Hi All,
Just checking my alternator as I am not convinced it is working ok...
So I get the red light on the dash when the key is turned before starting and on starting it goes out. Terminals on starter are all clean and earths are ok.
Before I start the car voltage at batteries is reading 12.6. when car is started they are 12.9, but red light isnt on on the dash unless its very slow idle, then it flickers.
It doesnt feel enough!, would have assumed they should be 13.5 or more.
Just wanted an opinion before I take it out to get it checked - have I missed anything?
Cheers
Roger
Alternator
Moderators: hot66, impmad2000, Barry, Viv_Surby, Derek, Mike Usiskin
Re: Alternator
Sounds fine to me. What's the voltage when the revs are at 2k?
'68 swb 911L
'62 Lotus 22
100cc Historic Karts
'62 Lotus 22
100cc Historic Karts
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Nine One One
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Re: Alternator
Get one of these off ebay, or similar, and plug in the cigarette lighter, plus you have a phone charger as well. It will show you how well the alternator is charging.
https://www.ebay.co.uk/itm/313642566244 ... gKv_fD_BwE
https://www.ebay.co.uk/itm/313642566244 ... gKv_fD_BwE
Re: Alternator
Its about 12.9 at anything over 2k revs
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Nine One One
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Re: Alternator
Regulator gone?
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Ralph
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Re: Alternator
I thought that it was meant to put out about 14v at over 2k?
Re: Alternator
What is the make of the alternator?
Bosch put out a different voltage to the SEV/Motorola type.
The SEV regulator is rated at 14.5V the Bosch is lower at around 13.5V if I remember rightly.
Do a test start and rev the car up above 2000 rpm measure the voltage and test again at idle.
If there is no change or minimal change in readings then you know it isn’t charging.
If the readings go above 16V when being raved then you are cooking the battery
Bosch put out a different voltage to the SEV/Motorola type.
The SEV regulator is rated at 14.5V the Bosch is lower at around 13.5V if I remember rightly.
Do a test start and rev the car up above 2000 rpm measure the voltage and test again at idle.
If there is no change or minimal change in readings then you know it isn’t charging.
If the readings go above 16V when being raved then you are cooking the battery
1966 912
1969 912/6 hotish rod in build
1974 914 Dead
LI 125 Lambretta (Yeah right it’s a 125)
Mini Cooper s
1969 912/6 hotish rod in build
1974 914 Dead
LI 125 Lambretta (Yeah right it’s a 125)
Mini Cooper s
Re: Alternator
Thanks all, will try that test later and report back. I have a feeling I need to just get on an take it out 
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Jonny Hart
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Re: Alternator
The charging system voltage depends on a few things..
1. How drained the battery is. 12.6 suggests you battery is fully charged so it won't need much more charging.
2. The temperature of the battery and cables
3. The voltage drop across the cable ( from alternator to battery incl. the earth)
4. The regulator voltage.
5. The electrical load on the alternator.
A better test is to switch lights, wipers and other higher power accessories on and then check the voltage. As long as it above 12.6 volts then the battery is charging. It is quite normal for a standard 911 to be below 12.6 at idle with accessories on. As soon as you get above 1000 RPM though, the voltage should rise.
1. How drained the battery is. 12.6 suggests you battery is fully charged so it won't need much more charging.
2. The temperature of the battery and cables
3. The voltage drop across the cable ( from alternator to battery incl. the earth)
4. The regulator voltage.
5. The electrical load on the alternator.
A better test is to switch lights, wipers and other higher power accessories on and then check the voltage. As long as it above 12.6 volts then the battery is charging. It is quite normal for a standard 911 to be below 12.6 at idle with accessories on. As soon as you get above 1000 RPM though, the voltage should rise.
Re: Alternator
Thanks Jonny,
I took my alternator out and had it checked and it was putting out 14v, the chap cleaned it up and saw 14.1 and good output. He asked about the voltage regulator, so I started Googling the voltage regulator and found and early 911s article that Marchal alternators should have Marchal voltage regulators otherwise they will drop 0.7v due to different construction. I have discovered I have a Bosch regulator so this may be the problem. I will take that to the guy checking it and see if that is the issue. If I got 13.7v out I would be happy!
Will report back
I took my alternator out and had it checked and it was putting out 14v, the chap cleaned it up and saw 14.1 and good output. He asked about the voltage regulator, so I started Googling the voltage regulator and found and early 911s article that Marchal alternators should have Marchal voltage regulators otherwise they will drop 0.7v due to different construction. I have discovered I have a Bosch regulator so this may be the problem. I will take that to the guy checking it and see if that is the issue. If I got 13.7v out I would be happy!
Will report back
Re: Alternator
....the conclusion was the alternator was ok, but when the voltage regulator warmed up it stopped outputting correct voltage. So new voltage regulator and all good.