Blue wire on alternator dead

bmwbeemer7 · bmwbeemer7's Garage

I have a 1979 928 and I just put on a new alternator but still not charging battery. My battery reads 12v but at the alternator the blue wire reads next to nothing. I checked it by turning the key on(not starting) and taking the blue wire loose and when I read it with the meter it only read .2. The battery is not charging at all. Can anyone help me please????

Pete R · Pete R's Garage

Is the little red light on at the bottom at the volt gauge? That circuit goes from the ignition, through that light (in parallel with a resistor) through the alarm to the alt. If the light is burnt out or you have removed you alarm the circuit won't be completed.

bmwbeemer7 · bmwbeemer7's Garage

The little red light is on. When I rev up the engine, the voltage read on the battery goes up to about 13.5 volts. But as soon as I let off the gas pedal it goes back down to almost 10 on the dash. I didn't know it had an alarm on it. Could this be the cause or can it be something else. Thanks

Pete R · Pete R's Garage

If the red light is on, the blue wire should be getting power to the alt, as that's it's ground. You say it goes to 10 "on the dash" but across the battery is it still charging? Is there a heavy drain (ie engine fan)or something else running. Is the belt tension good on the Alt? Alarm shouldn't be an issue as long as it hasn't been mess with. May be an alt. voltage regulator issue, but I'm merely guessing at this point. read "Euro 85 alternator not charging, signal wire popping fuses" on page 3 or so. The blue wire was talked about pretty extensively.

bmwbeemer7 · bmwbeemer7's Garage

I think it may be the battery. I checked the blue wire at the hotpost and it reads 1.4v and even at the blue wire on the back of the alternator, it reads the same. So I'll try and change the battery and see what happens.

JhwShark · JhwShark's Garage

'79 might not have an alarm. Your dash gauge is not reliable for testing mine behaves like yours. As suggested check belt tension and then voltage and continuity. You should be able to isolate the failure point through elimination. Odds are it is before the CEP.

You need to use your volt meter and get readings at a few points:
Key in OFF...at Bat, at Hot/Jump post, at ALT 30 (big Red)
Key in RUN (not started)...at Bat, at Hot post, at ALT 30 (big Red)
Key in RUN (started at idle)...at Bat, at post, at ALT 30 (big Red); rev over 1600 and check voltage again

All points should have same voltage within a 0.1-0.2 volts, Car Off 12.4v if under 12 could be a bad BAT cell; running at least 13.4v or near these numbers.

Also, you may have degraded insulation on the Blue lead as it runs from the 14 Pin connector at Pin 1 down to the ALT. The Large Red lead from the Jump post runs down the same harness to the ALT. Check continuity of both these wires from the 14 Pin and from the Jump post respectively.

Jon

Mrmerlin · 01-05-2011, 09:30 AM

if the blue wire is broken then the alt will self charge after about 2500 RPM,

To fix the blue wire remove it from the alt then connect an OHM meter to the end ,
then go to the hot post, at the 14 pin connector,
find pin #1 this should be a blue wire, see if there is continuity.
Usually the blue wire gets sheared about 5 inches from the connection point at the alternator,
( due to hanging the alternator by the wires) fix the wire if its damaged.
Also if the wire is good look for shedding insulation at the 14 pin connector above the hot post open the back of the connector.
Then with the meter set to Volts connect one of the cable to ground , and then touch the car side of the # 1 connector socket and turn on the key to the run position see if your getting 12V, you should have 12V

MPDano · MPDano's Garage

Great thread!

bmwbeemer7 · bmwbeemer7's Garage

Ok. I went ahead and checked the voltage at pin1 on the connector and it only reads 1.6v while connected to the alternator. Is that normal?

Mrmerlin · 01-05-2011, 10:02 AM

please follow the directions, this will take you back from the alternator.
When testing at the 14 pin connector it should be separated to test for continuity then for voltage

bmwbeemer7 · bmwbeemer7's Garage

I checked the blue wire with it off the alternator and it reads 11v just as it does on the top but as soon as I connect it back to the alternator, it the voltage drops down to 1.6v. Weird....Now I'm lost.

MPDano · MPDano's Garage

Not trying to be smart or anything, but are you sure it's being connected to the proper post? Aren't there a couple on the back of the Alternator? A Picture maybe?

MPDano · MPDano's Garage

Here is a pic of mine before I put it on:

JhwShark · JhwShark's Garage

Nice pic Leo.

D+ is for the Blue wire to PIN 1 and B+ is to BAT & Jump Post large gauge Red wires.

bmwbeemer7 · bmwbeemer7's Garage

Ok. It is on the correct post. I am 100% sure about that. Would a faulty battery make the voltage read only 1.6v when the blue wire is connected? I notice that the battery is not holding a charge. My alternator looks exactly like the one in the pic above and I have the blue wire connected to the smaller post right next to the big battery post. When I crank the car, and rev the engine, the voltage at the battery goes from 12v to 13.5 or more. But as soon as I let off the gas the voltage goes down to around 10v as if it wants to shut off. I'm thinking my battery may be shot.

neil30076 · neil30076's Garage

Quote:

Originally Posted by bmwbeemer7

Ok. It is on the correct post. I am 100% sure about that. Would a faulty battery make the voltage read only 1.6v when the blue wire is connected? I notice that the battery is not holding a charge. My alternator looks exactly like the one in the pic above and I have the blue wire connected to the smaller post right next to the big battery post. When I crank the car, and rev the engine, the voltage at the battery goes from 12v to 13.5 or more. But as soon as I let off the gas the voltage goes down to around 10v as if it wants to shut off. I'm thinking my battery may be shot.

Your battery may have a shorted cell - I would pull it and take to NAPA and have them test it. They also have a replacement that is around $115, last time i bought one.

bmwbeemer7 · bmwbeemer7's Garage

Thanks!! I'll take the battery out and have it checked. I'll repost as soon as I find out more. Thanks everyone!! This is my first 928. I've owned a 924 and 944. I'm really a BMW man. I've owned 11 in the past 10 years.

MPDano · MPDano's Garage

Yeah, I would bet on the Battery now. I bought mine from Autozone for about the same price. It's a friggin huge one too. Just fits inside the battery box. Also, maybe the battery is the wrong one/type. PO's do lot's of dumbass things to our cars. Make sure to have them look up the correct battery rather than try to match what you bring in.

Eric Drewes · 01-05-2011, 11:51 AM

I too had a similar issue a couple of months back where the voltage reading on the gauge would drop to 10 when idling and back up to 12 when I was in motion. It turned out to be a loose alternator belt, but I also had another indicator of this being the problem as my belt was squealing between 1500 & 2500 rpm. This happened in very cold weather and a belt adjustment rectified the situation. Hope your's is as easy a fix as that.

Pete R · Pete R's Garage

Okay, my last theory. Maybe someone can support or contradict it.
Since you light on the dash seems to work properly, I think it's okay to assume you have continuity and voltage and it's attached correctly.
My 79, the voltage goes low at idle if something fairly high draw is on like the radiator fan. The fact that your battery won't keep a charge, leads me to say , something may be stuck on. Maybe the defrosters or something. Could a relay be stuck closed and keep a circuit open? maybe check.

On a battery recommendation.. wal mart, they are made by Johnson controls, they make the #1 battery in Europe and south America.. they also make Optima batteries.. and they are cheap.