Hello all,

Electrical gremlin on my 2006 Grande Punto Active 1.2 that has me stumped.

When I turn on the rear fog light, the light comes on momentarily and then goes off. The bulb failure warning light then appears on the dashboard.

I understand that this light does not have a dedicated fuse but is served by the BCM. All other lights on the car work fine. However I'm unsure whether there is a dedicated relay (for example in the switch module) and how I would go about testing the system?

To date I have checked the following:
(1) Battery terminals clean and tight. Battery is fairly new and voltage is good (12.2 volts when off, over 14 volts when running)
(2) Earth mounts in the engine bay have been cleaned and greased (2 under ABS modulator, one on chassis leg, gearbox earth strap and ECU earth wire on rocker cover - shout out to Andy Monty for some excellent guides)
(3) Fog light wiring checked - no visible breaks (though previous repair is evident as both wires have been soldered and shrink wrapped; is it possible to get these the wrong way around?)
(4) Contacts cleaned and Bulb changed - same result
(5) Fusebox checked for water ingress (including underneath) - dry as a bone. Bottom half is wrapped in perforated cling film for protection and lid is closed tight.
(6) ABS modulator checked - also dry
(7) Fixed broken wire in the boot concertina hose. I don't think this is related to the fog light but thought I would mention for completeness.

I've read a number of threads related to rear fog lights on this site. They either allude to a non-working light or one that stays on. I've not encountered one that momentarily works like mine.

Thank you in advance for any assistance.

For completeness..

The engine doesnt switch off either

varesecrazy said:

For completeness..

The engine doesnt switch off either

Click to expand...

?? Charlie

Didn’t think the rear fog or reverse bulbs were connected to the bulb warning triangle,but could be wrong! ???

varesecrazy said:

For completeness..

The engine doesnt switch off either

Click to expand...

Thanks for reminding me! Italian electrics are not for the faint hearted. :bang:

multipete said:

Didn’t think the rear fog or reverse bulbs were connected to the bulb warning triangle,but could be wrong! ???

Click to expand...

It most certainly is. The fog light comes on momentarily, goes out and then the bulb warning triangle lights up. When I press the fog light switch again (i.e. turning it off), the bulb warning triangle disappears.

Does anyone know how the Grande Punto light switch works internally? Is there a relay built in to the switch or is this completely governed by the BCM?

Also I've read quite a few posts relating to earth cables in the engine bay and how these can degrade over time causing a range of electrical gremlins. The mounts on my car look good but it's impossible to visually determine the condition of the cables as they're sleeved.

Are there some tests I can run to determine the health of these cables?

Can I run some tests using jump leads from negative to body to see if ignition switches off?

You simply have to measure the voltage drop across the cable using a multi-meter set to VDC.

Since the cable resistance is very close to 0, there is almost no voltage drop when current flows through it: ΔU=R*I, 10amp x 0,01ohm = 0,1volt.
If the cable or its connection developed some resistance, the voltage drop might be so high that the remaining voltage wouldn't be high enough to e.g. actuate a solenoid.

Of course the highest the current, the bigger the ΔU, so big devices will be more sensitive(starter).


BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

OK, I'll give that a shot.

One other thing. Does the fact that the bulb comes on (albeit momentarily) rule out issues with the wiring?

(If needs be I'll chase the wiring to inspect for any signs of damage but do not want to waste time on this if unnecessary as some wiring is hard to get to).

There ain't any relay involved with the rear fog light, all is done in the BCM with positive coming directly to the bulb and return through the body's rear earth point.

Have you monitored the voltage directly at the bulb, with and without bulb installed ?

BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

I assume you changed the bulb itself?

Check the repaired wires for correct thickness, globe is correct rating. And the repair also. Wonder what caused need for repair originally?

Eddie.

Uncle Ed said:

Check the repaired wires for correct thickness, globe is correct rating. And the repair also. Wonder what caused need for repair originally?

Eddie.

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I was wondering this too Eddie, though it's odd that the bulb turns on momentarily then goes off. Both wires are soldered near the holder so I can only speculate that the holder was changed. The thickness looks ok though wires obviously stiff where soldered. Bulb is correct rating (P21W) and has been successfully tested in the reverse light. Reverse light bulb has also been tested in the rear fog light with the same results so unlikely to be a bulb issue. Inside, the holder looks clean and there is plenty of contact with the bulb base. As I didn't perform the fix I'm wondering whether the wires were connected the wrong way around or just badly soldered? Not sure if this makes a difference but I'm at the point where I may re-repair the wires just in case the original repair is not up-to-scratch.

