Technical 500L diesel high earth resistance

[Kgl]

Need help with my friends 2014 500L diesel.
At idle the battery voltage is approx 11.5 volts, 2000rpm it climbs to 12/12.5v.
I checked the earth resistance between the engine and the earth post and it is about 50ohms with no consumers rising to 190ohms with headlights, blower motor and hrw on!! Measured between the engine and the earth quick connector it is about 0.5ohm irrespective of consumers.
As the earth passes through the battery monitor (?) to the negative post I'm unsure as to whether those high resistance values are normal? Doesn't seem right to me, but my real area of expertise is classic Mercs.
I did take the earth lead off and checked it off the car (resistance and then voltage drop using an old starter and slave battery) but all good as expected. To me the fault appears to be in the battery monitor(?) between the earth lead and negative battery post.
The battery was replaced three weeks ago by the way.
All help will be much appreciated.

 

[the green vanper]

Hello,

The FIAT earth cables tend to rot on the inside, it's hard to see. But you wrote you've checked the resistance of the disconnected cable, so should not be the cause.

Another known problem is grounding point corrosion.

I couldn't really understand which cable are you talking about. Battery to body? Or body to engine? And what do you call the battery monitor? Can you post a photo?

 

[Kgl]

Hello,

The FIAT earth cables tend to rot on the inside, it's hard to see. But you wrote you've checked the resistance of the disconnected cable, so should not be the cause.

Another known problem is grounding point corrosion.

I couldn't really understand which cable are you talking about. Battery to body? Or body to engine? And what do you call the battery monitor? Can you post a photo?

Thanks for the reply.

I'm pretty certain that the cable is good, especially after the cobbled up under load voltage drop test. The earth cable is all in one, battery to chassis leg and then on to the gearbox. Resistance checked from engine to battery, then from a secondary earth point on the body to battery (same result) and the same body point to engine (less than 0.2ohm) .

The issue is the very high resistance between any earth point and the battery post/clamp after the little black 'box' that is part of the clamp (resistance is low on the cable side of that box). The resistance increases with electrical load. I never took a photo but the link below will show what I'm on about.

https://www.fiatforum.com/attachments/1668343987431-png.415036/

 

[varesecrazy]

Can you run a 2nd earth cable, and compare figures?

The innards if the Fiat cables get damp roadsalt in there,
turns copper strands to a green paste, doesn't conduct so well

 

[Kgl]

Can you run a 2nd earth cable, and compare figures?

The innards if the Fiat cables get damp roadsalt in there,
turns copper strands to a green paste, doesn't conduct so well

Hi! You're Cardiff/Oxford, I'm London living/West Oxon working.
I'm happy with the earth cable. To put it simply, if I disconnect the quick release earth and check resistance from the pin where it connects to the clamp and the other side of that 'box' it reads about 50ohms.

 

[varesecrazy]

So you don't doubt the cable,

But think it's the actual cast clamp?

Or the sensor that is fixed to it..?

 

[Kgl]

Or the sensor that is fixed to it..?

Exactly that but not knowing what that sensor is I have no idea if my readings are normal.

One further observation - leave it running for 30/40 seconds and the resistance drops by about 25%, then after another short period it drops to just under half of the original value. This state only lasts for a few seconds and then it reverts back to the original high value. And as I say, the more consumers running the higher the resistance.

 

[the green vanper]

It'd be good to see what's under that plastic. I assume the OP is saying about the thing on the right hand side of the photo, not the flat one to the left.

From what I've understood from other threads in which the photo (or similar) appeared, the flat thing with the Magneti sticker is the Start&Stop sensor.

The thing on the right with the red part is a quick release terminal.

If we're talking about this one, I see at least 3 connection points (marked with green) that might have corroded. Have these been cleaned / checked? The arrow points inside the quick release terminal.

Here's a post with a photo of the underneath of the terminal:

Fiat 500 Stop Start Battery Terminal | FIAT 500 (2007+)

Ah now I understand, so is the part attached to the terminal post, connected to the sensor via a nut underneath as it looks like it's a two part item ? Stupid question is the battery the correct one? Is it possible it's wrong one with the wrong size terminal ? Not that it would help now, try...

www.fiatforum.com

 

[Kgl]

Sooooo....... I went to work today and decided to spend an hour on the Fiat before getting on with work proper. Unlocked it and turned on the multimeters (which I had left connected) - no discernable voltage drop overnight but resistance from the engine to the battery post now zero ohms! Started the engine and the resistance slowly rises, system voltage also rises with the resistance.
Bit of research this evening and here is what I've learnt. The little black box is an Intelligent Battery Sensor. The two small wires connected to the side are 12v feed and a LIN bus signal wire. So now I need to find out if the fault is the IBS, the body module or the engine module.

Find out more about the IBS (in general) at the link below.

https://www.google.com/url?q=https://www.hella.com/hella-uk/assets/media_global/621_KI_IBS_12V_HELLA_EN.pdf&sa=U&ved=2ahUKEwj8xZ7ymfGKAxWRnf0HHVHKGpEQFnoECAQQAg&usg=AOvVaw3Efwph-_sXXQhz4tDQIOtj

 

[the green vanper]

So, it's the other part on the photo

I've found here on the forum that it's the Start&Stop sensor.

Is it the 1,3 or the 1,6 l diesel?

 

[Kgl]

So, it's the other part on the photo

I've found here on the forum that it's the Start&Stop sensor.

Is it the 1,3 or the 1,6 l diesel?

Hi!
Whilst its' output does influence the stop/start system its ' primary function is to monitor the battery. Via th e body module its output affects battery charging, fueling etc. Troublesome on BeeEmms, not unheard of to give issues on other brands apparently.
Back to work on Wednesday so I'll have a look then.

 

[the green vanper]

Yes, seems correct. It monitors the battery and if necessary, disconnects the Start&Stop system.

It is called battery charge status sensor in the FIAT eLearn.

Just uploaded those:

https://www.fiatforum.com/downloads/elearn-documentation-on-charging-alternator-power-supply-earth-and-battery-charge-status-sensor.538/

Should be available for download shortly.