jonny_five

i have a 1981 928 s and the battery does not charge on its own. i took it to a mechanic because i had to transport the car when i moved. all he did was put a switch in by the fuse panel that excites the alternator while you have the car running. you flip it on and then off and it maintians charge and the voltmeter shows the correct charge. he thought that the wire was disconnected under the dash somewhere. is there any specific place it commonly disconnects? also, is there a certain route the wire takes? should i just take the instrument panel out and try to follow the blue and black wires?

Garth S

You should find ( unless memory fails ) that the excitation current flows from the alternator charge indicator bulb ( red light at the bottom of the voltmeter in the pod) to the 14 or so pin connector located at the right side fender/hot jump start terminal .... on through the front engine harness to the small M5 terminal on the alternator.

Does, or did the pod light come on when the key was turned to the run position?? - and turn off once the engine was starter and the alt. began putting out ~14v??

Your problem may be as simple as that bulb being burned out ..... but a more likely issue to check first is for corrosion in the multi pin harness connector at the hot terminal.

joejoe

Garth is correct, check the bulb function first. There is also a resistor(?) on the rear of the cluster. With the cluster out it is easy to see (only one on back) there might be corosion there also. If light is not working you might be able to replace without removing cluster but you need prety small hands and use 'the force' as you probaly will not be able to see.

Stuie

Awesome... I am working the same problem on my '82 Euro.

I have 3 wires coming up from the area of the alternator. Two 10 ga red ones and about a 14 ga green one.

One red connects to the hot jump start terminal.

The weird thing is the other heavy red one is tapped into the smaller gage green one which terminates into the harness connection. I cleanly re-spliced the wires (the green one was corroded between the splice and the harness plug) and today, I the voltmeter worked the first try. I got excited I had fixed it and started zip tying the wires back in route. Tried again and no meter again. I am gonna pull the plug tomorrow and trace the wires to see which red is tapped into the green just so I know for sure. The fresh red on used to be green and still is about 6 inches from the harness connection.

Mrmerlin

Stu, it looks like you could use a 88S4 hot post cover this will help keep things dry, you should also open the back of the 14 pin connector and see if any wires have failing insulation a few pieces of heat shrink and some cleaning of the pins with a pink eraser should do the trick. DISCONNECT the BATTERY first

jonny_five

finally! i found the problem. someone (either past mechanic or previous owner) took the blue wire off the plug that attaches to the instrument cluster and replaced it with a brown one (by soldering)... turns out the brown one went nowhere and the blue one was just left hanging! hooray this is the first time i get to drive since my main car was destroyed in the tennessee tornados...

Stuie

Rockin' Johnny!

Congrats! My ride is still hosed... Working on it. I have a blue wire which has nowhere to go which went into the plug pictured above I guess. I dunno cause it's just cut. Ahh well.

I still have about 20 minutes of drive time after each battery charge from the A/C charger!

WallyP

An alternator converts rotary energy into electrical energy by moving a magnetic field (the armature) thru the coils in the housing. The magnetic field MUST be present for the alternator to generate electricity. It is initially created by the excitation current that is supplied thru the charging warning light and the parallel resistor. If the excitation current is too weak (bad resistor, bad bulb, bad connection), but present, the alternator will start charging ONLY at higher RPM levels. If the excitation current is not present at all, the alternator will never charge. The charge light must come on with the ignition switch on and the engine not running.


Once the alternator starts charging, it supplies its own field current. Since there is now voltage on both sides of the light/resistor combo, the light goes off. Once the alternator starts charging, the light/resistor combo has no effect. The light must go off with the engine running.


The power to excite the alternator has an elaborate route - this is specifically for an '82, but is similar on most early cars.
1) Battery to ignition switch.
2) Ignition switch to bus 15 (general switched power bus).
3) Bus 15 to terminal H7 on the central power panel. (Connectors are A-Z, left to right on the bottom of the panel).
4) H7 on a black wire to terminal 3R on the instrument pod.
5) Terminal 3R to the Generator light, and to a resistor mounted in parallel to the light. If the light is burned out, the alternator won't receive enough power to generate. Bad connection or bulb in the pod is a likely problem if the light never comes on.
6) From the light/resistor to terminal 11R in the pod.
7) Terminal 11R on a blue wire to terminal H8 on the central power panel. There are either two or three blue wires here - power goes to the central warning computer, and (on '82 models) to the mileage counter (for the oxygen sensor warning light) under the cover on the passenger door sill.
8)Terminal H8 to terminal Z6.
9) Terminal Z6 outside on a black wire to terminal Z1.
10) Terminal Z10 inside to terminal O8.
11) Terminal O8 on a blue wire to the alternator.

Later models have a similar current path, but the plug identities are different.

If the charging warning light is on when the engine is running, there is voltage on one side of the bulb and not the other, and there is a problem, usually in the alternator. If your charging light was coming on and staying on, but is now going off after tightening the belt, you may have already fixed the problem. If the light is still staying on, you may have a problem in the alternator.

cdbtx

Great write up Wally - where have you been hiding?

I discovered the same thing with my old Emergency Generator at home - it wasn't generating any power. I plugged an electric drill into the generator with it running, put the drill in reverse, pull the trigger and turned the "Chuck" by hand and *Poof* it starts generating power - cool trick, same theory...

Alan

So a practical outcome of Wally's description is that if your pod charge light doesn't come on - you can ground terminal H8 (for an '81 - varies) on the central electric panel with the ignition on and the car not running (don't do this with the car running) to test the charge bulb. If it illuminates the bulb is good - so instead there may be a connection issue to the alternator, or a fault in the alternator.

In that case test for connectivity through the rest of the loop to the alternator small terminal. The Z1 to Z6 loop seems to have been installed as a convenient point to break or monitor this circuit for diagnositcs (and is possibly where your switch connects in).

If the light does not illuminate digging into the pod is probably needed.

You can also monitor the exciter terminal voltage on any of these latter connection points - H8, Z1/Z6, O8

Alan