Technical BCM (Body Computer Module) ground and other electrical gremlins

[Mike1alike]

I just discovered the BCM grounding point on my Punto. The electrical system is very similar across most of the FIAT models, so this topic is not Punto exclusive.
What happened
I noticed idle dipping very short while engine was on warming up temp (40~70 ⁰C) and being stop at red lights. Dipping was not related to AC being on or off, but I discovered it was related to headlights being on. Headlights off = no dip.
I did noticed some little time before that that the headlights ground point on the chassis next to left headlight doesn't look good, nut rusted and much oxidation there and I had in mind I need to clean it. So this was the good time to do so.
That ground cable has many little wires coming together into one ring connected by 2 crimps. First I thought it's just headlights (and front electronics, fan, sensors etc) ground line, but is not just that, is the BCM grounding point there too.
So besides main battery-chassis-engine (gearbox) ground cable that we all know about (and which is shown in the first pic, sits very close to this second one) there is this second grounding point that feeds ground to BCM and other electrical consumers.
BCM is physically located inside cabin, besides fusebox panel. BCM stores the IMMO code too, same as ECU, so voltage dropping caused by oxidation on this cable can cause IMMO reading errors. I did find some of those stored in ECU over the time and some U errors (CAN bus communication) which I found odd because battery is good and charged and main ground cables as well, as main (+) cables are good too. Oxidation on this cable explains it now.
I will tell more about it in next messages on this thread.
I also am set to fix the voltage drop reading by ECU. ECU reads battery voltage 0.2 V less than it normally is. I know it's said 0.2 V is in good range but I will do some investigations to see and fix that.

 

[Mike1alike]

So, what happened there and my incidental and quite unplanned testing there.
I set to clean the contact ring and meeting surface, not by using the wire brush but by having the ring sit in hot solution of water+citric acid+detergent, about half an hour or so. Than rinse with clean water and dry.
As car was parked ~500 m away, I went to get it closer. But having sunshine over it in that parking lot I decided to unscrew the nut holding that ring, there. I did and let the nut at the end of stub and got the car closer. Now, cold engine, by when I got closer I had the rev dipping phenomena again, but this time even more pronounced, more powerful, dipping was harder and more intense repetition. Because loose connection means increased resistance. Then I got the nut and ring out, just sitting on air and went in for the cleaning solution. When I got back to the car I unlocked it by remote and... rear wiper started running by itself, made about 3 full turns. Then I got in the car and put the key in ignition on MAR. Rear wiper started again, did the same 3 turns and even front wipers started, but just a very short movement, like a second or so. I repeated that test, ignition off ---> MAR 3 more times and every time it did the same, rear wiper running 3 turns, front wipers jumping. Then I connected that cable ring to chassis, put ignition on and everything behaved, no wiper started by itself anymore.
So I concluded that BCM ground is indeed connected to chassis in that point. Cable disconnected made BCM having no reference ground which led to floating voltage between connected circuits and that voltage made the wiper start on its own.
Engine running, ECU communicates with all the system and voltage drop on oxidised contact on that ring affected the good engine running.
Why it shows up around ~40 °C (40~70 °C)
At that temperature the ECU reduces cold idle enrichment and switches more to closed-loop idle control. Idle becomes more sensitive, so a small voltage shift suddenly causes RPM dip.
My impromptu experiment confirmed it. When I loosened the nut, contact resistance increased, ground reference shifted slightly, ECU saw wrong values, idle dipping returned.
That is a textbook ground reference shift.
Why headlights make it worse
Headlights draw high current. Them sharing that ground point means current → resistance at corroded joint → voltage drop. That drop shifts the whole harness ground potential. So ECU thinks sensor voltages changed.

On the Fiat Punto Mk2 (1999–2010) with the Fiat FIRE engine (but also on many other Fiat models), ECU does not measure ground separately, instead it assumes the sensor ground network and chassis ground are the same reference. When corrosion appears at the chassis point, that assumption becomes wrong.
So what really happens then is this. How the ECU “sees” voltages: sensors output voltages relative to sensor ground.
Example (simplified): MAP sensor output → 1.5 V relative to its ground. ECU reads that voltage relative to its own ground reference. If the chassis ground point develops resistance, the whole sensor ground network can shift slightly.
Example:
Real sensor signal: 1.50 V
Ground shift due to corrosion: +0.20 V (due to voltage on ground line)
ECU interprets: 1.70 V
Now ECU thinks load or throttle position changed, even though the engine didn’t actually change.
That can affect idle air control, ignition timing, fueling corrections.
And that explains the idle dip I observed when electrical load increased.

