Thread: Why I hate Petrol Engines... (3.9 V8 ignition wiring)

should of tryed to convince him to but a isuzu in it

The 3rd wire soaks up interference energy from the alternator etc - returns it to -ve. Since the signal from the sensor inside the dissy has to travel to the amp, it can be pick up stray stuff.

You can live without the shielding - I presume that you have got the engine going... (When I swapped to a shielded line, I forgot to power the coil)

Originally Posted by langy

The 3rd wire soaks up interference energy from the alternator etc - returns it to -ve. Since the signal from the sensor inside the dissy has to travel to the amp, it can be pick up stray stuff.

You can live without the shielding - I presume that you have got the engine going... (When I swapped to a shielded line, I forgot to power the coil)

No - we haven't been able to start it yet. I soldered the blue and red wires back together and heat shrinked them, but have not reconnected the 3rd wire (so it is only ~ 1/3 of its original length).

There is a faint fuel smell from the exhaust while cranking, and a spark if you pull a plug lead - but it could be weak???

I'm not sure if this could be the reason why it isn't starting???

Originally Posted by isuzurover

What purpose does the 3rd wire serve? It is grounded to the body (heatsink) of the remote amplifier, however not connected at the other end???

Grounding is more effective in certain circumstances when it is only terminated at one end. This is probably the reason. Otherwise you can have a ground loop effect that actually causes interference or does nothing to reduce it.

Thanks

Steve

And speaking of grounding - make sure the amplifier is well grounded. There have been some instances of the amp not working properly and making weak spark if it's ground isn't right.

Originally Posted by isuzurover

No - we haven't been able to start it yet. I soldered the blue and red wires back together and heat shrinked them, but have not reconnected the 3rd wire (so it is only ~ 1/3 of its original length).

There is a faint fuel smell from the exhaust while cranking, and a spark if you pull a plug lead - but it could be weak???

I'm not sure if this could be the reason why it isn't starting???

I take it you have fitted the remote amplifier as well? When it is mounted to a good earth and you have power input tested, make sure the white/black trace or white/blue trace wire is connected to the coil negative. This drives the ECU. Also make sure the plug at the distributor is not fitted reversed. If it is flipped over and then runs you know what you have done.

Don't worry too much about the shielding, just make sure the blue/red wires are away from most other wires.

Originally Posted by bee utey

I take it you have fitted the remote amplifier as well? When it is mounted to a good earth and you have power input tested, make sure the white/black trace or white/blue trace wire is connected to the coil negative. This drives the ECU. Also make sure the plug at the distributor is not fitted reversed. If it is flipped over and then runs you know what you have done.

Don't worry too much about the shielding, just make sure the blue/red wires are away from most other wires.

We swapped over the whole coil/amp assembly, so that is all connected OK as the engine was running in the donor. The donor vehicle was running carbs though.

I think we will have to trace some of the wires to make sure the ECU has power. The PO (who built the 90) did a good job of the mechanical stuff, but the wiring needs tidying up.

Originally Posted by isuzurover

We swapped over the whole coil/amp assembly, so that is all connected OK as the engine was running in the donor. The donor vehicle was running carbs though.

I think we will have to trace some of the wires to make sure the ECU has power. The PO (who built the 90) did a good job of the mechanical stuff, but the wiring needs tidying up.

No probs, so long as you realise the tracer wire I mentioned is part of the vehicle harness near the coil, if I remember its the one with a male connector than plugs into a small female terminal at the coil negative. Its how the engine knows you want injection to happen. Most modern cars use a crank angle sensor instead.
To find if the engine electrics have power, just find any brown/orange trace wire at an injector and test for 12 volts with the ignition on. It also feeds the MAF in the air intake, same colour wire.