Picture from my '78 marchal, the large brown wire at the rear should be grounded at an engine case bolt. (probably the clean one in your picture) The three small browns are on a ring terminal together for another ground path.
Hope that helps.
Have the Alternator tested as well as your voltage regulator.
As I posted earlier, my alt was OK, the regulator wasn't.

http://forums.pelicanparts.com/uploa...1357023698.jpg

To the OP: You posted your voltage readings at various engine loads. But was that while the tach needle was jumping around erratically? (I'm guessing not.)

I had a similar problem, where my tach was jumping around erratically but only intermittently. But how to test the voltage across the battery for the occasions where that was happening while I was driving? So I used speaker wire to connect the battery to my multimeter which I wedged onto the dash. When the tach needle started jumping, I switched on my multimeter and found out the volts were surging from 16-18 volts!

So it was most likely the regulator at fault. I replaced it (and the alternator at the same time, as they're attached to each other in my car and I didn't want to have to do the job twice) and all was fine. Battery was fine.

I gave my worthless POS dash clock the float test...replaced it with a new fangled thing called a wristwatch, I check it for accuracy with the clock on the radio face plate.....the dash got a volt meter/head temp gauge.

The two failed alternators that I have had showed voltage spikes along with tach swings....

Noah930,

The voltage tests were done after charging the battery and the tach was steady.

timmy2,

I'm assuming one of my three small ground wires on the alternator goes to the fan housing via the coil loom?
Looking at the size of your large ground wire I will make up a fat ground wire and run it to the case bolt you suggested when I put it back together again.

Alternator and regulater are off to the auto electricians this week.

Cheers
Bill

Bill,
Download and take a look at the schematic diagram for the '78. It shows where the brown wires go.
None go directly to the case. The large brown is the only one that goes directly to engine ground. (It's actually not even shown as a brown wire, but is the wire coming off the bottom of the diode array in the schematic at lower track number 102.)
Pelican Parts - Porsche 911 Parts Listings & Diagrams

while waiting: get a soft wooden wedge and use it like a chisel to get that gunk off the motor

then use a solvent for further cleaning

- use a shop vac to suck debris for off of the cylinder fins

use vinegar (overnight) to clean all elec. connectors until shiny

look around at all accessible area for fixes, rust repair & cleaning (like the sound pad)

Timmy2,
I'm assuming I have to connect the large ground wire to the same terminal on the alternator as my 3 small ground wires as I do not appear to have another terminal for it?

Whats the minimum size (gauge) for this wire?

Cheers

Bill

You are correct, same terminal (B-), as it is the same potential. Use a number 10 AWG wire for the ground.

+1 Poraboynz regarding stranded/braided earth/ground strap. This strap must go from one of the bolts on the alternator hub extension to a ground on the case. It it's missing, that is a problem. See picture.

If you go with a rebuilt alternator, the following is what you need:

PartInformation

Call our host regarding application. The say they have on for 74 however, I think it will work on a 77 just fine.

http://www.pelicanparts.com/cgi-bin/smart/S-mart.cgi?command=add&weight=12&itemid=AL-9341-X&itemname=Alternator%2C%20Bosch%20%28Rebuilt%2C%2 0Core%20Charge%20Applies%29%2C%20External%20Regula tor%2C%20Motorola%2FSEV%2070%20Amp%2C%20911%20%281 965-74%29%20Brand%3A%20Bosch&itemprice=187.00&itemquan t=1



It has an external voltage regulator just like to have now. The bosch site should also tell you where you can find one. They are warranted for 2 years if you go through a Bosch supported seller. I just bought one in the States for under $200.

Having said all of this, I would have an electric repair shop that rebuilds alternators check out the old alternator. Many of them will do it for free. Could be the rotors in the alternator are bad? It's to bad you have the alternator already out because you could have had the electric shop tell you how to jumper the external voltage regulator to see if is was bad or not.

I would make sure my battery is good and will hold a charge before replacing anything.

As I have already stated, if the battery is good, then have electric shop test the alternator.

If the alternator is good you either have a wiring issue or bad voltage regulator.

Did you note what wire goes where?

Hope this helps and have a great new year


http://forums.pelicanparts.com/uploa...1357092314.jpg

The ground strap needs to be the same or greater guage than the large red output positive wire connecting to the alternator. On a 72 its a braided earth strap that runs from a housing mounting stud to a case bolt, but looking at the timmy2 photo there is a suitable stud directly on that alternator. I do not see a similar stud on your alternator so use a mounting stud. Make sure the spade connectors are clean and have good tension, the connector to the slip rings is very important, it provides the starter current that causes the alternator to produce a charge. I agree with cleaning the engine case and fins while the alternator is out, lots of degreaser and a water blaster, plus check the oil cooler is not gunked up as well. Did you check the slip ring brushes on the alternator???

