I have used the searched button for the above 2 terms because my 2001 996tt tiptronic with 45000 miles on the clock is suffering from a low voltage after a new battery. I obtained the following voltages using a voltmeter.

After fitting a new battery, I found the voltage to be

engine off = 12.6v
engine cranking = 10.6-10.7
cold start engine idle = 13.5-13.9v
rev to constant 2000rpm = 12.8v
engine on, idling, ac on, main bean on = 12.5v
driven for 20+ mins engine still on = 12.3v

Gathering all the threads that I have read, I am suspecting the culprit comes from the voltage regulator and not the alternator because the car starts strongly every time even after 2 weeks of peaceful rest. The battery connections are tight.

Is the diode built in the voltage regulator?

Would any kind fellow owners please share your thought?

Thank you very much!

James

 

I think a lot of alternator failures are indeed regulators in reality. Unfortunately I don't know if there is a way to test for that. As far as I know you would need to replace the regulator and reinstall the alternator and see if it works properly. A PITA solution.

 

Thanks Deckman! I gotta agree with you that it is a PITA solution.

I am just wondering if anyone has thought of a way to roughly estimate which is the one I need to replace regulator or alternator?

Moreover, I dont know where the diode is located, the regulator or alternator?

 

The regulator is attached to the end of the Alternator, it takes about five minutes to replace it. Think mine cost about £25
The hardest part is removing the Alternator, it is fiddly but perfectly doable as a DIY. I have done it many times.

 

Thanks mt10argyll,

I realized the regulator is linked to the alternator. I am just wondering where the diode is. Is it in the alternator or the regulator?

Does any reconditioned alternator comes with a new voltage regulator? Or, is it compulsory to buy a new voltage regulator?

Am I right to think that I will need the Bosch F-00M-145-350 for the 2001 996tt tip?


Thanks again!

 

The bridge diodes for the rectifier are mounted to the alternator itself I believe. If a diode drops out it will lower your terminal voltage but if that was your problem you probably wouldn't have normal voltage on initial fire up. Not sure if the diodes will even be that accessible but if they are you can always give each of them a continuity test both directions with a meter to see if they still function properly, but again IMO I really doubt this is your problem it's probably just the V Reg. I had to replace mine at ~30k miles and the brushes attached to the reg were quite worn. New alternator will have the V Reg on it if you decide to go that way. Sorry not sure about that model of regulator at the moment.
Cheers,
Rob

 

IF in fact the issue resides within the alternator and you choose to replace/rebuild it, you can review our DIY tech article which will walk you through the process - takes about 4 hours but pretty straightforward:

Alternator Replacement

Let us know how it turns out!

Mark / Pelican Parts

 

If you remove the alternator and take to an auto parts store they will bench test it for you. Question: if it passes yet you know it's not right, does that means its probably the regulator? That was my situation exactly. I replaced the whole thing because I didn't know any better at the time.

 

Thanks for the replies mates!

If my car can only charges the NEW battery to 13.0V after a long night sleep and it does NOT charges the new battery to 13.8-14.0V (like some members suggested).

Does it mean I am definitely going to need a new alternator and a voltage regulator?

My car is 2001 and have only done 45000 miles with neither the regulator or alternator replaced before.

cold start engine idle = 13.0V (it can no longer go up to 13.5-14.0V)

and that my headlights wont even come on after the engine is running with voltage at 13V. They do eventually come on after 30 secs or so.

Is this a clear sign that I will need a new alternator and a regulator?

I would be grateful if any fellow members can chime in for your prediction.

Thank you

 

I supposed the diode sits in the alternator while the voltage regulator is screwed to the shell of the alternator. So, you can replace the V regulator yourself but not the diode.

I hope it helps.

 

Changed my VR yesterday on my 02 turbo with only 13K miles. Pelican parts $37.50 plus shipping, arrived in 2 days. Chances are your problem is the voltage regulator, an easy replacement once you get the alt. out. In my case, voltage started at close to 13.5 volts and within several minutes dropped to under 11vdc. I suspect temperature related as the alt. is somewhat buried and probably starved for fresh air. Note, you can see the 6 diodes after you remove the VR cover. You should check'em with an ohmmeter while the cover is off.
Hope this info. helps.

Rich

 

The (6) diodes are in the alternator.

 

Hi gillrich,

thanks for the very informative info.

may i ask how i should test the diodes with my voltmeter? Which ohm setting should i be using?

but when i start the car, the highest voltage i see from a cold start is only 13.0V, never higher. It drops to 12.1-12.2v after 1-2 mins of driving.

thanks again!

 

Need multimeter with a diode check position if you have a digital meter or RX1 scale if analog type meter. I believe with a digital meter on the diode setting, you may just get a beep in one direction and none with the leads reversed signifying continuty and infinity. Be willing to bet your issue is the is the VR as my alt. behaved the same way as yours. Good luck.

 

Thank you gillrich, mt10argyll, deckman, The Pork Hunt and pelican.com

But, I am still in massive frustration after a a new full new alternator & voltage regulator swap and a new 85A battery. It took me a few hrs and a lot of sweat. But, however, after 10 miles of driving, the voltage hasn't changed except it now sits at 13.0V instead of 12.5V during driving with only headlamps on (AC off, music off). Am I now left with 3 possible culprits, possibly the slipping belt or pulleys or the stock earth cable?

Without doing all of the above, how may I find the ultimate culprit?

p.s. at least now when I rev the engine or put my foot down lightly during cursing, the voltage does climb up. But, if I dont keep my foot down, it can drop down to 12.3V or so. I checked the dash voltage gauge vs a proper voltmeter and I can confirm that the dash voltage gauge is indeed accurate. What should I do next? Will running the car as is damage any part of the car? Can I keep driving the car daily?

Many thanks again