1987 3.2 Carrera Backdate project - Sladeys hotrod

[Bruce M]

There is a chance you’ve fried a small earth wire (sensor earth or similar) as the starter motor earth would have taken any possible return path.

Haltech recommend earthing the ECU & all related components to the engine block for this reason. So there is no alternative path back to the battery, via ECU or related wiring (& no chance of false ground causing signal errors).

Thanks Bruce. Is there a chance that could have fried the small earth lead in the new alternator - there’s a big earth in there but also a small one. But actually they earth to the engine which in turn wasn’t earthed.


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Alternator doesn’t give the starter a return path to the battery, since it’s connected to the block & uses the same earth path that the starter uses. If you have trouble fault finding a simple answer, do consider how the starter could have found a path back to the battery negative. For example if the ECU is earthed to the chassis directly, then any signal/sensor earth terminals on the ECU would have been an alternative route to battery earth, if those are connected to the engine case via a sensor or similar. Hopefully not, but something to consider if you are struggling to find the cause.

[DustyM]

Mark,
If You haven’t already try swapping
Around those two black plugs.
Cheers
Jon



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[sladey]

Thanks Jon - Tried that but no joy. I had labelled up the other side of those so didn’t think I’d mixed them up

[sladey]

There is a chance you’ve fried a small earth wire (sensor earth or similar) as the starter motor earth would have taken any possible return path.

Haltech recommend earthing the ECU & all related components to the engine block for this reason. So there is no alternative path back to the battery, via ECU or related wiring (& no chance of false ground causing signal errors).

Thanks Bruce. Is there a chance that could have fried the small earth lead in the new alternator - there’s a big earth in there but also a small one. But actually they earth to the engine which in turn wasn’t earthed.


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Alternator doesn’t give the starter a return path to the battery, since it’s connected to the block & uses the same earth path that the starter uses. If you have trouble fault finding a simple answer, do consider how the starter could have found a path back to the battery negative. For example if the ECU is earthed to the chassis directly, then any signal/sensor earth terminals on the ECU would have been an alternative route to battery earth, if those are connected to the engine case via a sensor or similar. Hopefully not, but something to consider if you are struggling to find the cause.

The gearbox was earthed properly so I would think the starter would have earthed through that

[DustyM]

The injector loom has a separate plug on the crossmember behind the inlet
Manifolds, is that plugged in or is that different on your car?


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[sladey]

Yes that’s plugged in - is that the fairly big one with I think 4 connectors in it?


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[DustyM]

Yes that’s plugged in - is that the fairly big one with I think 4 connectors in it?


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Yes that’s the one.
So if you have fuel pressure and the pump is running, and you have a spark at the plugs, then the only thing stopping it can be the injectors not firing, which could be one of the crank sensors, or an electrical problem elsewhere. You’ve swapped your DME relay I presume?


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[sladey]

No I haven’t touched that Jon - good call - I think I’ve got a spare one in the glovebox.

I’m at parents house for a birthday now so will try that when I get back - the DME relay is the one under the passenger seat isn’t it?

From memory when the engine wasn’t earthed there was mad clicking which could have been from that

On the crank sensors although I’ve had stuff apart I didn’t touch them at all

[Bruce M]

The gearbox was earthed properly so I would think the starter would have earthed through that

Doh. When you said the earth wasn’t connected I assumed you meant the main earth strap.
Ignore me.

[sladey]

OK I swapped out the DME relay for the one in the glove compartment but no joy - exactly the same (great cranking but nothing firing)

I don’t know the one in the glovebox is any good - are there tests I can do with a multimeter to confirm that?

So I guess if it’s not that then it would be most likely to be one of the sensors?

Regarding the big plug on the top of the crossmember - when I was taking the engine out I’d forgotten to undo that and there was a cracking noise - I then unplugged it. When I checked it later the female part of the plug had come undone but it all looked fine and clicked back together. The plug plugs into there without any problem.


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[Gary71]

Maybe a pin backed out when the connector was stressed? Or maybe it broke a wire inside?

[sladey]

It’s a possibility but I did check it thoroughly when the engine was out and it seemed fine - everything clicked back together


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[Gary71]

The clicky bit might still work but a pin (male or female side) can be out of place. Can you get to it and have a look inside both halves?

[DustyM]

On the dme you can bridge three pins in the socket then it will be permanently live to the ecu and fuel pump. You’d need to Google which pins though I would need one in front of me to know.


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[DustyM]

The clicky bit might still work but a pin (male or female side) can be out of place. Can you get to it and have a look inside both halves?

As Gary says, a closer inspection needed, it does seem a bit of a coincidence that this happened on removal and now it would appear you have no fuel.

Have you removed a spark plug to see if they are wet or smell of petrol?


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