Hi all,

I need some help and advice. A couple of weeks ago my ’89 240DL died just as I pulled into the parking lot at work. The battery would not hold a charge – I tried jumping the battery – no luck. I purchased and installed a new battery and was able to drive home. However, as I drove home (16 miles) I noted that the Alternator Light (Battery Icon) on the dash was very dimly lit. I’ve not used the car since then and have tried checking for what the source might be. From prior experience, I know that the instrument cluster needs to be installed and connected in order for the alternator to charge the battery. (Earlier this year I had the dash apart to replace the heater blower motor.) Since the alternator was working fine before it’s not my 1st suspect. I'm guessing that the instrument cluster is not giving a signal to the alternator to charge the battery. I am suspicious that there may be an electrical connection or ?? that has come loose.

In attempting to troubleshoot the problem, I’ve noted the following:

When I turn the key from O (Off) to I (Intermediate or Accessory):

No warning lights – just as it should be

When I turn the key further to II (Drive or Run Position):

What should happen: All warning lights should go on.

What happens: Only Service Light (momentarily), Check Engine Light, Oil Pressure Light go on.

When I turn the key further to III (Start):

What should happen: All warning lights should go out after engine has started.

What happens: The engine starts and all lights go out except a very dim Alternator Light (Battery Icon).

Some ideas that I have to follow up on..............

Wiring from voltage regulator on alternator to "Alternator Warning Light" in instrument cluster?
Wiring/connection on instrument cluster?
Voltage stabilizer on instrument cluster?
Voltage regulator on alternator?

Thanks in advance.

Doug in Lakeview
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1989 240 DL sedan (USA model)
silver exterior & black cloth interior

Thanks for the info. I will be spending some time today in the garage checking out your suggestions. I will update you on the results - good I hope.

Doug in Lakeview
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1989 240 DL sedan (USA model)
silver exterior & black cloth interior

Glen has the right idea, except that the resistor is 68 ohms and there to provide a 200 mA path to the alternator case for lamp test. The trbl is not in your dash or fuse panel, it is in or around your alternator. Be sure the blue ground wire is intact between alternator frame and motor mount. Temporarily ground the

small

Check your alt first for good output and get the battery checked out. Those are the most likely culprits. Make sure you belts are good and snug also. If those check out there are couple more of things to check. And the cluster is as you already said is most likely the cause.

The alt needs an excitation voltage to start charging and that comes from the instrument cluster though the alt light to the back of the alt on the small red wire. If that voltage is not there it will start, but not charge the battery. Make sure the small red wire is properly connected to the back of the alt. It is very easy to knock the wire around getting the oil filter on and off. Start the engine with the small red wire loose from the back of the alt (do not let it touch ground). Check for 12 Volts from the wire to chassis with the engine running. Yes points to the alt or battery. No voltage and you going back inside the car.

The instrument cluster sends 12 volts to the Brake Sensor (octopus), oil pressure sending unit, the alt and so on when the ignition is engaged. No lights in position II is a red flag for no voltage going to the sensors, so you are on the right track. Bulbs works and we have voltage is the reason for that setting. And yes you will see a dim alt light if there is a problem in this area. Start by checking fuses for good connections and nothing blown. You will have to pull the instrument cluster for the next part. Check the connections on the back of the instrument cluster. Then check the link (little flimsy jumper between two screws just on top of the speedometer) on the back of the cluster to make sure it is not blown or not making solid contact. That will cause no lights and the alt not to charge because that sends the 12 volts to the rest of the cluster. You can add a small wire between the screws to see if that solves the problem. If anyone knows the current rating of that silly jumper it would be nice to know! Plan to add a small fuse someday and get rid of that silly thing. The voltage limiter is pretty bullet proof. But that will also cause your problem it is open.

Good Luck with your rabbit hunting and Happy New Year,

PT

Good advice re: checking the alternator excitation wire function, but the idiot lights work a little differently.

When the key is in position II but the engine is not running, 12 volts is of course supplied to the instrument cluster (as it is to all other parts of the car), but it's not really distributed from here. The idiot lights that illumnate at this point - bulb failure, brake failure, parking brake, charge - are due to the 12 volts being supplied to one end of the these bulbs (due to the ignition switch) and the ground path provided to the other end of these bulbs via the wire between the charge idiot light and the alternator. When the engine starts, this wire provides "excitation" current to kick-start the charging cycle. If the alternator is working properly, it immediately puts 12 volts on this line. With 12 volts now on both sides of these four idiot lights, they extinguish.

The bulb failure, brake failure, and parking brake idiot lights are diode isolated from the alternator idiot light wire, so that when the alternator is charging, they can still be illuminated via other means (ie. their intended function - true bulb failure, brake failure, or parking brake engaged). The charge idiot light on the other hand continues to perform the same duty of sensing a difference in voltage between the battery (which is directly supplying voltage to one side of most idiot lights) and the alternator output (which is driving the other side of the charge idiot light). Slight differences in these voltages (ie. when the alternator is not charging and is therefore at a slightly lower voltage than the battery) will cause a dim glow of the charge idiot light.

I would have thought that even with no brushes contacting the slip rings in the alternator, there should still be a path to ground inside the alternator for the four idiot lights when the engine isn't running. The fact that Doug is seeing the charge light dimly lit during driving appears to indicate that the wire between the alternator and the charge idiot light is in place, so maybe no idiot lights with the engine not running is the expected indication when brushes have failed. A good check would be to disconnect this wire from the alternator and ground it with the ignition on, to verify the cluster function. If all four idiot lights don't illuminate, then maybe the flex circuit you mentioned has a problem.


The oil pressure idiot light by the way is completely independant of the above functions. I simply gets 12 volts from the cluster on one side when the key is turned on, and the pressure sensor provides the ground path to illuminate it. Once the oil pump provides pressure to the sensor, the sensor switch turns off and the idiot light goes out.

--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto

Reasonable logic, this:

I would have thought that even with no brushes contacting the slip rings in the alternator, there should still be a path to ground inside the alternator for the four idiot lights when the engine isn't running.

The path to ground that the voltage in the thin red wire takes is in one brush, around the rotor windings, out the other brush to ground via the alternator frame.

Your point re: dim dash light indicates brushes making contact, is good. Brushes near the end of their life, worn below 3mm, might carry enough current to power the light but not enough to do their main job.

Since brushes/regulator unit is easy and inexpensive to replace, they are always a good place to start in a charging system detective job.

In the event that the brushes removed are good, then one has a decent spare.

Regards,

Bob

:>)

Take a voltmeter/multimeter and measure the voltage at the battery with the engine running and off. Running voltage should be around 14v.
--
1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

When the ignition light glows dimly with the engine running it's an indication that the brushes in the alternator are worn out. The voltage regulator assembly holding the brushes is on the back of the alternator secured by two screws - a replacement is quite inexpensive.

When you remove it you will see a 120 ohm resistor mounted beside the brushes - this was added to give the dim red light indication - not all aftermarket replacements have it, so be aware for next time if that's what you get.

My experience has been they last around 75K miles, usually only one of the brushes is completely worn out - don't know why. If you're really strapped for cash you can buy new brushes and solder them in, probably around $2.

Google Bosch voltage regulator 1 197 311 028

http://cgi.ebay.com/ebaymotors/Bosch-Voltage-Regulator-VW-Audi-Volvo-Mercedes-Saab_W0QQcmdZViewItemQQcategoryZ33577QQihZ016QQitemZ260054149695QQtcZphoto