alternator issues: update
 

well, while riding this afternoon and feeling somewhat baffled and dispondent about my alt issues, i decided to mash out a little,figuring i would get home just a little quicker! when i got about 85/ 90 mph and in the upper bands of the rpm around 35 to 4k, i just happened to check all guages and NOTICED my voltage rose from say 11(the orange above 10 ) to about 12+ and when i decelerated it went back to 10/ 11, is this a sign of something trying to charge or was i just seeing things in this scorching heat!:confused:

aha moment, while researching alternators i noticed in all pictures there is a pos, neg, white or cream colored voltage regulator, and some other black box that plugs into alt and has a screw in it to hold it onto alt, whatever that black box is,,,,,,,,,,,, MINE IS MISSING!!!! SO please someone tell me what it is and where can i get one!

Check your voltage at the jump post to see real numbers...

928 voltage gages read low sometimes...usually due to old wires, bad contacts, or bad ground. Also, the circuit board on the back of the instrument cluster may need its contacts cleaned as well.

I believe there are two different types of regulators and they look different. If you are getting higher voltage while driving and lower at idle (on your dash gage) it's probably OK.

thank you , but what do you know about the liitle box that plugs into the back of the alts, any info on what it is and where can i order one

928 alt's have a black cover that attaches to the back; this cover has a tube that goes forward connecting to a hose for cooling airflow.

Post a pic of what you are looking at or look up the part and give a part number.

both of these threads should be combined
http://forums.pelicanparts.com/porsche-928-technical-forum/548866-alternator-issues.html

As JhwShark says old wires, bad contacts, or bad ground, will give you voltage drops and a low Charge and or reading at the gauge. A old alternator or a bad battery will do the same.

Caution don't go spending money or taking anything apart until it has been properly diagnosed and you are sure what is the cause.

A simple loose connection at the alternator is cheep to fix and it sucks to find it after you already replaced a battery and then bought a new alt. with out checking it out.

DO NOT!! remove the battery cable while the engine is running or the key is on for that matter. Many people have and then don't put 2 and 2 together that the failed ECM and later a ALT is due to their poor diagnostic practices
One way to check the integrity of the alternator and diodes on Bosch alternators is to check the voltage readings at the D+ (blue wire) terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the alternator.

you might want to read this and then if you need help, someone here will walk you through it.
Charging System Checks (Alternator Testing)

Perhaps disconnecting the battery while the engine is running is a convenient way to finally flush out the weak components. Force them to die once and for all. *****ing junk.

No?

You have a couple options on the 84.

A rebuilt-by-Bosch Factory replacement can be purchased to replace your Bosch or Rhone alternator for $150. These are supposed to be complete rebuilds, not typical partials. You can order them from the better 928 suppliers or even from Advance Auto for overnight delivery.

You can covert to a the 105A Delco rebuilt alternator for an 88 Camaro. It just happens to fit with some very minor mods. I'm "currently" running two of them. They fit the 16V cars with minor shimming using 3 flat washers.

You can take yours to a shop for a rebuild.

Each of my 3 bad ones has acted as yours does, BTW. The one on my 86 car would respond to a light tap from a hammer and begin to fully charge, maybe for a week or so, before needing another tap. When operating poorly, it would sometimes give more voltage and move the gauge at higher RPM, just like yours.


There is an outside chance from your symptoms that the exciter wire is making intermittent contact. It runs from the alternator through the 14 pin connector on the fender well, then into the CE panel, than to the dash where it is in line with the alt bulb light. I've had one of these wires break at its solder joint on the back of the 14 pin connector and make intermittent contact. Symptom was stucatto movement of gauge.

The workshop manuals and auxillary documentation from Porsche, available in a collection by a rennlister on CD, have some pretty good circuit diagrams and troubleshooting sections, too, for this and other things.

Disconnecting the battery of a running car is a shadetree technique that needs to be avoided. On the 928, and other modern cars, the brain power lead is integrated into the hot lead. Disconnect the battery, what do you think will happen?

first sentences in top posting- an alternator can on some not cut in until 3000rpms yours just might not be working until then, as your seeing yourself the gauge that its voltages are rising. obv thats takes alot of speeding to get it to cut in a low cut in 1800 would keep you and all of us alive longer lol.

esp if buying used alt you dont know what anyone has put in it for parts/kits you definately want a low cut in so its charging at ignition start.

WAIT! thanks mrmerlin (combine threads) - in your other post you did put vr in so you may have bought put in a high cut in

When they start to fail they behave can behave like this in my experience.

headed to 928 specialties for alt!!!!!