 

[bugsymike]

So, what happened there and my incidental and quite serendipitous testing there.
I set to clean the contact ring and meeting surface, not by using the wire brush but by having the ring sit in hot solution of water+citric acid+detergent, about half an hour or so. Than rinse with clean water and dry.
As car was parked ~500 m away, I went to get it closer. But having sunshine over it in that parking lot I decided to unscrew the nut holding that ring, there. I did and let the nut at the end of stub and got the car closer. Now, cold engine, by when I got closer I had the rev dipping phenomena again, but this time even more pronounced, more powerful, dipping was harder and more intense repetition. Because loose connection means increased resistance. Then I got the nut and ring out, just sitting on air and went in for the cleaning solution. When I got back to the car I unlocked it by remote and... rear wiper started running by itself, made about 3 full turns. Then I got in the car and put the key in ignition on MAR. Rear wiper started again, did the same 3 turns and even front wipers started, but just a very short movement, like a second or so. I repeated that test, ignition off ---> MAR 3 more times and every time it did the same, rear wiper running 3 turns, front wipers jumping. Then I connected that cable ring to chassis, put ignition on and everything behaved, no wiper started by itself anymore.
So I concluded that BCM ground is indeed connected to chassis in that point. Cable disconnected made BCM having no reference ground which led to floating voltage between connected circuits and that voltage made the wiper start on its own.
Engine running, ECU communicates with all the system and voltage drop on oxidised contact on that ring affected the good engine running.
Why it shows up around ~40 °C (40~70 °C)
At that temperature the ECU reduces cold idle enrichment and switches more to closed-loop idle control. Idle becomes more sensitive, so a small voltage shift suddenly causes RPM dip.
My impromptu experiment confirmed it. When I loosened the nut, contact resistance increased, ground reference shifted slightly, ECU saw wrong values, idle dipping returned.
That is a textbook ground reference shift.
Why headlights make it worse
Headlights draw high current. Them sharing that ground point means current → resistance at corroded joint → voltage drop. That drop shifts the whole harness ground potential. So ECU thinks sensor voltages changed.

On the Fiat Punto Mk2 (1999–2010) with the Fiat FIRE engine (but also on many other Fiat models), ECU does not measure ground separately, instead it assumes the sensor ground network and chassis ground are the same reference. When corrosion appears at the chassis point, that assumption becomes wrong.
So what really happens then is this. How the ECU “sees” voltages: sensors output voltages relative to sensor ground.
Example (simplified): MAP sensor output → 1.5 V relative to its ground. ECU reads that voltage relative to its own ground reference. If the chassis ground point develops resistance, the whole sensor ground network can shift slightly.
Example:
Real sensor signal: 1.50 V
Ground shift due to corrosion: +0.20 V (due to voltage on ground line)
ECU interprets: 1.70 V
Now ECU thinks load or throttle position changed, even though the engine didn’t actually change.
That can affect idle air control, ignition timing, fueling corrections.
And that explains the idle dip I observed when electrical load increased.

I recall many years ago a Ford Cortina Mk1 that had a bad engine earth.
When the owner turned the ignition to start the metal choke cable glowed red hot in the metal dash.
I suppose the next question from younger car owners will be "What is a choke cable?"

 

[Mike1alike]

So it's worth mentioning that after cleaning that ground point the problem is fixed, no mor idle dipping when engine is on warming up temp.
I enjoyed discovering this BCM grounding point and fixing the problem.
And this opens another lead / line of inquiry / diagnostic path for very often questions asked about idle gremlins on different Fiat models. The BCM grounding point.

 

[Mike1alike]

My next quest in the electrical department, as mentioned above, is to find out about the battery voltage difference between measured by multimeter and seen by ECU.
ECU sees the battery voltage 0.2 V less than multimeter. I also noticed the cigarette lighter plug voltage is 0.2 V less than voltage on battery terminals. So I'm thinking of some oxidation, voltage drop somewhere on the line.
Looking at the inside cabin fusebox, the 2 red wires (little one is actually more peachy coloured) down right look like a good starting point, those look like 12 V feed lines. I very quickly (last night, was already dark) tried to measure voltage on them and it is 12 V. Measurement wasn't the best, some improper contact between cable connectors and lead, but voltage was not stable. Will do some more proper measurements there.

