Alternator Output Voltage

[Manxman]

Had the blown alternator fuse problem last night with the ABS light flashing then lights fading until the battery drained and the car stopped.. Have replaced the alternator fuse with an inline 100 amp as a temp measure until I get a new fusebox assembly. The alternator output is between 14.2 and 14.5 volts. Is this too high? If a new voltage regulator is required, then is is available as a separate from a complete alternator assembly. When checking the fuse retainer nuts they all needed nipping up so possibly some heat has been building up from a bad connection causing the damage..

What do you think?

KF

[chromedome]

The output voltage sounds about right. If all the electrical connections were loose then extra current would have been drawn and this may be why the fuse blew.

[Manxman]

The output voltage sounds about right. If all the electrical connections were loose then extra current would have been drawn and this may be why the fuse blew.

Thanks for that... Wasn't sure about the correct voltage but being a 1.9tdi with heated front screen I suppose the alternator needs to shove plenty out..

[Bobd]

The most likely cause of your blown fuse was over heating and not over current. I had the same problem (as have many others) and I suspect that the cause was either a poor connection in the bolt that folds the fuse or the connector on the end of the cable. Because of the current flowing through this even a small resistance will generate a lot of heat. For instance with a current of 80A and resistance of 0.01ohms it will be dissipating 64 watts.

 

The fuse box is an off the shelf item and costs about

[bj2004]

yes agree output is spot on

[Manxman]

yes agree output is spot on

Thanks for the input.. I will fit a new fusebox assembly and make sure all connections are clean and tight.. Cheers..

[Willie Krashitt]

And while you are doing it, PLEASE give the cable that goes off into the main loom very close inspection at the connector that goes into the fusebox. Any black on the cable means that you will be doing the job again very soon. The only fix is to cut this wire back and join in a new piece and also a new connector. My previous galaxy went back over 4" before copper coloured wire was found, while I was lucky with my current car as it only went back about 2".

 

George.

[Manxman]

And while you are doing it, PLEASE give the cable that goes off into the main loom very close inspection at the connector that goes into the fusebox. Any black on the cable means that you will be doing the job again very soon. The only fix is to cut this wire back and join in a new piece and also a new connector. My previous galaxy went back over 4" before copper coloured wire was found, while I was lucky with my current car as it only went back about 2".

 

George.

 

Yes, good advice George..... I am suspicious of the cable . It feels sort of brittle near the terminal.. Might be as well to make up a complete new lead...

[chromedome]

Just to clarrify my point earlier. A bad connection causes more current to flow. More current equals more heat so the failure is caused both both excess current and heat.

 

My house fuse board nearly caught alight because the feed to the electric cooker became loose and the cable started burning back.

 

I think you are right to try and replace the brittle cable.

Edited November 12, 2009 by chromedome

[Bobd]

Sorry to be pedantic but chromedome is only half right. A bad joint can

[Saif Rehman]

Sorry to be pedantic but chromedome is only half right. A bad joint can

[RADIOTWO]

Sorry to be pedantic but chromedome is only half right. A bad joint can’t cause the current to increase but the increased resistance of the joint will produce more heat, in fact the slight increased resistance of the joint will cause a slight decrease in the current (current = voltage divided by resistance (ohms law)). To expand on the earlier figures I quoted, if the current was 80A and the resistance of the joint were 0.01ohms the dissipated heat would equal 64watts (power = current squared x resistance, 80 x 80 x 0.01 = 64 watts), if the resistance increases to 0.02ohms the dissipation would be 128watts. This is further exacerbated by the fact that the resistance of most materials increases as they get hot (Positive temperature coefficient of resistance). If the item is unable to dissipate all the heat generated you get thermal runaway.

 

Hi Bobd

 

You are poss correct with your thoughts but its no use confusing folk with all this increase in resistance etc, at the end of the day it usually down to a BAD connection !

 

so make sure all connections are tight and more so the ones that carry high current, like battery to starter/Alternator

 

Radiotwo

[RADIOTWO]

Sorry to be pedantic but chromedome is only half right. A bad joint can’t cause the current to increase but the increased resistance of the joint will produce more heat, in fact the slight increased resistance of the joint will cause a slight decrease in the current (current = voltage divided by resistance (ohms law)). To expand on the earlier figures I quoted, if the current was 80A and the resistance of the joint were 0.01ohms the dissipated heat would equal 64watts (power = current squared x resistance, 80 x 80 x 0.01 = 64 watts), if the resistance increases to 0.02ohms the dissipation would be 128watts. This is further exacerbated by the fact that the resistance of most materials increases as they get hot (Positive temperature coefficient of resistance). If the item is unable to dissipate all the heat generated you get thermal runaway.

 

Hi Bobd

 

You are poss correct with your thoughts but its no use confusing folk with all this increase in resistance etc, at the end of the day it usually down to a BAD connection !

 

so make sure all connections are tight and more so the ones that carry high current, like battery to starter/Alternator

 

Radiotwo

[RADIOTWO]

Sorry to be pedantic but chromedome is only half right. A bad joint can’t cause the current to increase but the increased resistance of the joint will produce more heat, in fact the slight increased resistance of the joint will cause a slight decrease in the current (current = voltage divided by resistance (ohms law)). To expand on the earlier figures I quoted, if the current was 80A and the resistance of the joint were 0.01ohms the dissipated heat would equal 64watts (power = current squared x resistance, 80 x 80 x 0.01 = 64 watts), if the resistance increases to 0.02ohms the dissipation would be 128watts. This is further exacerbated by the fact that the resistance of most materials increases as they get hot (Positive temperature coefficient of resistance). If the item is unable to dissipate all the heat generated you get thermal runaway.

 

Hi Bobd

 

You are poss correct with your thoughts but its no use confusing folk with all this increase in resistance etc, at the end of the day it usually down to a BAD connection !

 

so make sure all connections are tight and more so the ones that carry high current, like battery to starter/Alternator

 

Radiotwo

[Bobd]

Radiotwo

 

Point taken, but I do feel that if you have a better understanding of why something fails, you have a better chance of fixing it.

 

Bobd

[tim-spam]

Exactly right!

[big_kev]

Wow, i knew i should have studied harder at physics at school. :wacko:

 

You went to school ! :o

Edited November 12, 2009 by big_kev