I am having a strange problem with my 1990 240 wagon that might be a problem with the charging system but I'm not sure.
First of all, only the "check engine" warning light is lit when the key is in the "accessory" position.
Next I drove it last week and it only made it a half mile down the road when the engine lost power and eventually died, with a dead battery after having trickle-charged it overnight.
Sometimes the headlights will stay on after removing the key from the ignition.
The belt on the alternator is sufficiently tight.
These are all strange problems and I'm wondering if they could all be related to a bad alternator/bad voltage regulator or if I have other problems as well, such as a wiring problem.
Would appreciate any tips.
Thanks.

 

sounds like a bad dashboard ground to me. on a 240, theres a ground up against the firewall that most all the dashboard stuff is connected to.

also, those tin fuses suck, and get really flakey after a few years. I'd get a complete set of the brass or copper ones (ebay for 'mercedes w124 fuses' and there's a couple vendors selling big sets of fuses for mercedes W124 cars (late 80s/early 90s 300E etc) which are the same fuses as the 240. clean the contact fingers with a pink pencil eraser, and replace ALL those fuses to solve lots of erratic electrical gremlins.

 

Without the instrument cluster working - namely the alternator charge light (looks like a battery) - then your alternator will not charge your battery.

Have you had the instrument cluster out recently? If so, check all of the connections to make sure they were put back properly.

Is the speedometer working? If not, it could be the fuse on the back of the speedometer is blown. This fuse looks like a small plastic circuit board on the back of the speedometer. It's held on by two screws. If this is blown, then you won't get any cluster indicators.

 

First check the basics. As a quick check measure the battery voltage. A charged battery should read at least 12.6 volts. Start the car and you should read 14 volts running.If the voltage does not increase from 12.6 volts you have a fault in the charging system.You may have other problems but you must start here. Hope this helps.

 

I checked the battery voltage and it was 12.6 volts before I started the car and 11.7 after starting it.
Obviously I have at least a charging system problem.
I will have it towed to my garage since I cannot trust it for the 30 mile trip.
Thank you for all your tips.

 

If you do not drive the car daily, there is a good chance that your battery is bad. You can have a few bad cells and still get a good voltage reading from the good cells. The car can start and run on a bad battery. I drove my car around for months on a bad battery.

 

if the voltage isn't going UP after starting, his alternator isn't charging.

since he said his dashboard idiot lights aren't coming on before starting, there's a very good chance that the alternator light circuit is somehow open... without this light connected between to D+ on the alternator, the alternator can't bootstrap and charge.

(without reading the whole thread here, I think we're talking about a 240?) the circuit in question goes like this.

1) red wire from positive terminal to ignition key pin 30
2) red-black wire from ignition key pin 15 to instrument panel at Speedometer pin 3 (right angle connector)
3) (inside instrument panel) connects that +12V to alternator light
4) (inside instrument panel) alternator light to connector 31 pin 11 (round connector)
5) red wire from connector 31/11 to alternator D+

wire 5 goes through an 8 pin connector somewhere under the dash.

if ALL the lights are out, I'm wondering if that connector 31 (the round connector behind the temp/fuel gauges) isn't disconnected.

 

Turns out it was the voltage regulator that caused almost all of the problems.
The headlight issue was solved by replacing the headlight relay behind the heater controls.
Sometimes you get lucky and the fix is easy (and inexpensive).

 

each cell on a 12V battery is 2V (ok, really more like 2.1V when fully charged, giving 12.6V for the whole battery, but this is temperature and charge state dependent)... any dead cells will show up as a measurably lower voltage (assuming the cell is shorted) or 0V (if the cell is open).

a weak cell can put out ~2V at no load, resulting in a 12V total but if you put any significant load across the battery, like your headlights (maybe 10 amps for both headlights plus another amp or so for the 4 parking lights), the voltage will plummet as the weak cell can't deliver any current. a proper battery tester not only measures the voltage, it puts a heavy load across the battery and checks how fast the voltage drops, the newer digital ones can calculate from this drop how much battery capacity remains.... but the headlight test is pretty good. cheap-n-easy version. turn ignition on without starting car, note how bright dashboard lights are, and turn on headlights and see how much dimmer the lights get and whether they keep getting dimmer and dimmer or hold the initial 'dim' ... you can back this up with a volt meter to quantify things.