SRafferty said:

I assume you changed the bulb itself?

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Yes, see point 4 in the OP. In fact I tried a new bulb and testing with the bulb from the reverse light (which works fine). All 3 bulbs produce the same result - rear fog light comes on momentarily before going out.

Bernievarian said:

Have you monitored the voltage directly at the bulb, with and without bulb installed ?

BRs, Bernie

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Hi Bernie,

I've dug out my multimeter but could do with some guidance on testing the voltage.

As I don't have a piercing probe do I have to expose wiring in order to test or is there another way to tackle this (e.g. probe the connector)?

Both wires going into the rear fog light holder look identical. How do I know which to connect the negative and positive probes to? Presumably if I connect them the wrong way around I'll get a negative reading on the multimeter?

Finally I will test with and without the bulb installed as suggested. Should the engine be running or not?

Thanks,

Anthony

Bartoosh said:

Hi Bernie,

I've dug out my multimeter but could do with some guidance on testing the voltage.

As I don't have a piercing probe do I have to expose wiring in order to test or is there another way to tackle this (e.g. probe the connector)?

The "repair" should have been somehow insulated, I'd measure there or at the connector with bulb removed

Both wires going into the rear fog light holder look identical. How do I know which to connect the negative and positive probes to? Presumably if I connect them the wrong way around I'll get a negative reading on the multimeter?

Yes, you'd have a negative reading, but it's ok

Finally I will test with and without the bulb installed as suggested. Should the engine be running or not?

Doesn't matter if engine runs or not, voltage will be different though (12.6 vs 13.8 ish VDC)

Thanks,

Anthony

Click to expand...

BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

OK finally got round to some testing.

Quick recap of the symptoms - When the rear fog light is switched on, the bulb lights up momentarily before going out. There were signs of a previous fix as wires near the bulb holder were soldered and insulated (my guess is a previous owner changed the bulb holder hoping this would fix the problem).

To start with I measured the battery voltage to ensure it's healthy:
12.88V with the engine off
14.14V with the engine running

I then performed 3 tests with ignition and headlights on (but engine off):
(1) Measured voltage on the wiring just before the fix
(2) Measured voltage on the area of the fix
(3) Measured voltage in the bulb holder (by sticking a couple of pins into the rear of the holder and connecting the probes to the pins)

All 3 areas produced similar results. With the headlights on but fog light off, voltage reading momentarily fluctuates between 2-6V before eventually settling at around 5-6V. When the fog light is turned on, the voltage again fluctuates between 2-7V before settling at 5-6V.

Unfortunately during testing, one of the wires snapped at the point of repair. As a result, I cut the other wire and stripped back the repair to expose clean wires. I then re-tested on the wires - same result 5-6V.

Testing so far has led me to draw the following conclusions (please do correct me if I'm wrong):
(a) There is insufficient current to fully power the rear fog light hence why it blinks.
(b) The bulb holder is not the cause.
(c) The repair was not the cause.

Before I continue testing, I'd appreciate some advice.

(1) What are my next steps? How do I continue testing?
(2) Is it normal to see a reading of 5-6V when the fog light is off? Is this some residual current, for example from tail lights (as the headlight has to be on to power the fog light)?
(3) How do I test for drop along the rest of the wire (given that so much of it is hidden from view)?
(4) Does the switch itself on the fascia require testing and, if so, how?
(5) Does the BCM come into my investigation or can I rule this out?

Thanks,

Anthony

Bernievarian said:

BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

Click to expand...

Hi Bernie,

I'd be grateful if you can comment on my test results. As I'm a rank amateur I'm unsure whether I tested correctly or if my conclusions are sensible.

For starters, I tested with both probes on the wires and connector. As I have no idea what voltage to expect, do those readings look normal to you?

Also how would I test voltage drop? As the fog light is at the back and battery at the front, the only way I can think of connecting the multimeter is to construct a long wire with crocodile clips. Is there an easier way?

Thanks,

Anthony

Hi Anthony,

seems you did it right up to now, gime some time to check what's involved in the rear fog light and I'll be back to you..

BR's, Bernie

Bernievarian said:

Hi Anthony,

seems you did it right up to now, gime some time to check what's involved in the rear fog light and I'll be back to you..

BR's, Bernie

Click to expand...

Thanks, Bernie, appreciate it.

In the meantime I have located another switch panel. I'm not holding my breath on this being the answer but at £6 it was worth a punt.