Battery light on, alternator OK, but battery not being charged.

[gav10]

Hello all,

My car is a 1995 Audi 80 1.9 TDi Avant (the air bag model).

A few weeks ago my battery light came on whilst driving. I drove the car straight home, put my volt meter across the battery terminals and there is definitely no charge reaching the battery, the voltage reading is only 11.8 volts there is also no difference when engine revved.

Anyway i bought a second hand alternator off ebay (the seller assures me it was a good one, and no reason to doubt his word).

My mechanic has swapped the alternators over and the battery light is still on and there is still only 11.8 volts at the battery terminals. Either both alternators are faulty or there is another problem in the cars charging circuit somewhere else. Anybody got any ideas of things to check.

My mechanic is in the process of sending my original alternator off to a place where it will be put on a testbed and checked out. In the mean time I thought I would ask on here for any ideas. I have been tinkering with cars for years and my gut feeling tells me both alternators are working fine. I also can't see any visible broken wires.

Thanks in advance.

[baltiboys2]

Just measure the voltage from the back of the alternator then that will tell you if the unit is faulty ?? also you just need the buy the regulator which screw into the back of the alternator as it is uncommon for the actuall amrmature and commutator to fail

cheers Raj

[micheal balbrig]

Hello all,

My car is a 1995 Audi 80 1.9 TDi Avant (the air bag model).

A few weeks ago my battery light came on whilst driving. I drove the car straight home, put my volt meter across the battery terminals and there is definitely no charge reaching the battery, the voltage reading is only 11.8 volts there is also no difference when engine revved.

Anyway i bought a second hand alternator off ebay (the seller assures me it was a good one, and no reason to doubt his word).

My mechanic has swapped the alternators over and the battery light is still on and there is still only 11.8 volts at the battery terminals. Either both alternators are faulty or there is another problem in the cars charging circuit somewhere else. Anybody got any ideas of things to check.

My mechanic is in the process of sending my original alternator off to a place where it will be put on a testbed and checked out. In the mean time I thought I would ask on here for any ideas. I have been tinkering with cars for years and my gut feeling tells me both alternators are working fine. I also can't see any visible broken wires.

Thanks in advance.

What I would suggest is to try for continuity in small
cable / terminal . Reason for suggesting this is as I had a
problem like yours on an agri machine that was not charging
but when put on test bench was ok. Problem was a faulty push
in connector (joiner) on this small cable which was eliminated
after cleaning contact points.! Unlikely to be your problem
but worth eliminating anyway. Cable could be broke internally
possibly at point where it joins terminal

[gav10]

Thanks for the advice chaps.

I have reached the conclusion I must have 2 faulty alternators. As you say Raj, it is hopefully just the voltage regulators on them. I got under the car this morning and put my voltmeter across the alternators main output and the cars chassis (earth). I got the same voltage as I get across the battery terminals. So from that I am concluding the alternator is not throwing out any power. I have check the spade connector and it has a good clean connection on it micheal.

Just out of interest, alternators have 3 connectors on them. The thick lead which is the output, used to power the car and charge the battery. The second one with the spade connector is for the battery light on the dash and the third one which I believe is connected to the cars computer and used for the car to say to the alternator give me more or less power. If there was a break in this lead, would the alternator just shut down? With the engine running the voltage on this lead and the chassis (earth) measures 0.32 volts does that sound about right?
Also if there was a break in the lead between the alternator and dashboard light so that the dashboard light is always ON, does that shutdown the alternator or is this circuit just cosmetic. What I am trying to say here is it possible to have the lamp on the dash on saying your charging system is not working when it actually all is fine?

Every car I've ever owned develops a strange electrical problem at some point. I hope this isn’t the next one. I prefer them to just breakdown and then you can usually fix the problem. There is probably just a fuse blown somewhere. Bloody cars, eh.

[Alan02]

Hi lads,have you had your battery tested? an alternator will never charge a faulty/dead battery.Also you say your ignition light works ok,which it needs to to start the charging process but do you have the wires on the correct terminals on the Alt?
TERM.15 is the Ign,feed to the Alt.from the switch.BATT= main live,heavy wire.
Also, the regulator brushes could just be worn out,or carbon on the commutator?just a few ideas.

[gav10]

Got my cars alternator fixed at last. Hurray. My mechanic was stumped by it, so we ended up leaving the car with a local auto electrician.

After fitting 2 new voltage regulators to the alternator and them quickly blowing the auto electrician finally figured out what the problem was. There was a short happening between the 2 thin wires that come off the alternator this was causing the alternator's voltage regulator to blow almost immediately after the engine was fired up.

The short was happening at the point where the 2 thin wires off the alternator run up behind the air filter housing. Over the years the metal plate under the air filter housing has been rubbing against these 2 wires eventually chaffing the plastic off the wires and causing the bare wires to short against each other. The bare wires have been fixed with a bit of insulation tape around the chaffed bits and for extra security, the wires have been moved an inch to the right so they don’t actually touch the back/bottom of air filter housing anymore.

Actually, I think the wires are back to where they should be. I could be wrong as I don’t know anybody else with an Audi 80 TDI to compare mine against. I suspect a dodgy repair has been made in the past and this wire was in put back in slightly the wrong place and hence it was rubbing against the air filter housing. Unbelievable, my initial gut feeling was a stupid electrical fault somewhere and I was bloody right. This is the fifth car I have owned and the fifth car to develop a stupid electrical fault. I hate car electrics.

[micheal balbrig]

Got my cars alternator fixed at last. Hurray. My mechanic was stumped by it, so we ended up leaving the car with a local auto electrician.

After fitting 2 new voltage regulators to the alternator and them quickly blowing the auto electrician finally figured out what the problem was. There was a short happening between the 2 thin wires that come off the alternator this was causing the alternator's voltage regulator to blow almost immediately after the engine was fired up.

The short was happening at the point where the 2 thin wires off the alternator run up behind the air filter housing. Over the years the metal plate under the air filter housing has been rubbing against these 2 wires eventually chaffing the plastic off the wires and causing the bare wires to short against each other. The bare wires have been fixed with a bit of insulation tape around the chaffed bits and for extra security, the wires have been moved an inch to the right so they don’t actually touch the back/bottom of air filter housing anymore.

Actually, I think the wires are back to where they should be. I could be wrong as I don’t know anybody else with an Audi 80 TDI to compare mine against. I suspect a dodgy repair has been made in the past and this wire was in put back in slightly the wrong place and hence it was rubbing against the air filter housing. Unbelievable, my initial gut feeling was a stupid electrical fault somewhere and I was bloody right. This is the fifth car I have owned and the fifth car to develop a stupid electrical fault. I hate car electrics.

Good to read that your problem is sorted
There may be a problem where these cables are routed
as I came across this before on a 94 model which had chaffed
cables under the plastic air box. I did not know what the cables were feeding but after insulating them I was hoping to get rid
of the juddering problem. You can always expect electrical
problems to be very elusive but the satisfaction of finding
the problem makes it all worthwhile