 

[Mike1alike]

Other thing, one detail I want to mention about this BCM grounding point is that that BCM ground point is the point where all those cables on that ring are being fed ground! They get it from the inner ring, main ground cable being connected very close there on the chassis.
So, if corrosion appears between the ring and the stud, all those circuits lose a good reference.
One interesting detail is that this ground point often becomes a single failure point. A little oxidation there can cause many seemingly unrelated electrical symptoms.
That’s why cleaning it can solve some electrical gremlins problems.

 

[puntodad19]

Choke cable reD hot, seen similar with a BMC mini...

Wiring diagrams show numbered earth points and earth diagram shows locations.

What about the earth point at the back of the engine which is subject to flexing, work hardening, fatique then make and break/poor contact ?

Would have to look up that earth point number and systems reliant on it....

I have often wondered About that easy to move earth connection on the battery negative...

Anyways, great thread mike....

 

[Mike1alike]

I must continue to post here as I did some more measurements and tinkering in the magical world of car's electricity.
First off, turned out lower voltage on cigarette lighter socket compared to battery terminals was just battery surface charge. Although I think I saw the difference while engine was running, too. Anyway, I'll consider this point and go further, cleaning the contacts might have helped.
So I tested around with engine off. And after having headlights on for a little while leads on battery terminals would show exact same voltage as cigarette lighter socket, no surface charge shown. On battery it was shown on first contact but then immediately jumped. I think measuring through wiring to the socket would not pick up the surface charge. This is a video with it on battery. Showed 12.40 V on first contact then jumped to 12.18 V (~0.2 V).

 

[Mike1alike]

Then... as they say, you find out something new everyday. I did!
For a long time, when measuring different electrical stuff inside cabin, I used the key barrel for ground. But there is no ground line connected to it! It usually gave me correct voltage with red lead on (+) circuit but that was only a coincidence.
So that's why my earlier measurement on the 2 wires inside cabin fusebox panel showed me unsteady voltage, because I used the key barrel for ground. Measuring correctly, voltage is steady there and matches the voltage at the other end, inside engine bay fusebox panel and voltage on battery.
Anyway, while at it I did clean the contacts on the cables and on engine bay fusebox panel and plugs and also cables connectors on battery terminal. That improved things a bit, voltage shown while driving is a little higher, 14.4 ~ 14.5 V compared to 14.1 ~ 14.3 V before.
I did not hookup MES to read ECU's battery voltage after this, but before the same difference was on. It's 0.2 V less with engine off and about 0.3 V with engine on.
I must say, about 5 years ago I made a new ground cable and also new (+) cable from battery to fusebox. I let the (+) cable between alternator - starter motor - battery (+) terminal be, as it goes through harness, but I also added a new one there.

 

[Mike1alike]

So, I am getting closer to the cause of 0.2 V less voltage.
I mesured the ECU plug and there are 2 power suply lines, Pin 4 and Pin 17.
Pin 4 is OK, voltage is the same as on battery terminals, Pin 17 is 0.2 V less, this all with ignition key on MAR.
ECU's plug A where I did the measuring is the blue plug, sitting in the right side as looking at the windshield from the front of the car and connected to ECU's side where supporting screws sit closer to each other.

Physically I noticed the ECU plug harness goes to engine bay fusebox panel and is connected under the panel through the big black plug with purple safety stripe, right side in picture.
I will check a diagram, but I'm guessing that wire goes through main relay that sits in that panel.
Relay is as old as the car, 20 years.
Question is can the age of the relay be the cause of 0.2 V drop?

 

[bugsymike]

So, I am getting closer to the cause of 0.2 V less voltage.
I mesured the ECU plug and there are 2 power suply lines, Pin 4 and Pin 17.
Pin 4 is OK, voltage is the same as on battery terminals, Pin 17 is 0.2 V less, this all with ignition key on MAR.
ECU's plug A where I did the measuring is the blue plug, sitting in the right side as looking at the windshield from the front of the car and connected to ECU's side where supporting screws sit closer to each other.

