alternator not charging after start until 3000 RPM - fixed
 

I have seen this somewhere but can't find it.

Just noticed this today. Not showing a charge at the battery after start unless I rev above 3000 RPM.

Is there a fix for this?

It may be charging just fine, but your gauge may not come alive until it gets a spike of volts from the ramped up RPM.

Use a voltmeter on the jump post right after you light it off without touching the throttle, see what it says.

You may need to do some scrubbing on your grounds, especially in the POD.

If low voltage light not on with key turned to on then needs replacing. It may also be the resistor(?) on the rear of cluster.

I was using a MM at the battery. No low volatage light on the guage. The guage shows 11v when I am reading 13.8v at the battery on the MM.

My volt gauge follows the voltage changes perfectly, but is always one volt low.

Check and clean the 14 pin plug by the jump post.

The jump post should have a cover fitted to keep the water out.

here are a few tests you can do .

Turn on the key so all of the warning lights come on , do you see a red light on the charge gauge?
If not the bulb is blown replace it..
If the light does come on then
start the car and without revving the engine check the voltage at the hot post it should be above 13.5, .
If not the rev the engine and see if the charging system is then giving this reading.
if it does then the is probably a break in the charge wire that goes to the alternator it runs through the 14 pin connector.

remove the blue wire from the back of the alternator and separate the 14 pin connector see if you have continuity from the engine side of the wire to the blue wire on the back of the alternator.
If not then the wire is damaged and it will usually be broken about 4 inches up inside the red sheathing from the alternator this is from hanging the alternator on its wire harness, find the break and rejoin the wire

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 (mounted on the back of the voltage gauge). 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.

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.

If the charging warning light is on, there is voltage on one side of the bulb and not the other, and there is a problem, usually in the alternator.

The alternator coils produce AC voltage, which is not useful for our purposes. The alternator includes six charging diodes (one-way check valves for electricity) that convert the AC voltage to slightly-pulsing DC voltage. If one diode is failed open, the charging amperage will be reduced. If one diode is failed shorted, there will be AC ripple in the output voltage, and you will get whine in the radio. As more diodes fail, the conditions will worsen.

The charging circuit is shown on Pages 97-31 and 97-45 of the Factory Workshop Manual Wiring Diagrams:

The gray area at the top of the page is the Central Electric Panel.
- All lines represent the wires behind the Panel. Connected wires are shown by dots where they cross.
- The Roman numerals show the relay positions. These are identified on the panel, but you will need a flashlight to read them. Go to our web site. Click to enter. Click on the + by 928 Fuse/Relay Charts. Click on 77-79. You will find a diagram of the Relays and the Fuses.
- The relay diagrams show the terminal numbers (which are standardized), and the basic mechanism inside the relay.

The numbers at the bottom of the gray area show the Plug terminal connections. The plugs are NOT shown on our relay charts. The plugs are lettered left to right, starting with A on the left end. There is no I (i) plug. The shorter plugs in the center are out of sequence - they are Y and Z. The terminals on each plug are arranged:

4 8
3 7
2 6
1 5

Therefore, "E6" is the next-to-bottom terminal on the right side of the 5th plug from the left end.

The wiring in the car is shown by solid or dashed lines. The solid lines indicate a solid color - for example all unswitched ground wires are solid brown. All switched ground wires are dashed, and indicate a brown wire with a colored stripe. Other dashed wires are different base colors with a thin stripe of a different color. For example, the wire that supplies power to the turn signal flasher is a dashed line marked "bk/ye", showing a black wire with a yellow stripe.

The gray boxes in the main field are devices in the car, such as the emergency flasher switch. The numbers around the box show the connecting terminals (again, the same standardized numbers).

The small boxes with Roman numbers/Arabic numbers show a circuit jump to a different area of the diagram. For example, there are several such boxes connected to the emergency flasher switch. One such box is marked "IX/1". This means that the brown wire leading to the box continues on Part IX in area 1. The areas are the numbers at the bottom of the page. The box that we are discussing is located at III/20. You will go to Part IX, which is on Page 97-47, look in area 1 and find a brown wire. This show the continuation of the circuit.

The line at the bottom of the page shows chassis ground. The numbers in circles are the major ground points, where you will find several brown wires connected to the body or the engine.


> The power to excite the alternator has an elaborate route.
> 1) Battery to ignition switch. (97-31)
> 2) Ignition switch to bus 15 (general switched power bus). (97-31)
> 3) Bus 15 to terminal H7 on the central power panel. (Connectors are A-Z, left to right on the bottom of the panel). (97-45)
> 4) H7 on a black wire to terminal 1R on the instrument pod. (97-45)
> 5) Terminal 1R 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. (97-45) There is a tech bulletin calling for the installation of a 68 Ohm resistor in parallel with the charge light on the early cars - hopefully, yours has that installed.
> 6) From the light/resistor to terminal 2R in the pod. (97-45)
> 7) Terminal 2R on a blue wire to terminal H8 on the central power panel. There are two blue wires here - power goes to the central warning computer, and to H8. (97-45)
> 8)Terminal H8 to terminal Z6. (97-31)
> 9) Terminal Z6 outside on a black wire to terminal Z1. (97-31)
> 10) Terminal Z1 inside to terminal O8. (97-31)
> 11) Terminal O8 on a blue wire to the alternator. (97-31)

While there are numerous possible failure points in this circuit, some of the more common include:
- Burned out charge light bulb.
- Failed parallel resistor (back of the voltage gauge).
- Bad connection in the 14-pin connector located in the engine compartment on the right fender liner, just under the jump start terminal. Be careful - this connector can fall apart while you are cleaning it!
- Broken wire, as describe by mr merlin.

