Bmw635

Car was running fine until I noticed v-meter showing around 10v. I assume bad ground like BMW so I continue to drive until it ran the battery down. Jump it then it died, no charging when engine is running.

Got it home, charged full battery then start it. I don't see the alt light under the 10v gauge with ignition on so I thought revving above 3k rpm would excite the alt to start charging but it's not.

If blue alt wire is broken, alt doesn't charge but Volt light should still lit? If v-light bulb is burnt, alt should still charging once above 3k rpm is excited?

I don't have time to go through these diagnosis yet. Tia.http://www.stuttgartperformanceengin...SHPROBLEM.html

GlenL

Ignore the dash gauge numbers and use a voltmeter on the jump post for voltage measurements.

Remove the alt and have it tested at a parts store. If it's good then you can worry about the pod.

Bmw635

I confirmed the voltage dropping while engine was running with v-meter at the jump terminal by the 14 pin. It went from 12.4v to 11.8v when I shut if off. Alternator were just rebuilt 100 miles ago when I did the WP & TB.

I'll test alternator tomorrow when it's on the ramp so I can get under to measure with v-meter after revving above 3k rpm. Would this method work or I need to have it bench tested? Thx.

Alan

If there is a break in the blue wire circuit the charge light will never illuminate and it will never excite the alternator. Some (most) alternators with residual magentism in the rotor core will self excite at higher RPM - this may not work on a brand new alternator though (or brand new rotor core).

Alan

Bmw635

Thanks for the tip Alan. Hopefully I can find this break on the blue wire from pin 1 of 14 pin to alt tomorrow so I can get the car smogged. It died on the dyno today during testing.

Another saga of reviving my S4. Just got the MAF to fix the high idle, rough running, lean mixture so I can get it pass smog only to fail onthe dyno.

Bmw635

Update, checked the blue wire and it was good so I pull the recently rebuilt alternator to test since it was easier than tearing into the pod. The voltage regular got burnt so shop is rebuilding it. There was sign of electrical surge that left burnt mark where the brush touch the armature. Hopefully everything will be good to go once I install the alt back tomorrow .

Thanks for all the help to save me from going on goose chase.

Peter

Bmw635

I noticed that if the car sits longer than couple hr, the voltmeter doesn't come alive for couple seconds after starting. Is it supposed to show voltage when key is in 2nd position or until after engine started ?

Makes me nervous when I don't see the v-meter move right away after engine started. TIA.

Tazzieman

All my old Porsches do that , even the 928 which has a (new alternator) but harness crying out for replacement. They are like old nags , a quick whip of revs and they start off OK.

Alan

There is no intended delay - as soon as the ignition is activated it should start to read battery voltage...

Alan

Bmw635

Sounds like I need to look into wirings, contacts. Thx.