Help with short and/or bad earth.

Hi,
I'm getting a bit lost trying to diagnose a short or bad earth or maybe even both. I have multiple circuits with faults.
The car in question is a 2010 Alfa Romeo Mito Cloverleaf which probably isn't familiar to a lot of the members here.

I have error codes from the engine ECU and the body computer and a lit engine management light. Codes are:
Engine
P0615 - Startup motor relay command - Short to +V or open circuit
P0230 - Fuel pump relay - Short circuit to +V
BCM
B1032 - Brake fluid level - Short to +V or open circuit
B1053 - Inertia (FIS) switch command Engine Control Node - signal/mesage invalid

The other symptoms are that the brake fluid level warning message is on. When I use indicators strange things happen in that if I use the left indicator, the left side indicators front and rear flash as expected, but the left side repeater glows dimly and the right hand side flashes instead. Plus there is bonnet open message flashing on the dash (in time to the indicator pulse) and the fuel pump relay also pulses on and off in time to the indicator pulse. If the engine is running it misses to the time of the indicator pulse.

The car starts fine though and other than the warnings, errors, indicator and fuel pump problem I haven't noticed any other symptoms.

So far I have been trying to locate the wires involved in the circuits mentioned by the faults and their respective earths. The main earthing points physically look fine, strong connections. And I haven't yet found any broken or damaged wires.

If I do a voltage test with my meter betwwen the battery negative and the earth point in the passenger foot well where I think these circuits are earthing (I say think as I don't have a wiring diagram) I get 11.75V there, so my thoughts are something is leaving voltage in the earth side or a short is putting voltage into the earth side. I did the same test on the fuel pump relay on the control circuit earth side to battery negative and I see the same 11.75V, and it's also present in the earth side of the side repeaters.

Any suggestions welcome. 
Thanks
 

Seems like you have a bad chassis/body ground. When the path back to battery negative is high resistance or open, the voltage backs up to where the break or resistance is.

Take those big cables with the clamps on that you use to jump cars with and connect it to battery negative and just clamp the other end to anywhere on the body/chassis of the car.

Yeah, it's not in the main earths at the battery, engine, etc. I have already dismantled and cleaned the battery terminals, battery to chassis earth, engine to chassis earth, etc. Those are all good, I'm 100% confident in them.

There is a chassis earth point in the passenger side foot well that a lot of individual black earth wires all coming back to and this is where I see weirdness. For instance I was trying to isolate some of the wires to work out what they did and picked a wire that was thicker then the others, this led me to the horn circuit (which doesn't seem effected by the problem) as it was the only black wire I could see that was the same thickness. With my meter on resistance I measured at each end and there was 0ohms so was happy that that was the horn earth wire, but then (while the other meter probe was left connected to this wire) I put the meter probe on the other wires in this location and found some with 10M ohm resistance, one with 15M ohm and some with 1k ohm and some were open circuit as I was expecting. I wasn't expecting any other wires to be showing a circuit as both ends were disconnected.

 

If I'm following you correctly, you unplugged the horn ground wire at both ends but your meter found it has a path to the other ground wires in the passenger side foot well?
If so then the horn ground wire was not fully isolated. Without a wiring diagram I don't know if anything could be spliced into that ground.

Anyway, those ground wires should NOT have high voltage on them like you're describing. I recommend you use the voltage that's in the circuit that's loaded (by loaded, I mean the component is on and current is flowing) to diagnose wiring issues, not the ohm meter. You must be careful as a wire that ohms out as 0 on your meter may not be good enough to carry any current, but since your horn works, I think that wire's good enough.

wrote:

If I'm following you correctly, you unplugged the horn ground wire at both ends but your meter found it has a path to the other ground wires in the passenger side foot well?
 

Yep, that's right. In my mind I had isolated the wire at each end so was expecting a circuit and found one, but then also had other wires completing the circuit albeit it with varying (higher) resistances.

I was only using the ohm meter in this instance to try and find the wires in question.

So I'm thinking I have a bad earth and I'm getting voltage being left (or back fed?) in the earth side. But I don't think it's necessarily the circuits that have a fault on them or doing weird things as they are a victim of this lingering voltage. It's just not making sense to me where these volts are coming from and how I'm going to actually find it. I also have yet to track down a wiring diagram which isn't helping and not sure one actually exists outside of FCA's eLearn system.

If I do a voltage test with my meter betwwen the battery negative and the earth point in the passenger foot well where I think these circuits are earthing (I say think as I don't have a wiring diagram) I get 11.75V there, so my thoughts are something is leaving voltage in the earth side or a short is putting voltage into the earth side. I did the same test on the fuel pump relay on the control circuit earth side to battery negative and I see the same 11.75V, and it's also present in the earth side of the side repeaters.

You have unwanted resistance between the ground stud and the ground wire, that's why the voltage is backing up like that.

I'm getting the voltage without load or supply being present as well though. For instance on the side repeaters:
With ignition on, both side repeaters have a dim glow.
With left hand indicator on, the left side repeater pulses with a dim glow, the right side repater flashes brightly
With the right hand indicator on, the right hand side repeater pulses with a dim glow, the left side flashes brightly.
Voltages measured as below.

 

 

When the source voltage is switched off and the bulb is still there, other circuits tied to that bad ground will find an alternate path to ground through the bulb. It's a back feed as a consequence of a bad ground.

When you have the ignition on, there's a circuit wanting to use that ground, but it can't so if finds another path to ground. Instead of the current going down the bulb, it's going the other way, maybe through another bulb, then finding a ground. That would be why both side repeaters glow dim when you turn on ignition.

I wish I had a wiring diagram for your car

Here's a video with a bad ground and a backfeed.

Okay, yeah that makes sense. Do you think I'm looking for the bad ground in the earth wires where I'm seeing the back feed or look else where? Maybe in wires that are on that same earth terminal?

 

One of the best educational video on bad grounds:



 

It's fixed! I found the bad earth and feel quite stupid now as I had completely missed this one. There was a big clump of black wires mounting to the chassis hidden underneath the ABS pump. I couldn't see it from above or below it was only when I removed the battery and fuse box that I saw some wires disappearing into the abys. When I undid the terminal nut and saw the state of the corrosion on the stud and the terminal I knew this was the one!.

Thanks for everyones help.