924Board.org

[sermeussteven]

Hello guys

i'm facing some problems with the generator of my 924 from '79 and i could really use some help since i'm already struggling with it for a few days.

Main problem is that the generator is not charging the battery.
To explain the problem, i will use the following:

B+ terminal = output from alternator which should be directly connected to the positive side from the battery.
D+ terminal = the terminal on the alternator which excites the alternator an 'reads' the battery voltage, wired through the instrument cluster indicating lamp.
Terminal 7 on the instrument cluster is the supply to all the indication lamp, coming from the ingintion switch No.15
Terminal 11 on the instrument cluster is the output of the generator indication lamp and is normally wired to the D+ terminal.

The battery voltage was 12.6V measure at the battery terminals.
If I switch on the ignition, I measure 12.2V between the D+ terminal and the engine block (ground)
If I start the car like this, the generator is giving 30V! (measured between the B+ terminal and engine block, without the B+ connected to the battery terminal)

However if I bypass the dashboard and pull a wire directly from the battery to the D+ terminal on the generator (so 12.6V on this wire), the generator is giving 13.1V. Which is still quite low but already better...

So I pulled out my instrument cluster, pulled a wire from the positive side of the battery to an on-off switch, to the lamp inside the dash (Terminal 7 to 11), and then to the D+ terminal on the generator. So on this one I'm measuring the battery voltage while passing the 'idiot ' lamp. However, if I start the car, the generator is giving 35V!

So now i'm wondering:
1) How the f*** is this possible? And what to do about it?
2) Why is it losing 0.4 V after the ignition switch in normal condition?

Side notes:
-The generator was completely revised in a reputable workshop.
-If I start the car in normal condition (everything connected like in the book) the generator indication on the dash is dissapearing so not indicating a problem while there clearly is a problem.
-For the tests of the voltage on the generator, it was never connected to the battery because then the battery voltage would interfere?

If anybody could help me, would be highly appreciated!!

Thanks!
Steven

PS: sorry if I used alternator/generator throughout the text, you know what I mean ;)

[BTRacing]

I suggest checking really closely your wiring harness from alternator/starter to the plug on the passenger side . Harness is routed under the exhaust close to the starter. It works in very hot environment and has hard life. We have seen many issues that are caused by it.

http://www.924board.org/viewtopic.php?p=382464&sid=a21692f78e1df9c5fc7052ff7c33e1d4
_________________
www.btrpalmbeach.com
1979 931 GTR Le Mans replica to be
2008 Audi S4 Avant daily driver
2008 Aston Martin Vantage
1974 Moto Guzzi Cafe Racer
1979 Glastron CVX 20

[sermeussteven]

The wiring to the starter had been completly renewed, the cable from the starter to the alternator is new and the cable from the connector near the battery to the D+ terminal is also renewed (slightly larger size)
So normally should not give any problems...

[BTRacing]

OK if it is new then most likely is fine if was done correctly.
_________________
www.btrpalmbeach.com
1979 931 GTR Le Mans replica to be
2008 Audi S4 Avant daily driver
2008 Aston Martin Vantage
1974 Moto Guzzi Cafe Racer
1979 Glastron CVX 20

[Paul]

Check alternator to block ground.

Try a different regulator.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[sermeussteven]

alternator to block resistance is 0.01 ohm

Replaced it today with one i knew that was working is doing the exact same,

Might this be the dashboard indication lamp?

[Paul]

The lamps primary purpose is to energize the field upon startup.

Make sure you have a good ground strap from the alternator to the block.

Do not run the car with the battery leads disconnected, any readings taken this way are meaningless and can damage the alternator and other electronics.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[sermeussteven]

Yes with the battery lead (coming from starter) not connected to the B+ on the alternator.
Thats what I meant with the voltage readings.

[staticsan]

I can't see where you're measuring the voltage the alternator is laying on the battery. Nor how old the battery is.

I had a charging problem for ages, but never tried to measure the alternator with the main charging wire disconnected. With everything hooked up, though, I was seeing 13.2v on the battery. The problem was, though, is that the battery uses a modern chemistry that has calcium doping instead of the older antimony. Four roadside servicemen said it should be charging at 14.5v. Two auto-electricians said 13 is enough to charge. It wasn't. I was putting a charger on the battery every three or four trips or it would never start.

But my alternator was original. I bought one of Ideola's alternators and it now charges the battery at 14.5v. All solved.

Wade.
_________________
'82 British NA - Which I think I've been very lucky with!

[sermeussteven]

Thanks for the suggestion. The battery is brand new (bought from an internet site though)
With the voltage measurement I mean the following:
- The alternator is installed
- D+ terminal connected
- B+ terminal diconnected on the alternator BUT battery connected to starter.
- Start the car, runs on battery only
- Measure the voltage between the B+ terminal and the alternator housing/ground.

This voltage can go to 30V or is this an inproper way to test it?

[Paul]

Yes it is the improper way to test it. The proper test is to hook everything up, start the car and measure the voltage between the battery terminals.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[sermeussteven]

I've managed to get a little bit further but the problem is still not solved:

The voltage over the battery terminals is just the battery voltage when the car is running... so the alternator is still not function properly.

So i pulled off one battery terminal (i am fully aware that you normally shouldn't do this) and car keeps on running so i measured over the battery terminal connections without the battery connected. Then i measure between 7 and 8V. Peaking at 10.5V by increasing the engine speed.

So what i did then is pull out the instrument cluster and the run a cable from the battery to the a switch to the light and then to de D+ terminal on the alternator. More or less like this:

Battery terminal ----- Switch -----dashboard bulb ------ D+ terminal alternator

However this give the same results as before. The lightbulb is brand new and a resistance of 8 Ohms is measured over it.

All the grounds are cleaned and alternator passed a bench tested this morning, anybody has a clue what can be the issue?