BMW 2002 FAQ - Question ? for Electrical guys only

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Thread Topic: Question ? for Electrical guys only

Date: 4-30-08 10:26
From: Tiger75 in Shreveport, LA

Subject: Question ? for Electrical guys only

I have an electrical question.

I was doing a test drive and getting my car in shape for a trip this weekend of about 800 to 900 miles to Mid America Fest in Arkansas. I am not really having any trouble except that my L (red ) came on on my test run. I only saw it cause I started the car in the dark with no lights on. When it stays on, it is very, very faint. Only 10 % to 15 % of normal brightness. It is on at idle but stays on when I rev it up.
If I turn off all acces and electrical sometimes I can no longer see it at all.

I ran most of the tests that are listed on FAQ - results below.

My electrical system is not stock. I have added Red Top Battery in the front Tray of my 1974 TII. I have also added additional grounding wire from Alt to block and body. Last summer I replaced my Tii alternator with one from a 318 that has an internal voltage regulator and disconnected the stock voltage regulator.

Engine off - static battery voltage 12.77
Engine at idle -900 rpms- all acces off - Battery Voltage 13.21
Engine at Idle - AC on - Front pusher fan on - battery voltage 12.60
Engine at idle - head lights on -Battery Voltage 12.73
Engine at idle - Alternator output (at B+) 14.03

It got to midnight so I quit for the night.

To me it seem that the Alt is putting out and the Regulator is regulator is regulating, but I have something bleeding a small amount of residual current through the ignition to the Service light.
Could this be because of the internal voltage regulators?
Any tips or tests would be greatly appreciated.
Tim Abendroth
Tiger75
Timbucktiger@comcast.net

Date: 4-30-08 10:49
From: Mars in Pennsylvania

Subject: Re: Question ? for Electrical guys only

Hmmm...

Trivia: The "L" is short for "Ladung", which is German for electrical charge :)

Your alternator is clearly not happy. We'll start with the easy stuff. Check for any loose connections or frayed cables going into the alternator. Bad wires and connectors in the plug that goes into the back of the alternator could cause this.

Check for a loose alternator belt although you mentioned the light stays on even when you REV the engine...

Check for loose grounds.

Check for a bad battery too although your numbers are within tolerance so we'll assume that is ok.

If you jumper/switch the wires on the voltage regulator does the light not come on at all? Manuals tell us usually if the light goes out immediately the voltage regulator is faulty (or vice versa). There's a full on test procedure but I don't know it completely from memory. Would have to look it up tonight.

When the motor is running the voltage should be greater than when the motor is OFF. 12.5 volts motor off should be 12.7v > when running . Glowing light on dash with lower running voltage is bad diode in alternator. If the voltage is lower it is time to replace the alternator.

If the battery, wiring, ground, belts, etc. all check out your suspicion of the regulator is logical. If you can bypass that somehow temporarily hook up the old external regulator and see if the problem goes away.

Keep us posted.

Date: 4-30-08 11:53
From: TobyB in Seattle

Subject: Re: Question ? for Electrical guys only

Quote:

Engine at idle - head lights on -Battery Voltage 12.73
Engine at idle - Alternator output (at B+) 14.03

You've got a pretty impressive (1.25 v) drop across the charge wire.
Take a good look at that red wire- it's supposed to go from that B+ bolt
to the bat+ terminal, and shouldn't have much drop across it. Less
than half a volt, even at real charge levels.

The other thing to check is that alternator ground wire- that will let the
alternator pull it's case above ground, causing exactly the symptoms
you're having.

Often, it's worth reworking those wires as the ends are 30-40 years old...

fwiw,

t
_________________
I have a car disease. There is no cure.
I'm not even managing the symptoms very well...

Date: 4-30-08 08:19
From: Tiger75 in Shreveport, LA

Subject: It was after midnight

and i could see under there too good. Remember the Tii alt is mounted low.

