Today i tried to replace the battery in my Seicento.
It's a 2001 model with 1.2 engine.

The original battery is 12 Volt / 40 Amps (Magneti Marelli).
The new battery is 12 Volt / 44 Amps (Exide).
It's a little bigger, but fitted perfectly.
(This was the most similar battery available in the local shop)

After the replacement the following occurred:
The engine starts perfectly, but right after that it starts revving.
Just like i'm tapping the gaspedal in and out.
It keeps revving up and down like crazy.
(Radio and other parts worked perfectly)

I was scared something was broken.
After connecting the old battery again, everything worked perfect again.

Isn't it possible to use an battery with higher capacity?
I though that more amps means that is gets less fast exhausted?
Or do I really need an battery with exactly the same specs?

Any help / advice is greatly appreciated!

i dont know much about this kind of thing but id try looking for air leaks. you may have knoked a pipe off the air flow sensor easy done

Do you have a multimeter? Is the new battery fully charged? I know on some cars if the battery voltage is low then the ECU will give it a load of revs to produce more power from the alternator.

ChrisSei said:

Do you have a multimeter? Is the new battery fully charged? I know on some cars if the battery voltage is low then the ECU will give it a load of revs to produce more power from the alternator.

Click to expand...

the output from the alternator should be constant either at idle or red line

Ash

Thanks for all quick responses ! Problem solved !

I added this guide:
https://www.fiatforum.com/cinquecen...cento-2001-1-2-battery-replacement-guide.html

ChrisSei said:

Do you have a multimeter? Is the new battery fully charged? I know on some cars if the battery voltage is low then the ECU will give it a load of revs to produce more power from the alternator.

Click to expand...

You completely solved my problem. First time I did not charge the battery at al. The battery indicator was green, so I assumed it was fully charged. After charging the battery the revving did not happen. So, despite of the green indicator, the battery was almost empty. Stupid me

So the ECU did probably trigger the revving in order to speed up the battery charge.

whitz said:

the output from the alternator should be constant either at idle or red line

Click to expand...

I don't know much about alternators, but when the engine makes more revs, you might think the alternator delivers more power because it rotates faster? The dynamo on my bicycle delivers more power when going faster (brighter lights). Or is that a silly comparison?

Now that is an impressively clean engine compartment. Is this car still in the showroom?

As whitz said, the alternator should provide a constant supply.

It’s not a totally silly comparison but they are different beasts.

In a nut shell, an alternator provides a constant supply while a dynamo does not. It provides this via its regulator, which varies the voltage supplied to its rotor, thus varying the magnetic field which induces an EMF into the stator which provides the power to charge the battery and run the other car systems. They provide a three phase AC voltage, which is converted to DC via a full wave bridge rectifier.

The dynamo’s you normally find on a bike does not have a regulator, so the faster they spin the more power they deliver and it is generally DC, without the need for the rectifier.
:life:

This was the problem I had with my old MK1 ford escort. The battery kept going flat due to the load and the fact that I just pottered around town in it.
Change it for an alternator off of a later model Cortina and the problem when away.

This does not really answer your original question as to why the engine speed was hunting (going up and down). Maybe the low voltage was confusing the ECU, but I would have the that the alternator would have provided enough to charge the battery and power the ECU.

Are you sure it’s charging the battery?

John

The Professor said:

Now that is an impressively clean engine compartment. Is this car still in the showroom?

Click to expand...

Whahaha. it's still a mystery to me why this Seicento stays so pretty inside. The mileage is actually quite low. But I would expect more dirt inside after 35.000 KM. My other Seicento from 1998 with 98.000 KM looks pretty nasty inside. And both cars are used in exactly the same way.

hamman said:

In a nut shell, […]

Click to expand...

I always thought 'alternator' and 'dynamo' were exactly the same. This makes much more sense of course. Thanks for the explanation!

hamman said:

This does not really answer your original question as to why the engine speed was hunting (going up and down). Maybe the low voltage was confusing the ECU, but I would have the that the alternator would have provided enough to charge the battery and power the ECU.

Click to expand...

Hunting is the right word, good you mentioned it.

The alternator seems to be working perfect. Also the battery is being charged without any problems. Maybe I touched something near the battery that caused the hunting? Any part you can think of?

No sorry, no idea. The best I can suggest is to check all the earth connections.

Maybe one of the others can suggest something, maybe an air leak as suggested earlier. Check the pipe to the MAP sensor. Apparently if that’s leaky it can cause hunting.

Best regards
John

I checked all wires and tubes. As far I can see there is no air leak. So, I asume everything is oke and that the hunting was caused by low battery (at least, it stopped after the battery was charged).

Thanks everyone for your time and response!