What Wally said.

Wow,

Thanks Wally. I do have the WSM wiring diagrams and your fuse and relay charts.

I think I had/have several issues going on.

I had pulled the pod yesterday to do an ignition switch swap as I had some long standing issues with switch position and over the last few days some odd warning light issues as well as intermittant start issues. After replacing the ignition switch I quickly plugged in the instruments in and the warning light issues seem better.

Not sure I have the 68 ohm resistor installed. Looking at other write ups the one on my pod seems much smaller and does not indicate 68 ohm on the resistor. The circuit card is in pretty rough shape as well.

Good time to refresh all an eliminate those as possible issues. Thanks for the in-depth response!!

Far left hand grayish item is your resistor.

I have a Bosch alternator that will not start charging until I get to 2000 rpms.

I built a 46 ohm load board out of spare instrument cluster lamps
(turns out 3 in a row is pretty darn close) and replaced the exciter wire
with this setup.

I can watch them glow until I hit 2000 RPM or so -- instrument gauge acknowledges
increase in charging and the lights go out.

My gauge is unreliable at best -- I wasn't getting the lamp coming on,
but I found it easier to test by bypassing the lamp and building my little load rig.

I may move this little setup into the cockpit and bypass the charging lamp completely.

I also replaced my battery ground cable -- and the difference at the volt meter
on the instrument cluster is way more pronounced.
I also gained 1V at the battery terminals -- so my confidence in my charging system
is high now.

Mike

The (non) 'charging lamp' should not be bypassed - it is important to monitor - eliminating it doesn't help in any way electrically.

Sounds like your regulator has an issue - it seems the Alt is still not initiating until it self-excites - you need to get the alt fixed - or investigate the exciter circuit more. If you disconnect it from the alt the charging light should go out (ign on)
if you ground it the charging light should go on...

Alan

Alan,

If you are talking to me.

Alternator is brand new -- in fact it is the second to behave exactly this way.
(the first one was destroyed by an idiot putting the battery in backwards)

I have not electrically removed the circuit -- I have simply created my own and
disconnected the blue wire, thus removing the factory circuit.
At the moment it's located under the hood.

So far I just live with the fact I have to goto 2000rpms to get it to ignite,
then it will stay charging until I shut the car off.

My circuit was a clear cut way to prove the alternator was working properly, or
at least would start charging and stay that way after it was excited.

My own igniter lamp circuit may be moved into the cabin where I can
monitor it all the time, however since I now actually see the voltage move
on my dash gauge (cleaning ground points, replacing battery ground cable, fixing instrument cluster),
I'm not quite so worried about it anymore.

You have to realize I'm an electrical engineer so I was comfortable
mocking up this relatively simple circuit and I was tired of chasing multiple
problems at the same time -- I have been systematically chasing down the
issues. Sometimes that required me to isolate the systems one at a time,
or at least compartmentalize them.

Maybe someone could be helped by "temporarily" doing the same.

mike


Mike

Yes - point is it still isn't initiating - its just self-exciting.... right?

What you added only made it a little bit better - it still doesn't work.

If it isn't the alternator - then its the exciter circuit itself - Test the circuit -something is still wrong... (and since you are an engineer - you know this). If nothing is left connected to the alternator - then there is indeed a problem with the alternator or your new exciter circuit.

The alternator should reliably initiate at 1000 engine rpm or lower with a suitable exciter crcuit

Now your new circuit seems a odd choice - bulbs alone don't make a very good resistor for this application. I assume 1.2w bulbs and in series - so you will have probably about 40 ohms cold - but more like 360 ohms hot (though you will never get there with 3 in series) in any case the filament resistance is hugely non-linear with voiltage - not really a great thing here.

Alan

As I understand it -- the igniter circuit supplies a 46-48 ohms load to the igniter pin
on the alternator and battery voltage (positive). Found the circuit diagram on here on
the forums. Verified by measuring the ohms of the instrument cluster.
Duplicated the ~48ohm using 3 16 ohm lamps in series.

Internal to the alternator the other end is electrically grounded.
This creates about a 250mA current flow -- and the lamp (or in my case lamps) light.

When the alternator develops a sufficient field the electrical ground is now connected
to regulated voltage and since the other end of the igniter circuit is connected to the battery,
no voltage is dropped across the lamp(s) and they do not illuminate.

Aside from the 46 ohm path -- everything else is internal to the alternator.

So either the replacement alternator doesn't self ignite at 1000 RPM, the pulley configuration
is such that 1000RPM engine is not 1000 RPM alternator, or my alternators just
don't self ignite at 1000R RPM.

Now once it has ignited -- I'm good at whatever RPM.

YMMV, but I can live with this configuration.

Mike