The B+ voltage at idle was 13.56

The 14.03 volts was at the D+ terminal.
So my wire from alt to battaery is good.
I had replaced it with I added the ew alt.

I found that alt has to excite =on initial start up. L light burnd bright for a few seconds until alt gets going. Once it is going red light goes out - everything A ok. Then if i add a big load like turn on blower and head light , I get that slight, very slight glow in the L red light.

I think it might be a diode starting to go bad, but alt was checked by a reputiable shop before i installed it.
Anyboby got any other ideas?

Date: 4-30-08 08:48
From: reallyoldgeezer
Subject: Re: Question ? for Electrical guys only

Looks like your alt is overdrawn under load. Didn't mention if you had extra heavy lights / or fan / or other axxes but that could be it. You said the alt was from 318i....could have been tired unit then.....

or/ could be a bad diode in internal rectifier. Look at alternator output and measure AC (some call it 'ripple.') If there is a lot of AC "on the DC,"
a bad diode in the rectifier is indicated. Compare your readings with a known 'good' vehicle. If you have a 'scope, you can view it clearly....
Remember, at idle, your DC is really choppy anyway, to the untrained eye could masquarade as AC.....so, if it looks choppy, rev it up and see if it smoothes out. At the higher revs, it is simpler to tell the DC component from genunie unrectified AC (bad ripple.)

Good Luck.

Date: 4-30-08 08:52
From: Harv in Willoughby Oh.

Subject: Re: It was after midnight

Tiger75 wrote:

I think it might be a diode starting to go bad, but alt was checked by a reputiable shop before i installed it.
Anyboby got any other ideas?

Bingo!

A decent shop should be able to replace the bad diode for you.

John
_________________
"Chicken make no good house pet!"

"Less brake, more accelerator"
#1665778

Date: 5-1-08 08:22
From: Tiger75 in Shreveport, LA

Subject: Faint L light - Red

I went to my local expert and have emailed Chris B who wrote the elec test quide on FAQ.

Problem is triodes - probably or one or more of the diodes in the rectifier.
I went to my local expert this am.

He said if light was not out when running then alt was not keeping up. He reccomended to replace unit before my trip not during my trip.

The whole rectifier will probably have to be replaced since he said it was almost impossibe to replace the triode(3 diodes) in this model because it is so tight.
He bench tested my spare alt and it is great so I am going to swap them tonight.
It ain't easy on a TII cause the alt is so low and you have to take the front tension bar off the get the alt out.

But I got till 7 in the morning.
I guess that what I get for using Arkansas parts!!!!

Date: 5-1-08 09:19
From: PatAllen in Quebec

Subject: Re: Faint L light - Red

as an automotive electronic designer, what i would do is to measure TRUE current draw from all theses accessories.

If the current draw is less than the max amp out (45 amp originaly ?) then if there is voltage drop i would assume bad/improper wiring or defective alternator (diode, regulator,...).

I recently refurbished all the wiring for my automatic project. I was realy surprised to see 10awg wiring from the alternator to the big splicing (that is almost unvisible) under the brake fluid reservoir.
It splices there and goes to the batt and into the car, still 10awg...
thats is realy marginal even for 45 amp.

The voltage drop yo measure from the alternator demonstrate the weakeness of the wiring.

As a fast test, you could splice up another 10awg from the big post on the alternator directly to the positive post at the battery. You will bypass then probably 3' of weak oem wiring.
Thats what i did on my turbo 02, and then spliced up all the powers FROM the battery, not from the splice, especialy my fuel pump and coil pack/efi circuit.

hth
_________________
1976 turbocharged 2002, 14psi, T3, 70%LSD, urethane bush, 36x15 intercooler, 63mm tby,SDS EM4F, 5sp overdrive. Bilsts. Tech edge wb02, datalogger, recaros.
1974 2002 auto (under resto)
1992 325i
1991 318is
1985 XR4TI, 22psi
1986 Toyota pick-up

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