Alternator or Voltage regulator?

[md944s2]

H i, I have a 1991 944 S2, which has recently been giving some problems with the battery going flat after as little as a few days of being laid up. It starts ok if I jump start it, or remove and charge the battery. I reckon the battery is faulty (despite only being a year old!) as there is nothing draining it when the car is off. I checked this with a clamp type ammeter.

My concern is about the charging when the car is running. When it is running with no electrics on the voltage at the battery is 13.8 Volts, with 20 Amps. When all of the electrics are on full this drops to 12.8Volts and 3 Amps - is this normal. I think I can accept the current being lower at the battery under full load, but woul dhave thought the voltage would stay the same. Does this sound like a potential problem with the alternator or voltage regulator, and is replcing either of these especially difficult?

Any thoughts/ideas/suggestions would be greatly appreciated.

 

[Neil Haughey]

Voltage when running sounds spot on which means the regulator is fine. Sometimes bad earths will cause voltage drops around the car that are a bit to much but 12.8v is very close to mine after I remade most of the cars earth connections. Bizarrely the interior fan causes the biggest drop. Your testing says to me there is nothing wrong with your charging circuit and your battery is stuffed (note that I haven't checked current because my multimeter will blow at 10 amps).

If you have completely charged the battery using a charger and then it is drained in a few days then sounds like battery is kaput. However on my car I have the original PA1000 alarm system and that will drain a full battery to virtually nothing in 2 to 3 weeks i.e. not enough juice to start. It has always been this way for me so if the alarm was faulty it wouldn't surprise me if it drained the battery faster.

 

[sawood12]

A classic symptom, though i'm sure not the only one, of a voltage regulator on its way out is irratic voltage output. This happened to mine. Every now an again my electrics would go crazy, all my lights (exterior and interior) would go ultra bright, the in-dash volt gauge would go off the scale and the fan would go crazy - sounds like twice the speed of setting 4. It would hold this for a few seconds before settling down again. It happened randomly and sporradically. If you leave it too long then it can cause damage to your battery as well. I bought a 2nd hand alternator and VR for £45 from the breakers and so far no probs.

I was told that at idle with all the electrics off then the voltage accross your battery terminals should be about 14.5v. With your electics on it shoulsn't drop below 12v at any time. If you are not getting this then my first suspicion would be the alternator. The VR tries to maintain a constant voltage, therefore a varying current, but if it is not being supplied with enough energy in the first place then it will simply do the best it can. If your voltage is simply low but steady then i'd suspect the alternator.

 

[geoff Ives]

A very simple check.
When you next park the car for a couple of days remove the earth lead from the battery. That will make sure that there is no leakage but you will loose any antitheft device during that time and you will have to reset your radio when you reconnect the battery.

I have this problem so bad on my 911 that I always remove the earth lead when I leave the car for a period. Mine will go flat overnight or it can stay charged for a couple of months.

 

[xenon]

A clamp ammeter will not be sensitive enough to measure a typical current drain that will drain the battery after a couple of days. A 6w courtesy light which is not switching will draw 0.5A - probably not enough to register on a clamp-type ammeter. Having said that, a fully healthy batter should be able to sink 0.5A for 4-5 days. Your charging figures sound OK so it's either an iffy battery or a current drain that you'll need to measure in series.

 

[md944s2]

Thanks for the replies - I reckon I'll replace the battery first anyway (or get it tested at least) and take it from there.

 

[colinbythesea]

If you do need a regulator and it is bolted into the alternator, take it out and look at the part number. Motor factors can get Bosch ones and they only cost a few pounds.

 

[Neil Haughey]

This is what I did with mine, it had gone crazy and was putting out mega high voltages (ISTR measuring 28v !), local autofactors just said if it looks like this one here then it is, and it was. ISTR it cost about 20 quid.

BTW this was Sheltune in Northampton.

 

[Guest]

A local firm that rebuilds alternators told me a few years ago that by far the majority of Bosch alternators that come to them are only suffering from a duff regulator pack. The rest of the unit is built to a very high standard.

When I converted the Allard from dynamo to Bosch alternator (s/h and ex-Vauxhall Astra) I fitted a new regulator pack (£18) and new bearings for peace of mind although I could not detect any play in the originals. In case of regulator failure I always carry a spare ex-VW Golf/Passat regulator in the glovebox. As it is only held on with 2 screws it takes only minutes to change.