mk85911

1985 911. Dead battery , only one year old. Charged it yesterday, and voltmeter shows 12 volts, with car not running. 12 volts today, also. However, after starting up the car, with engine running, voltage was the same 12 volts. This suggests to me that the charging system is not charging, because IIRC a properly charging voltage level should be at least 14 volts with the engine running. Any thoughts?

Mike

theiceman

yup you should be putting out a min of 13.6 volts.

if your belt is tight it is likely your regulator. it can be replaced and is part of the alternator. id try that first..

if you are pulling it to replace the regulator though i would take it to get it bench tested.
one question though .. when its running is the gen light on ??? i thought it used that bulb as part of the exciter circuit. maybe look at that too if its not lit when the ignition is on ,

autobonrun

I am uncertain for 1985 but you are correct for the older cars. My 1979 has a red light that comes on when it’s not charging. It lights dimly when the charging power is too low. Like you said, check the belt first.

mk85911

Thanks iceman and autobonrun, I appreciate your help. I did check the belt first thing. Funny thing, I had just been thinking, that it was just about time for the alternator to crap out because I’ve had the problem twice before, and each time the alternator lasted about twelve years and 40,000 miles. I’m coming up on 36 years of ownership and about 120,000 miles, so it is on schedule.

there’s a diode that goes bad sometimes.

the light is not lit now, and I think it may just flash briefly when the charging system craps out, depending upon which component has gone bad.

my plan is to give the battery a good charge and rely upon the battery to provide enough power to keep the engine running while I drive to my mechanic.

thanks again, Mike.

autobonrun

Nice. I bought my SC in 1984 as well. If I recall correctly, the alternator light is lit in the accessory position, along with the oil pressure light then both should go out when running. Does yours light up when key is in accessory position?

r911

4 diodes or more rectify the AC from the alt. and regulate the voltage

your VR is built into the alt.

I expect that you will be pulling the fan/alt. out - so start trying to find a good electrical shop & if it specializes in German cars, even better

obviously avoid the past places

autobonrun

I had to pull my alternator 3 times for one repair. All this happened the same week. First the solder at a diode got old and cracked causing intermittent failure. Fixed that then a spring holding a brush down against the commutator stuck in the holder; fixed that then one of the brush springs actually snapped . It’s made it about 15 years since that repair.

r911

a good elec. shop will go thru it comprehensively, incl. the bearings - then run it at different rpm on a machine to check output

mk85911

neither oil pressure light nor alternator light lit when in the accessory position. When running, oil pressure light briefly lights up, but accessory light does not light up at all. Mike

theiceman

the alt light should be on when just ignition is on .. fix that first.
always fix what you can see before you fix what you cant see..

oldskewel

Based on how my '85 works (and also every other car I'm aware of), yes the charging (alt / battery) light should be on when the ignition is ON and the engine is not running. Same with the oil pressure light.

I had an alternator failure on that '85 (with a Paris-Rhone alternator) which somehow resulted in that light staying off all the time, including when the ignition was on and the engine was stopped. In my case, everything was fixed when I replaced the alternator with a new one.

So it is possible that (only) a failed alternator could case your situation.

Oil pressure light though ... that's another story. I don't know. You might want to check the circuit diagrams to see if there is any way ...

autobonrun

Sorry but I don’t have diagrams for a 1985 model. These are for the 911 SC alternator light and oil pressure indicator. Yours may be similar.

theiceman

i agree if thats the case .. get that alt out.

mk85911

Thanks , all. My mechanic, who has a Porsche, Audi exclusive shop and dozens or years experience will check it out next week. he’s good, but wouldn’t do an alternator rebuild in house. Actually, I probably have more experience along the rebuild line because I was an electronics technician in the Navy in the mid sixties, and replacing brushes and that sort of thing was routine . But that’s history.

looking more closely at my records, original alternator lasted about 50,000 miles, and it may have been replaced by a rebuild (not rebuilt by my mechanic, probably a core swap). That lasted about 30, 000 miles, and next it looks like a new Bosh unit went in. Part of this is based upon my memory. If I get 40,000 miles out of a replacement now, I’ll probably get to the finish line. Might not be driving in another dozen years; or breathing, for that matter.

I will have to drive it to the shop relying on the charged battery, about a fifteen minute drive. My mechanic says
that will be ok, but he offered to let me use a portable battery pack he has, just in case. I’ll take him up on that, because I don’t have 100% confidence in the battery charge, at this point.

Another thing, I’m glad I reviewed my records; I have them all. Other than the alternators, I have not had a single significant issue in 35 years of ownership of this marvelous vehicle. I maintain it well, 3000 mile oil changes, etc. , so I guess that paid off.

Thanks again, Mike

r911

IIRC, the alt. bulb provides a resistance for the D+ circuit

- the D+ circuit is used to energize (kick-start, or bootstrap) the alt. charging circuit, so that would prevent charging