Alternator Testing... Need advice

[Elite1967]

Hi all,

I need an advice on how to proceed.

I own a C2 MY05.

I changed the battery last week and the technician told me that my alternator is running too low.

So here are my readings of my alternator and battery:

(The reading of the alternator is taken from the back of the alternator itself - I took the alternator out and connected a cable to the back of it and then reinstalled...)

Running @ 1500-2000 rpm with lights on, A/C on and radio on.

Cold engine:

Alternator: 14,30V

Battery: 13,80V

After 5 minutes:

Alternator: 14,00V

Battery: 13,60V

Hot engine (After 15 minutes):

Alternator: 13,80V

Battery: 13,30V

Questions:

1) is it normal to have a 0,5 volt drop from the alternator to the battery?

2) Is it normal that the alternator drops from 14,30V to 13,80 as soon as the temperature raises up?

Should I change the regulator perhaps?

3) What are your readings at the battery terminals?

Are mine readings in the average or is my alternator too low?

4) Should I consider changing the cable from the alternator to the battery?

Thanks a lot

[logray]

One straight forward and relatively easy thing to do is remove the alternator again and take it to a parts store such as Napa.

In about 5 minutes they will bench test it on their very expensive machine and perform a lot more tests than what just a simple multi meter can do.

They will even get you a print out and tell you how well it is performing/etc and whether it needs to be replaced (or how soon).

Even though you are reading OK voltage, there are other factors to a properly working alternator such as amp draw, etc.

Edit: see you are in Italy, but at least in the USA we are able to take our parts to a car parts store and have these things tested for free.

Edited August 19, 2011 by logray

[PAULSPEED]

Your volts look good so I would try looking at

other areas then the alternator. Maybe

new alternator cables?

Paul

[JFP in PA]

Along with voltage testing, we also like to test the alternator's response under load conditions. While we use a load tester for this, you can do the same thing by turning on the high beams and other electrical loads while monitoring the alternator's voltage response.

[wvicary]

When I test a suspect alternator I turn on every electric consumer in the vehicle and monitor the alt output over a period of 20 mins or so. I've had alternators fail under these conditions of extreme heat and loading.

[JFP in PA]

With an actual load tester, you can see the diodes "ripple" under load, confirming everything is correct. With a problematic alternator, usually the alternator starts dropping diodes pretty quickly (within seconds) under load.

[Elite1967]

Thanks for all your replies.

Before removing the alternator again, let me understand:

What is your voltage reading at the battery terminals with the engine on and with headlights and a/c on?

And does it drop as soon as the engine warms up? To what value?

Thanks.

Edited August 19, 2011 by Elite1967

[JFP in PA]

It should still be over 14V..............

[Elite1967]

This is what I suspected...

Should it change with the temperature?

And what should be the voltage at the back of the alternator? 14,5-15,0V

Is it normal a drop of 0,5V from the alternator to the battery?

Anyway, I'll get the alternator checked very soon.

Edited August 19, 2011 by Elite1967

[JFP in PA]

Temps should not really effect the output, and the voltage loss between the altenator and battery could be a bad cable.

[PAULSPEED]

That is pretty interesting that the diodes wear out.

Ever try changing the diodes with a soldering iron.

I still for the time saving buy a new re-built alternator.

Paul

[JFP in PA]

I have never tried changing the diodes; I leave that to shops that specialize in these repairs. Quite often, changing out the regulator is a simple and relatively cheap fix. It is a bolt in item and common to Audi, Mercedes and even VW; often a better source for the part as they only charge about $40-50 for it.

[tunetec]

Alternator packed up on my C2S as well. Could have got away with just a new regulator, most common fault if still kind of working. However once it was out i had the whole thing rebuilt with new bearings, regulator etc from Auto Electrics shop, came back like brand new, 120 UK pounds, worth the money.

[minny]

It should still be over 14V..............

I recently had the alternator on my turbo rebuilt with new bearings and a new Bosch F00M 144 136 regulator. When cold, it charges at 14V at idle and engine revved (with or without accesories turned on).

After driving for 30 minutes, it drops to about 13.5V at idle and engine revved. If i turn on the aircon, both heated seats, air blowing at max, all lights including main beam and fogs and rear demister on, the voltage at idle drops to 12.4V. If I rev the engine it increases to 13V. I used my Techno tools battery tester and it said that the charging was fine and ripple was 96mV. It gave the alternator the thumbs up.

Is this normal behaviour?

[JFP in PA]

A perfect (e.g.: brand new or totally rebuilt with new diodes, etc.) will idle over 14+V with nothing on. As more stuff is switched on, it starts to load the alternator and pulls the output down until the voltage regulator kicks it back up, but it still would stay in the high 13's to low 14V range. Your alternator sounds like it is OK, but not fully up to snuff as it is dropping the voltage down to near battery only voltage levels (12.4V) with a heavy current draw on it, or 13.5V after the car is warmed up, but without load. 13.5V is right at the bottom of the acceptable range.

I would be taking a look at both your power and ground battery connections, looking for signs of corrosion or any significant resistance (use a digital multimeter to test); these cars are well known for having cable end problems. If memory serves, I think there was a TSB on the subject as well as "improved" battery and starter cables released to address the resistance problem.

[minny]

Thank you. It is what I was not hoping to hear but suspected.

[JFP in PA]

Thank you. It is what I was not hoping to hear but suspected.

I'd be looking at those cables, we replace them fairly frequently due to high resistance; plus they are cheaper than a new alternator, which still would not work properly on bad cables.

[minny]

Being a fool, I found this thread via the search facility and forgot to mention that my car is a 996 turbo tip. I know it also suffers with similar problems. I heard that on the turbo tips the engine may have to be dropped to change the alternator to starter power cable.

[Elite1967]

Hi,

I wanted to change the cable From alt to starter/battery as well, but it looks very complicated.

Is there a tutorial somewhere?

Edited December 14, 2012 by Elite1967

[JFP in PA]

Depends upon which cable is bad, none of them are any fun to change, but you can ususally get at them without dropping the entire engine. Small hands, patience, and getting some things out of the way helps. Not a difficult process, but a time consuming one.

[JFP in PA]

Hi,I wanted to change the cable From alt to starter/battery as well, but it looks very complicated.Is there a tutorial somewhere?

Not sure on that one, may want to do a DIY search here to see what others have posted.............

[minny]

Thank you JFP for the advice.

[sullivas]

I believe someone documented cable replacement on Rennlist, with pictures. Long job.

[AP 997S]

If you have a early 997 (ie 2005) and you're experiencing slow cranking (especially when your car is warmed up) I'd look at the alternator/starter motor cable assembly. Its a fairly common issue; crimp or cable is offering too high a resistance. An upgraded replacement harness cost about $100 and it solves this problem. The harness comes in a 6sp and Tip version so make sure you get the correct one for your car (if that's the problem). I had my Indy shop replace it; about a 4 hour job for them.

[0356kid]

My cable was replaced by the dealer under warranty (08C2S). When I took the car in they asked if I noticed that it was cranking slower than usual and I said no. I thought that was the way it always cranked w/o problem until they put the new one on and now it turns over much better.