I went through all of the quick thoughts; shift through gears and hard back to park, rock steering wheel, etc. it started after all of this. Drove it home and after shutting it down it wouldnt start again no matter what.
I started checking from both ends (ignition to starter motor) and I end up at the relay in the middle...Meaning; I have power from the ignition through fuse 23, ignoring anything from fuse to relay because I have constant power at the SM relay first blade (blade nearest the firewall) as well as power on the second blade (closest to engine) of the SM relay when in ignition position III. I have constant ground at the SM relay 3rd blade (furthest forward) and I have power leaving blade 4 (nearest RF fender)on the SM relay in ignition position III. This is where it gets confusing. If I jump power to solenoid the starter turns the engine fine. Follow the solenoid lead back to the fuse block under hood and it is on a plug with one other wire (fuel pump lead is second wire maybe). I can jump to the lead in the fuse box and activate starter so i know everything down line is fine. however there is no power coming to that lead in ignition III or any other time for that matter. From what i can tell the last stop for power in this circuit before the solenoid lead is at the SM relay??? if this is the case it would appear that there is a break in the circuit in the fuse block. Physical inspection shows no reason that power would make it from relay to plug but I know power is leaving the relay and not making to the plug. To further complicate; I can jump from constant power blade in relay to solenoid lead and it starts,I can jump from the momentary power in blade on the relay to the solenoid lead and it starts, if I jump from the momentary power out blade on the relay ( the one I assume normally sends the signal to the solenoid) it excites the solenoid but not with enough power to engage the starter motor...I have swaped the relay with around with other relays in the box and the issue stays with circuit and doesnt follow the relay (indicating the problem isnt the relay itself) . So I am left with power to and leaving the relay eliminating issues ahead of the relay, confrimed circuit from the fuse block plug to the solenoid eliminating issues there, the issue lives between the relay and the plug for the solenoid in the fuse box unless I am missing something. The appearant low voltage from blade 4 on the relay (most near the RF fender) just serves to wrinkle my forehead more. Any help would be apreciated.

 

Think negative thoughts. The relay gets positive volts from different directions, one of them being fuse 23 and position 3 of the ignition switch. That would be on pin 86, one side of the coil. The other side of the coil, pin 85, goes to the BCU. This would have a plus volts reading (to chassis earth) back thru the coil, but dropping to almost zero when BCU decides to ground that line. If immobilzed, BCU might not ground that spot. Also, if BCU and ECU are not talking to each other, it won't respond and other things can inhibited.

Red alarm light on the dash lit up? Lock/unlock five time in a row with key fob? Arm/disarm truck with EKA sequence?

 

Thanks..I had just discovered that I sould have been looking for ground to the final blade..It looks like the BCU is not granting earth. the alarm light isnt on.. have tried locking and unlocking. disconnecting battery etc...

 

Can you get to the BCU and pull off the plug and try to ground the pin?
Or don't pull it off the BCU but slip in a wire and see if you can ground the wire and fire the relay.
As long as that is activated by grounding.

The plus supply to the relay - has to be able to support closing the relay.

I would...
Pull the relay and find out which wire on the relay goes to the BcEM grounding wire.
Meaning the BcEM will ground that wire and cause the relay to close.

This is called an open collector circuit as there is probably an open collector power transistor in the BCeM.

Either the open collector transisitor in the BcEM can't ground it enough..
Or -

the voltage supply to the relay is not enough current.

I would pull the relay.
Put a thin wire on the pin that goes to the BcEM ground and plug the relay back in.
Then ground that wire and see if the relay and starter fire up.

If they do - then the BCEM can't ground it enough.
If they do not - then the supply to the relay is not enough power meaning current is low from resistance.

 

Thanks-

I tried bringing ground to the relay on the feed from the BC. I just broughtit in at the relay rather than at the BC itself. When I ground it everythingfunctions normally. As a younger man I would have just made a permanent directground circuit to the relay and called it a successful repair . Unfortunately I have a hardtime not putting things back they way the manufacturer intended so I am leftnow needing to find out why the BC isn’t granting ground to the relay? I thinkits either a software confusion that may require some sort of reboot or resetor there is another system that isn’t giving the BC satisfactory info. Myoriginal thoughts were neutral safety switch but there no other symptoms there,the digital in dash indicator for the gear selector is working and accurategear shift functions normal etc...second was the alarm...There again, no othersymptoms, the truck locks and unlocks normally, the alarm light comes on whenset goes off when deactivated..Ive tried resets there with no change...anythingelse I should be doing?

Savannah Buzz-

You mentioned the EKA..Not sure how to execute on this truck...Is thatsomething that I should try you think? even given there are no other alarmsymptoms?

Thanks again fellas,

Steve

 

If the BCU and the ECM have lost communication with each other, or the ECU has forgotten the identity code from the BCU, then you would have a problem. Locking and unlocking like 8 times with the key fob forces the two computers to try to talk again, and the EKA code may help. Usually guys with alarm / immobilization issues have more problems if they have no working fob to start with. Shops with a dealer level computer can resync the BCU and ECU.

The BCU will also stay immobilized if it lost communication with the IDM. This can be reset by leaving key in position 2 for 10 minutes.

From RAVE:

The IDM communicates with the BCU and the instrument pack via a serial interface. If the BCU or the IDM is replaced,
the communications link between the two units has to be re-established. This can be done either by switching on the
ignition and leaving it on for five minutes, or by using TestBook. The vehicle immobilisation will remain active until the
communications link between the BCU and IDM has been re-established.

and

For immobilisation, the BCU disables the starter motor relay. When the engine is cranking, the ECM looks for a coded
signal from the BCU. If the signal is not received within one second of cranking, the fuel supply to the engine is
stopped and the injectors are disabled. This also prevents unburnt fuel from entering the catalyst.
If the BCU is disconnected, the engine starter motor will remain isolated by the starter motor relay and the ECM will
remain immobilised. The main wiring for the system is contained within the main harness which is relatively
inaccessible, so preventing intruders from disabling the system by cutting the wires for the immobilisation system.
Once the immobiliser has been activated, destruction of the trigger device or the wiring to it will not disarm the system.
The RF transmitter communicates to the BCU via the RF receiver using a 70 bit code. Pressing the unlock button on
the transmitter will re-mobilise the vehicle. The RF transponder is integrated into the metal key assembly, inserting
the key into the ignition switch will induce a signal in the exciter coil to re-mobilise the vehicle. Note exciter coils not on most NAS vehicles.