Earth Question

[Veero 1]

Right I have just finished building my new Mi16 loom (since part of the old one melted for some reason), it is sat in the car ready to be plugged in and have the cabin-side of things hooked up.

 

Question is will I create some sort of loop by having 2 earths from different places connected to the same spot? I have the trailing lead from the battery -ve going to the gearbox (as well as the wing) plus a trailing earth from the loom. If these are both connected to the gearbox earth bolt will this cause any problems?

 

Any advice appreciated.

 

Cheers

Veero

 

P.S. I am slightly s*itting myself at the prospect of starting the car tomorrow on the off chance something blows up or melts, I have thoroughly tested the loom and everything goes where it should and as far as I can tell nothing goes where it shouldn't. This is what is about to be tested:

[DrSarty 113 1 Cars]

You need 2 earths for an ECU loom.

 

One is the gnd/earth for the sensors, which are back at the ECU itself. They still get grounded to the chassis but these are through the ECU via one or more of its pins( and include its casing) but they must not be local grounds to the sensors that the ECU needs.

 

I'm just clarifying here that there is one 'different' ground, despite it still being a ground and eventually going back to the battery neg via the chassis.

 

The gearbox stud, providing it is cleanly linked to the chassis/shell is the main (power) ground. If your loom has 2 non-sensor grounds, and as long as the main ground is good then there will be no issues.

 

Technically an earth is an earth (a ground is a ground) as they all go back to the same place, BUT, a sensor ground really needs to be back at the ECU so the ECU is reading off sensors that all share a common ground right back at the ECU. The power ground(s) can really be where you like, although it's best to have that/those at the gearbox stud, which is also linked to the chassis/shell.

 

I know I've repeated myself but it's quite difficult to explain. Power grounds aren't sensitive, but sensor grounds are.

 

To give an example or two:

 

The ign amp requires a power gnd, so can be a ring terminal to a chassis bolt, perhaps holding the heat sink plate (with paste on it) to the chassis. The same is true with the coil; it is a power gnd and can be local.

 

The ECU's power gnd wire can also go to a bolt on the chassis, or steering column or back to the gearbox stud - your choice.

 

The TPS, ECU Coolant Temp Sensor, AFM etc - all sensors which talk to the ECU - need to share a common ground back to a pin on the ECU. Those sensor gnds can even be joined at the sensors so only one wire goes back to the ECU. But that sensor gnd wire goes to a pin on the ECU and NOT a ring terminal to the gearbox stud or chassis anywhere. The ECU will gnd that itself for you.

 

I hope that helps Tris, you or anyone else venturing into loom building.

 

BTW, the shield wire around the CAS positive signal wire only connects at the ECU end as well. It's an ordinary gnd. It does nothing at all at the CAS, it just terminates inside the plastic molding. The CAS doesn't even have a sensor gnd really, as it has its own 2 dedicated pins in the ECU plug.

Edited September 23, 2009 by DrSarty

[Veero 1]

Thanks Doc, I will have a go through all that tonight. I have 2 earths on the loom, one to the gearbox (the S-shaped wire on the upper half of that picture) and another to come into the cabin (which is going to the bottom right hand corner of the picture).

 

Anyway tried starting it last night, starter turned over fine, unsure of spark or not but it certainly wasn't getting any fuel as there was no 12v coming from the switched portion of the fuel pump relay. The switched live was working correctly as there was switched 12v to the coil.

 

The injection relay fired as I turned the ignition to second click but absolutely zilcho from the FP relay. Bad light stopped play last night so hopefully I will have the ruddy thing working tonight.

 

Any pointers?