Jump to content

Welcome to RennTech.org Community, Guest

There are many great features available to you once you register at RennTech.org
You are free to view posts here, but you must log in to reply to existing posts, or to start your own new topic. Like most online communities, there are costs involved to maintain a site like this - so we encourage our members to donate. All donations go to the costs operating and maintaining this site. We prefer that guests take part in our community and we offer a lot in return to those willing to join our corner of the Porsche world. This site is 99 percent member supported (less than 1 percent comes from advertising) - so please consider an annual donation to keep this site running.

Here are some of the features available - once you register at RennTech.org

  • View Classified Ads
  • DIY Tutorials
  • Porsche TSB Listings (limited)
  • VIN Decoder
  • Special Offers
  • OBD II P-Codes
  • Paint Codes
  • Registry
  • Videos System
  • View Reviews
  • and get rid of this welcome message

It takes just a few minutes to register, and it's FREE

Contributing Members also get these additional benefits:
(you become a Contributing Member by donating money to the operation of this site)

  • No ads - advertisements are removed
  • Access the Contributors Only Forum
  • Contributing Members Only Downloads
  • Send attachments with PMs
  • All image/file storage limits are substantially increased for all Contributing Members
  • Option Codes Lookup
  • VIN Option Lookups (limited)

Alternator Testing... Need advice


Recommended Posts

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

Link to comment
Share on other sites

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 by logray
Link to comment
Share on other sites

  • Moderators

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.

Link to comment
Share on other sites

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 by Elite1967
Link to comment
Share on other sites

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 by Elite1967
Link to comment
Share on other sites

  • Moderators

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • 1 year later...
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?

Link to comment
Share on other sites

  • Moderators

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • Moderators

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.