ECU fuse blowing, Car wont start

[fenderbender]

Car is a 2000 4.5 and has been running extremely well. A recent 500 mile drive across Europe was achieved with no problems and I decided to wash the car to get rid of the salt which it had collected on the run from Portsmouth to Oxford on Sat - this was in blizzard conditions. Took her out for a quick blast round the block to dry the disks off after washing and she died on me while making a rh turn at a junction about half a mile from home.

Initial diagnosis was that fuel pump was not running, so I read all the threads about this and was all ready to crawl under the car to hotwire the darn thing when logic reminded me I should check all the fuses in the box. I found fuse 2 had blown (20A ECU).

If I replace this fuse the fuel pump whirrs into action but I can hear the ECU fuse blow as soon as I turn the starter.

In my experience things like this often result from recent work done, or from incidents on the road which precipitate a crisis (hitting a massive pothole, for example). In this case I can think of nothing except for the snowstorm last saturday. Car has had no work done to it except normal servicing and has never been modified from new.

Anybody got any ideas?? I will, of course, strip out the ECU to check for poor connections.

[chim]

Hi
I also have a 2000 4.5 and it's doing almost the same the fuse blows before I turn the starter. Carl Baker has had a look so far no joy.

Chim

Edited by chim on Monday 31st March 11:29

[fenderbender]

D'OH!

[spend]

Hotwire the starter solenoid, and see if that cures it?

Dave

[350Matt]

Check the big fuse on the chassis rail or behind the battery box

Matt

[fenderbender]

Starter turns motor over even if the fuse has blown. That's how I got the beast back into the garage after the short tow home (2nd gear with starter churning).

There must be a short circuit somewhere which is causing fuse 2 to blow. There is no apparent heat damage to ECU componentry. I have ordered a couple of new relays which I will instal, but I cannot find a good wiring diagram which shows which components feed off fuse 2. All the "bible" shows is a connection to a termination block which does not help at all.

Here's what I know so far:

Fuse 2 repeatedly blows when I try to start car. This happens sometimes when ignition on & before I turn the starter, and sometimes after
Starter works fine whether fuse 2 is intact or not
Fuel pump works fine both when fuse 2 is replaced and when hotwired. I've cleaned the terminals too
No obvious damage to ECU internals
No damage to wiring between fuel pump and inertia switch
Inertia switch has not tripped

[blitzracing]

Assumming (???) TVR dont alter the loom from the Range Rover , the fuel pump supply also feeds the lambda probe heaters. The wiring is pretty exposed around the front suspension and manifold, so Id take a carefull look at the wiring to the probes . Also Just unplug them to to see if the fault clears. Also does the fuse blow if the pump is disconnected ?

Mark

[fenderbender]

blitzracing said:

Assumming (???) TVR dont alter the loom from the Range Rover , the fuel pump supply also feeds the lambda probe heaters. The wiring is pretty exposed around the front suspension and manifold, so Id take a carefull look at the wiring to the probes . Also Just unplug them to to see if the fault clears. Also does the fuse blow if the pump is disconnected ?

Mark

Brilliant idea. I'll go see! Based on my homily about trying to work out what recent experiences the car has had ... the sensors just could have been affected by water as the car has not been out in any kind of wet weather in the past 5 years (Easter Saturday excepted when I hit those blizzards)

Anybody know if anything else hangs off fuse 2?

UPDATE: Tried out all the above and can report that fuse blows as soon as ignition is switched on, whether starter is turned over or not and also when fuel pump is disconnected. No obvious problems with lambda wiring.

All I can say is I am eternally grateful that this fault manifested itself at home and not at 8.30pm in the pouring rain whilst on my way from La Rochelle to Caen... or on the way up to Oxford in the blizzard the following day!!

Edited by fenderbender on Monday 31st March 16:45

Edited by fenderbender on Monday 31st March 16:49

[spend]

The Ecu is from the fuse box, power to the main & fuel relays is separate.. Ecu just 'switches' them on and they power the lambda heaters etc (in varying configs of the 2 relays). So a fault in the Pump / lambda / purge .. should blow the fuses on the fly leads with the relays..

Dave

[fenderbender]

spend said:

The Ecu is from the fuse box, power to the main & fuel relays is separate.. Ecu just 'switches' them on and they power the lambda heaters etc (in varying configs of the 2 relays). So a fault in the Pump / lambda / purge .. should blow the fuses on the fly leads with the relays..

Dave

Would this blow the ECU fuse as well?

[spend]

Not as far as I can see, the ecu powers up the coil in the relays which switches a completely separate circuit to the power hungry bits. If your relays are foobarred, or you have put some incorrect relays in it is possible..

Dave

[fenderbender]

Thats what I thought... I have to find out why the ECU fuse keeps blowing.

