73.5 alternator or VR problem
 

Hi

I sent my fan out for repair (thanks Ben McFarland), and had fan and housing cerakoted.
After reinstall, I get red light on >3k rpm?
Ok, maybe I made error reconnecting alternator, so out it came and the connections are correct.

Alternator is Marchal SEV
VR is external solid state replaced ~10.22

I’ve looked over the following links:
http://forums.pelicanparts.com/porsche-911-technical-forum/777345-marchal-alternator-trouble-wiring-ground-3.html

http://forums.pelicanparts.com/porsche-911-technical-forum/172131-alternator-issues.html

http://forums.pelicanparts.com/porsche-911-technical-forum/783329-73-charging-problem.html

I’ve cleaned transmission and battery grounds and added a jumper from VR ground to body ground behind fuel console.
I cleaned 14 pin connector.

I’m checking blue wire circuit:
IGN OFF
Battery 12.97V

IGN ON, ENGINE OFF
VR blue wire terminal 1.53V
VR black terminal 0.8 V

IGN ON ENGINE ON
battery 12.4V
VR blue D+ wire terminal 2.3V
VR black DF terminal 1.6 volts and fluctuating

With engine off I measured resistance from disconnected black wire to ground- steady 9.4 ohms
I think this means alt windings and brushes are ok, and implicates VR (w hall link above)

From above links it also suggests that voltage at D+ should be > battery voltage (mine is < battery), and that this would indicate bad excitation diodes.

Lastly I disconnected blue wire from bulb socket in gauge, and measured resistance to disconnected blue wires at VR - I got -0.3 ohms.

So, I’m still not clear if it is VR, Alternator, blue wire circuit.
And I’m certainly not sure how I created the problem in the first place?

Hoping someone better at diagnostics can shed some light on this.

Charles

not saying this is your problem but the fan housing is necessary as a ground for the alternator to the engine case. if you paint the housing you must leave the contact between the housing and case free of paint for a good ground connection. you also need a ground wire coming from the engine case directly to the back of the alternator. when you delete the fan housing ground with paint it can kill the alternator, cdi box, and cause other weird issues

I’ll file this under lessons learned!

Right you were - I pulled alternator, sanded housing bottom contact points, and ring where alternator nests, and added a ground from alternator to case.

Result- red light no goes out after starting- yeah!

I admit, I was getting tired of pulling it out !

Thanks

Charles

It’s amazing how many people paint their fan housing and essentially insulate them. It’ll charge with a ground wire to the case but it really needs both

That doesn't make electrical sense. If the alternator ground pole is grounded to the engine with the braided strap or thick brown wire, the ground connection through the fan housing doesn't matter. It could still have been a ground problem, but by removing the fan duct, sanding it, and reinstalling it, you managed to improve the ground strap connection. Or maybe your ground strap connection is poor, and now you are getting ground through the fan duct.

Make sure that the wires from the alternator to the VR are clean and have zero ohms resistance.

However, the fan duct SHOULD have a clean connection to the engine, because the coil is grounded to the fan duct.