Do you have a source for this?

This source has them for $6.60.

Alternator Parts - Starter Parts - Alternator & Starter Rebuild kits

Porboynz,
I did clean up the case and fins a couple of days ago but havn't checked how to get to the oil cooler yet. I have cleaned and tightened the terminals and reinstalled the alternator but it still was not charging so have removed it again. This was before the lack of ground to case was pointed out to me. I imported this car last april and have put 5000 miles on it since and have only just had a non charging issue. The only time I have had to put a charger on this car was on Christmas morning after nearly running the battery flat while out on a drive? I get that it (large ground wire) should be there and will fit one but do not understand how it could work the past 8 months without it if it is necessary. When i took the alternator out the first time I saw the three ground wires and assumed that one was to ground and the other two were ground pick up points to somewhere else but now assume all three are pick up points.
My question now is do I fit the ground wire and reinstall the alternator again to test it as is or have it rebuild first? Is the lack of this ground wire likely to be the non charging issue?

Cheers

Bill

Like you point out, if the car had been fine for 5000 miles, it's not likely that your lack of a proper ground wire is the culprit for your current problems.

The alternator would have been finding an earth return path via the fan housing which is clamped to the engine housing by the strap around the outside. Trouble with that is its not a great electrical connection and when you are dealing with 55Amps plus, even a small resistance is going to cause a significant voltage drop. (V=I x R) (Magnesium forms an oxidised surface layer that is not a great electrical conductor) Running an earth strap from the alternator case or the alternator mounting stud ensures a low resistance return path for the charging current. It may not be the cause of your no charge issue but if it went high resistance even momentarily while the engine was running and the alternator charging then you can toast the diode pack. Alternators do not like having the battery removed when charging, bad things happen from the resulting voltage spike. I presume you have checked the slip rings and brushes? You can see the top of the oil cooler in the rhs front of the engine with the alternator removed, shine a torch in there.

I have fitted a earth strap from the alternator to engine case with no change to my charging issue so am about to pull the alternator for the third time and will take it and the regulator up to Sydney to a Bosch auto electrician for testing/ rebuilding.

I'll have a look at the engine oil cooler once I get the alternator out again.

Cheers

Bill

I suggest you dump a full can of Super Cheap engine degreaser into the top of the oil cooler followed up by a water blaster to wash the solvent out the bottom along with any oil and crud. So the slip ring carbon brushes are good? Rebuilding the alternator should not be a major, at worst its a new diode pack, skim the slip rings, new brushes and new bearings. You have to really unlucky to lose a winding.

Have just made not one but two newbie mistakes from not paying attention.

One. When testing the charging with the new ground wire attached I have just realised right after removing the alternator for the third time that I did not have the regulator plugged in during the test.

Two. Forgot to disconnect the ground from the battery before removing alternator. I noticed this as I got an arc from the new ground wire at the alternator end to the backing plate of the alternator. What I am calling the backing plate is the horseshoe shaped plate that the two B+ posts protrude from. Should that plate be live? The ground wire was bolted to one of the six studs that bolt the rear air cowl and the alternator to the housing. I guess I should have chosen a stud that is not next to the backing plate?

http://forums.pelicanparts.com/uploa...1357852683.jpg

That horseshoe shaped plate is (was) the diode pack mounting plate that doubles as a heat sink. Those 3 round metal shapes are the backs of 3 of the 6 x stud diodes in the pack. The earth wire is usually insulated from accidental contact with the diode mounting plate because it is supposed to be attached to a stud on the outside of the fiberglass fan ducting that bolts to the rear of the alternator. Running the alternator without the regulator connected is probably harmless, not so earthing the diode heatsink. All you can do now is reconnect everything correctly, connect the battery and test the voltages with the engine running and hope. My bet is the diode pack will need replacement, but you might be lucky. So the slip rings and brushes were good then?

Porboynz,

Slip rings fine, brushes not, so I had new ones fitted today at a local old school auto elec shop.

From your description of the of the diode heatsink I have had a lucky escape. I did have the ground wire attached to a rear stud for the test but like an idiot used one adjacent to the heat sink and brushed against the heatsink with the ground wire when removing it early this morning. Of course this wouldn't have happened if I had removed the ground from the battery first!

I will use a stud on the other side when I put it all back together tonight.

Regarding the oil cooler I just happen to have a case of "Supercheap Auto engine degreaser" as you seem to have guessed so will do that before replacing the alternator.

Cheers

Bill