[Cider Andy]

If we take a list of components controlled by the ECU, according to the 'bible' we have:

Lambda sensors
Injectors
Fuel pump
Purge valve
AFM
Throttle pot
Fuel temp sensor
Coolant temp sensor
Stepper motor
Speed transducer
Main relay
Fuel pump relay
Coil (but no mention of ignition amp)?

If we assume for the moment that the components powered by the relays are not at fault, this leaves:

Throttle pot
Fuel temp sensor
Coolant temp sensor
Stepper motor
Speed transducer
Main relay
Fuel pump relay
Coil/ignition amp?

Try disconnecting all of those components. If the fuse remains intact, reconnect each one until the culprit blows the fuse. This is not a perfect approach as it would not affect a short circuit in any of their wiring, just in the component itself or its connector.

Note that it's not clear whether the coil and ignition amplifier are powered by the ECU, also that the AFM has an ECU connection in common with the throttle pot and temp sensors.

[blitzracing]

I think you are making things over complex. If the peripherals like AFM or TPS where drawing that sort of current, you would vapourise the ECU components, and you would smell the burning. The only "heavy" current switching the ECU does is the grounding the injectors, and as Spend says these have the 12v side supplied by a relay, so the grounding current is limited by the resistance of the injectors. The clue is in when the fuse blows. If its just with the ignition on, but BEFORE the engine starts, its likely to down to the lambda heaters, or fuel pump relay area / wiring. If it happens as you try and start the engine, it could be injectors (short circuit) or injector wiring.

Mark

Edited by blitzracing on Tuesday 1st April 11:43

[fenderbender]

Thanks for all your help thus far, guys.

The precise moment the fuse blows is when either (a) alarm unset and ignition switched on, or (b) ignition key switched on and alarm button blipped - my normal procedure to start the engine.

The fuse pops whether the fuel pump is connected or not, (wiring disconnected under the car) but I will check we have no grounding on the wiring between inertia switch and pump.

[spend]

Pulling out the ECU relays & fuses (on the trailing wires) will not stop you using the starter. You could try this just to eliminate 'unusual' chaffed wire / barfed injector.. type problems.

I would pull the fusebox and have a look at the wiring behind, it is common for it to get chaffed against the footwell fibreglass.

Dave

[fenderbender]

spend said:

Pulling out the ECU relays & fuses (on the trailing wires) will not stop you using the starter. You could try this just to eliminate 'unusual' chaffed wire / barfed injector.. type problems.

I would pull the fusebox and have a look at the wiring behind, it is common for it to get chaffed against the footwell fibreglass.

Dave

How is this done? Remove visible bolt and wiggle past those steel rods above the battery case?

[blitzracing]

That tells us its in the fuel pump supply, not the injectors. Next stage is to remove the fuel pump relay and power on the ignition with say a 10 amp fuse fitted. This will isolate the lambda probe power, wiring , and fuel pump wiring (we know the pump is OK -leave it disconnected.)If this is OK, disconnect both lambda probes, and refit the relay and try again. If the fuse pops, its down to wiring or connectors to pump or probes as we now have only the loom. If thats OK, then connect the probes one at a time, and see when it fails. I am making the assumption here that TVR have not hung anything else onto this circuit- just my disclaimer !

Mark

[spend]

blitzracing said:

That tells us its in the fuel pump supply, not the injectors. Next stage is to remove the fuel pump relay and power on the ignition with say a 10 amp fuse fitted. This will isolate the lambda probe power, wiring , and fuel pump wiring (we know the pump is OK -leave it disconnected.)If this is OK, disconnect both lambda probes, and refit the relay and try again. If the fuse pops, its down to wiring or connectors to pump or probes as we now have only the loom. If thats OK, then connect the probes one at a time, and see when it fails. I am making the assumption here that TVR have not hung anything else onto this circuit- just my disclaimer !

Mark

I don't understand what you are saying / trying to achieve? the fuel pump, injectors, lambda heaters, purge valve are all powered by those 2 relays/fuses.. whether from the main or fuel relay is pretty indeterminable (unless you look at the specific car) - they moved around during various incarnations at both LR and TVR. IIRC there is also a power feed from the relays back into the ECU (just so it knows they are working I suppose)?

Dave

[spend]

fenderbender said:

spend said:

Pulling out the ECU relays & fuses (on the trailing wires) will not stop you using the starter. You could try this just to eliminate 'unusual' chaffed wire / barfed injector.. type problems.

I would pull the fusebox and have a look at the wiring behind, it is common for it to get chaffed against the footwell fibreglass.

Dave

How is this done? Remove visible bolt and wiggle past those steel rods above the battery case?

Assuming you are asking how to pull the fusebox...

Remove battery, there is a bolt above & below the fusebox mounting box that goes through the wheel arch. Undo these two bolts and the box can be drawn out, there is about 1m of loom stuffed up inside the inner wing which can be pulled out (carefully withdrawn) like an umbilical cord.

Dave