Physically I noticed the ECU plug harness goes to engine bay fusebox panel and is connected under the panel through the big black plug with purple safety stripe, right side in picture.
I will check a diagram, but I'm guessing that wire goes through main relay that sits in that panel.
Relay is as old as the car, 20 years.
Question is can the age of the relay be the cause of 0.2 V drop?

Just out of curiosity are they the same pins you can test from the other end using a breakout box on the EOBD port.

 

[Mike1alike]

And I must say, battery voltage is a little low while resting. Battery is good, alternator good, charging system good, ground good. No parasitic consumers.
I'm suspecting the cause of this lower voltage is the starter motor's brushes. They are the original ones, so quite old. Last time I checked them, 5 years ago, they looked ok, not very much wear and I decided to let them on.
I do have a new brush holder plate with new brushes (well, 5 years old now, but not used) and I will get them on. I just need to get to them, I have them a little far away, about ~600 km.

 

[Mike1alike]

Just out of curiosity are they the same pins you can test from the other end using a breakout box on the EOBD port.

Surely there is a connection to the EOBD port because that does connect directly to the ECU.

 

[Mike1alike]

Just out of curiosity are they the same pins you can test from the other end using a breakout box on the EOBD port.

Bugsy, how about this? Can the age of the relay be the cause of 0.2 V drop?
I've been told to drop it (pun intended! ) because is "within specs".
But... I kind of would like it showing the same voltage as on battery terminals directly.
Everything is working fine, it's true. So natural coming question is 'why fix it?'! I think... just because! To be at its best!

 

[bugsymike]

Bugsy, how about this? Can the age of the relay be the cause of 0.2 V drop?
I've been told to drop it (pun intended! ) because is "within specs".
But... I kind of would like it showing the same voltage as on battery terminals directly.
Everything is working fine, it's true. So natural coming question is 'why fix it?'! I think... just because! To be at its best!

It would certainly be interesting to compare with a good quality new one, contacts deteriorate over time.

 

[Mike1alike]

It would certainly be interesting to compare with a good quality new one, contacts deteriorate over time.

Outside contacts, the ones that plug into the panel, I cleaned. There was some oxidation, of course. Question is about the inside. I think next I'll have the relay pulled out a bit, just sitting in so it makes contact and try measure the voltage on its contacts.

 

[Zardo]

If the battery is connected to the car, there's always a small current. However, with modern electronics, with ignition turned off and no electrical consumer running, this current might be very small - too small to draw surface charges. Simply turn the ignition on, the radio on (no volume), ignition off. Radio stays on for 20 min or so, and will draw all surface charges. Or switch cabin lights on.

Alternatively use a voltmeter with an integrated battery tester (a small resistance in a parallel circuit).

Also, there's no voltage on the cigarette lighter plug with ignition switched off.

 

[Mike1alike]

Also, there's no voltage on the cigarette lighter plug with ignition switched off.

That's right. Voltage there only with ignition on MAR.

 

[Zardo]

Yes, now let's look at the voltage drop:

U=R*I

first, if the earth connector has a 0 Ohm resistance, there won't be a voltage drop, and the full 12 V are available to consumers:

Volts	Earth [Ohm]	Current	Voltage Drop	Volts remaining	Resistance Consumer [Ohm]
12.00​	0.00​	0.00​	0.00​	12.00​	infinite
		0.25​	0.00​	12.00​	48.00​
		0.50​	0.00​	12.00​	24.00​
		0.75​	0.00​	12.00​	16.00​
		1.00​	0.00​	12.00​	12.00​
		1.25​	0.00​	12.00​	9.60​
		1.50​	0.00​	12.00​	8.00​
		1.75​	0.00​	12.00​	6.86​
		2.00​	0.00​	12.00​	6.00​
		2.25​	0.00​	12.00​	5.33​
		2.50​	0.00​	12.00​	4.80​
		2.75​	0.00​	12.00​	4.36​
		3.00​	0.00​	12.00​	4.00​
		3.25​	0.00​	12.00​	3.69​
		3.50​	0.00​	12.00​	3.43​
		3.75​	0.00​	12.00​	3.20​
		4.00​	0.00​	12.00​	3.00​
		4.25​	0.00​	12.00​	2.82​
		4.50​	0.00​	12.00​	2.67​
		4.75​	0.00​	12.00​	2.53​
		5.00​	0.00​	12.00​	2.40​
		5.25​	0.00​	12.00​	2.29​
		5.50​	0.00​	12.00​	2.18​
		5.75​	0.00​	12.00​	2.09​
		6.00​	0.00​	12.00​	2.00​
		6.25​	0.00​	12.00​	1.92​
		6.50​	0.00​	12.00​	1.85​
		6.75​	0.00​	12.00​	1.78​
		7.00​	0.00​	12.00​	1.71​
		7.25​	0.00​	12.00​	1.66​
		7.50​	0.00​	12.00​	1.60​
		7.75​	0.00​	12.00​	1.55​
		8.00​	0.00​	12.00​	1.50​
		8.25​	0.00​	12.00​	1.45​
		8.50​	0.00​	12.00​	1.41​
		8.75​	0.00​	12.00​	1.37​
		9.00​	0.00​	12.00​	1.33​
		9.25​	0.00​	12.00​	1.30​
		9.50​	0.00​	12.00​	1.26​
		9.75​	0.00​	12.00​	1.23​
		10.00​	0.00​	12.00​	1.20​

