Checking alternator wiring 2004 PD
I am confused by your post - I cannot figure out for sure which wires you are referring to.
Let's use some concrete terminology. Do you have a Bentley's manual? If not put it on your shopping/gift list. Connectors are named such as T32 for a 32 pin connector, and T32/3 means pin 3 of the 32 pin connector.
Clean the major grounds under the battery and in the ECM plenum. They must be clean.
Verify that T32/9 and T32/24 on the connector for the instrument cluster control module both have zero resistance to ground.
Verify the battery light comes on when you turn the key to on. This lamp must come on for the voltage regulator to work properly.
Verify fuses 11, 15, 22, 23, are not blown. Take them out and polish the conductors on the end. Then put them in and out several times to clean the clamping contacts.
There should be a blue wire coming in to the L terminal on your alternator. This wire comes from the indicating light on your dash, and supplies the field. This comes from pin 12 of T32 - the 32 pin terminal on your instrument cluster control module, via pin 2 of the T4 connector near the starter.
There should be a brown and red wire going to the DFM terminal on your alternator (Was this the "return" wire you were speaking of?), this wire feeds to and from the ECM via pin 1 of the T4 connector near the starter. The function of this wire is not well documented, but I think it allows the ECM to control the alternator. It sounds as if it is possible that the ECM or built in overvoltage system is turning off your Alternator due to an overvoltage condition. Exactly how high is the "over 14 volts" you are getting immediately after start up?
Check 1) verify that you have Battery voltage on the large connector on the back of the alternator, marked B+.
Check 2) Turn on your key and verify that terminal D+ on your key's electrical connector is at battery voltage.
Check 3) Verify fuse 11 has battery voltage on both sides of it, with the key on.
Check 4) start your car with the voltmeter hooked up on the B+ terminal of the alternator and the positive terminal of the battery. Try to catch this reading during the initial burst of alternator output. This is to verify that you don't have a high resistance contact somewhere in this path that is causing an overvoltage condition on the alternator.The closer this reading is to zero the better. If this is much over zero, clean all the contacts between B+ on the alternator and the battery positive terminal. This would include fuse S177(fuse 5 in the bracket above the battery), the common conductor to one end all five of these fuses, the large wire to the positive battery terminal, and the positive battery terminal itself. It sounds as if you have already tried to clean these, but this will check them. If you need to, you can hook up a jumper cable from the B+ connection on the alternator directly to the battery Plus terminal. Take great care not to short this to ground, or you will start your own arc welding class. If this jumper cable allows the system to work, it is a sign of a high resistance contact.
If all of these checks are good, then check for good conductors between T32/12 on the instrument cluster control module, and the L pin on the alternator. Make sure you do not have dirty contacts, check it for resistance to ground (should be infinite resistance when unplugged at both ends), check it for zero ohms resistance from end to end. You can insert a pin or needle into the wires near the ends (to give you another place to measure from rather than the end connections) to help you determine if there is a bad connector on one end or the other.
Perform the same checks for the brown and red wire from the DFM pin on the alternator to T94/pin64 on the ECM. This passes through T4/pin1 near the starter, and T10w(white) pin 5 in the ECM plenum connector protective housing on the left side. Check that it is not grounded and that it has low resistance from end to end.
This is quite a chunk of work. Hopefully it will surface the problem.
Dan