Hard to find electric problem
Discussion
Hi !
Every now and then I have the MIL light coming on and the ECU reports battery fault. Sometimes it happens once a day. Other times it can work for a week and then the problem is back again.
I have done the following:
- Alternator taken apart, cleaned inside, bearings checked etc. everything looks fine
- Changed positive wire from alternator to big fuse
- Changed negative wire from engine to chassis
- Changed negative wire from battery to earth point under dashboard
- Changed the big fuse and the holder for it to a gold plated heavy duty version
When the problem occuors the voltage drops from 14 volts to 13.7 volts at idle if measured with a digital mutimeter. Today I had the opportunity to get the problem when I visited my friend who works with industrial control systems. He had a advanced multimeter that could make a hi resolution graf of the voltage measured. What we could see was spikes of up to 8 volts up and down from the measured 13.6 volts. The spikes where like less then a thousand of a second long. Probably it is the spikes that triggers the ECU to say that it is a battery fault, but what is it that causes the spikes ??? I have a thought that it could be the voltage regulator in the alternator but I am not sure.
Is there anyone else that has had a fault like this ? What was the fault then ?
Peter
Every now and then I have the MIL light coming on and the ECU reports battery fault. Sometimes it happens once a day. Other times it can work for a week and then the problem is back again.
I have done the following:
- Alternator taken apart, cleaned inside, bearings checked etc. everything looks fine
- Changed positive wire from alternator to big fuse
- Changed negative wire from engine to chassis
- Changed negative wire from battery to earth point under dashboard
- Changed the big fuse and the holder for it to a gold plated heavy duty version
When the problem occuors the voltage drops from 14 volts to 13.7 volts at idle if measured with a digital mutimeter. Today I had the opportunity to get the problem when I visited my friend who works with industrial control systems. He had a advanced multimeter that could make a hi resolution graf of the voltage measured. What we could see was spikes of up to 8 volts up and down from the measured 13.6 volts. The spikes where like less then a thousand of a second long. Probably it is the spikes that triggers the ECU to say that it is a battery fault, but what is it that causes the spikes ??? I have a thought that it could be the voltage regulator in the alternator but I am not sure.
Is there anyone else that has had a fault like this ? What was the fault then ?
Peter
Sounds like regulator to me.
Not delved into the cerb alternator myself but alternators can incorporate in-built smoothing (not necessarily a part of the regulator block). May be worth sticking a capacitor across it, check that the spikes have gone or been reduced, and see how it goes. If it cures it you can decide whether to remedy or live with the cheap fix.
ETA, the battery acts as a large smoothing capacitor, but not necessarily for frequency and strength of these peaks. Although battery voltage may be good it could be on it's way out, and worth getting discharge tested.
Not delved into the cerb alternator myself but alternators can incorporate in-built smoothing (not necessarily a part of the regulator block). May be worth sticking a capacitor across it, check that the spikes have gone or been reduced, and see how it goes. If it cures it you can decide whether to remedy or live with the cheap fix.
ETA, the battery acts as a large smoothing capacitor, but not necessarily for frequency and strength of these peaks. Although battery voltage may be good it could be on it's way out, and worth getting discharge tested.
Edited by Buffoon on Friday 26th September 09:44
SimonSparrow said:
Could also be worth checking the voltage at the ECU (you might be doing this already....) as there is a lot of cable and connections between the battery and the ECU. A few people have rigged up a direct battery feed to the ECU using a relay tripped by the original wiring.
SimonSparrow said:
Could also be worth checking the voltage at the ECU (you might be doing this already....) as there is a lot of cable and connections between the battery and the ECU. A few people have rigged up a direct battery feed to the ECU using a relay tripped by the original wiring.
That is another thing that I have done during the past months. It seems as everything I docures the problem for a little while then its back again.At the moment I am trying to find a new alternator since the local car electric workshop doesn´t like to work with the Marelli stuff.
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