but if the Earth ground connection has 1 Ohm resistance, the voltage drop will behave like this:

Volts	Earth [Ohm]	Current	Voltage Drop	Volts remaining	Resistance Consumer [Ohm]
12.00​	1.00​	0.00​	0.00​	12.00​	infinite
		0.25​	0.25​	11.75​	47.00​
		0.50​	0.50​	11.50​	23.00​
		0.75​	0.75​	11.25​	15.00​
		1.00​	1.00​	11.00​	11.00​
		1.25​	1.25​	10.75​	8.60​
		1.50​	1.50​	10.50​	7.00​
		1.75​	1.75​	10.25​	5.86​
		2.00​	2.00​	10.00​	5.00​
		2.25​	2.25​	9.75​	4.33​
		2.50​	2.50​	9.50​	3.80​
		2.75​	2.75​	9.25​	3.36​
		3.00​	3.00​	9.00​	3.00​
		3.25​	3.25​	8.75​	2.69​
		3.50​	3.50​	8.50​	2.43​
		3.75​	3.75​	8.25​	2.20​
		4.00​	4.00​	8.00​	2.00​
		4.25​	4.25​	7.75​	1.82​
		4.50​	4.50​	7.50​	1.67​
		4.75​	4.75​	7.25​	1.53​
		5.00​	5.00​	7.00​	1.40​
		5.25​	5.25​	6.75​	1.29​
		5.50​	5.50​	6.50​	1.18​
		5.75​	5.75​	6.25​	1.09​
		6.00​	6.00​	6.00​	1.00​
		6.25​	6.25​	5.75​	0.92​
		6.50​	6.50​	5.50​	0.85​
		6.75​	6.75​	5.25​	0.78​
		7.00​	7.00​	5.00​	0.71​
		7.25​	7.25​	4.75​	0.66​
		7.50​	7.50​	4.50​	0.60​
		7.75​	7.75​	4.25​	0.55​
		8.00​	8.00​	4.00​	0.50​
		8.25​	8.25​	3.75​	0.45​
		8.50​	8.50​	3.50​	0.41​
		8.75​	8.75​	3.25​	0.37​
		9.00​	9.00​	3.00​	0.33​
		9.25​	9.25​	2.75​	0.30​
		9.50​	9.50​	2.50​	0.26​
		9.75​	9.75​	2.25​	0.23​
		10.00​	10.00​	2.00​	0.20​

Therefore, to see a voltage drop, current drawn by the electrical consumer must be high, and for that, the resistance of the electrical consumer must be low. You won't see much of a voltage drop if there isn't an electrical consumer behind which draws current.

So a 1 Ohm resistance on the Earth point won't allow your car to start, since the starter motor (which has a very low resistance and draws lots of current) will see basically no volts and almost all of the voltage drops on the bad Earth point (or cable).

This can be simulated with Excel for other Earth point resistances, too. The formulas are simple.

 

[Mike1alike]

Ground connection is good, no resistance.
That's what got me into checking that 0.2V voltage drop ECU sees. The fact that it happens with no consumers. Engine off and just ignition on MAR.
I'll go now and test the relay. Let's see the resistance on its circuit with coil charged and voltage drop on its 87 pin.