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'93 300CE, '84 300D, '75 450SL, '82 240D-sold,
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Discussion Starter #1
Dear Forum Members,

It has been a year since I last posted. I lament as I write this as I am going on a business trip Monday for five days and leaving my dear wife without an operable car. She'll be fine as she has friends who will give her a ride as needed. We just returned from a 900 mile trip to Austin, TX and back and the 1993 300CE performed flawlessly and the LSU Tigers left Austin with a win.

Yesterday, 09/14, the battery was dead as in 7 to 8 volts. So I charged it up and started it up. The battery light did not illuminate with the key on, but it started up just fine. Upon checking running voltage, I found 11.5 volts (and dropping) instead of the normal 13 to 14 charging voltage. I pulled the dash out a bit and checked the battery indicator light bulb (major PIA) and the bulb was fine. I got her up on the ramps and pulled the undercarriage shield and renewed the voltage regulator (@ 186K miles) and found the old brushes were very short, so I was hopeful. After getting it all back together, I started it up and still the battery light did not illuminate with the key on and while running, there was still no charging going on.

Six months ago I renewed the battery, OVP relay, three coils, six plugs and all plug wires. I am looking for the next step, which without any further advice will be a new alternator. Oh yeah, I'm already in the market for a new car for my dear wife.
 

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1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
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7,380 Posts
the dashboard battery light goes to the small screw on the 3-stud terminal X4/10 on the back side of the spring tower in front of the battery and below the grounds, and from there it goes to the D+ pin on the alternator.

when you turn the key on before you start the engine, you should be seeing some voltage on that small stud.... if you see a full battery voltage there, then the connection from that stud to the alternator D+ is bad. if you don't see any voltage, then the connection from the dashboard to the stud is bad.

here's the wiring diagram for 93-95 124 6-cyls...

A1 is the battery light, X4/10 is that terminal strip.
 

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'93 300CE, '84 300D, '75 450SL, '82 240D-sold,
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303 Posts
Discussion Starter #3
Thanks, LeftCoastGeek I'm back from my business trip to FL and before I got started troubleshooting, I wanted to charge the battery to normal voltage via an overnight, 2 amp trickle charge. Saturday, I noticed the voltage was out of the expected range while charging and when I turned on the key it made some funny noises (relays, perhaps). This morning it was only up to 11 volts so I removed the charger and disconnected the battery and it was only at 8.5 VDC. At this point I'm starting to think about ecojunk wiring in the lower wiring harness. So, I took a look at the wires over by the oil filter where they run into the plastic sheathing and they are almost all bare and they have to be making contact with each other. So at this point I am going to remove the lower wiring harness and either buy another one or rebuild this one. Does anyone know if this harness can still be bought?

The car performed flawlessly (A/C included) during our 1,500 mile trip to Spring Branch, Austin, Fredericksburg and Luckenbach, TX. This included 1.5 hours stuck in 98 degree, downtown Austin traffic getting to the LSU-TX football game and again as we tried to depart that madhouse after the game (Geaux Tigers). So, I'm thinking the excess heat and wear and tear during that week-long trip promoted further degradation of the ecojunk harness.

FYI - the upper wiring harness and the throttle body have already been changed, as well as most of the the suspension components (LCA, struts/mounts, shocks, tie rods, sway bushings and rear end mount bushings). So this is a sweet car my wife has been driving for almost three years. Wish me luck.

BTW - I do that week-long business trip to Key West, FL every ten weeks and I will not leave my dear wife without transportation again, so we are we are scheduled to test drive a mid-sized SUVs from Ford, Dodge & Chevy this week. So, I may be selling the 1993 300CE.
 

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Premium Member
1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
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7,380 Posts
well, a battery thats been discharged below 9V has been permanently damaged.

a 2 amp trickle charger will take about a week to charge a good battery thats 50% discharged. To fully charge that 50% discharged battery overnight, you want a 3-stage 'smart' charger that's around 8 amps.

the lower harness is pretty simple, I believe its just the alternator wires, the starter circuit 50 wire, and the oil pressure sensor wire. the connectors are mostly soldered. I don't know if its still available.
 

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95" W124/ E220 MasterPiece and 01" W210/ E200 KOMPRESSER
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281 Posts
I think the D+ wire to the X4/10 line would be the main issue.
You could detach the small wire connected to the generator and make/fabricate a new wire connecting to the X4/10. Its spec is 0.75MM2 and equivalent to AWG18.

When you doing that, it would be much more convenient if you could remove the battery.
This wire is embedded and covered in the generator cable set and biodegradable!! Over the years, this wire skin self-destructs and becomes pieces and needs to be changed to a non-bio wire material. I have been changing several sets already.... A simple non-bio wire can save you lots of money and bring the function back to normal.

Main wire of the generator cable set is non-bio material, but sensor wires are biodegradable...
Same setup to the starter power cable as well!!
 

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1989 W124 260E
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2,132 Posts
No battery charge light on start up means your bulb is shot . And with this bulb out you can charge up the battery with a battery chatrger all you like , but it will not charge up the cell . Replace the bulb and get that working first if the problem is sill the same when the charge light is showing then do a voltage check with meter . But lots of problem do get solved by replacement of alternator regulator ..
 

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'93 300CE, '84 300D, '75 450SL, '82 240D-sold,
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303 Posts
Discussion Starter #7
First of all, thanks to LeftCoastGeek and a few others for their response and the help on this. Since I first started this “no battery light, no charging thread, I have:

1. With a battery voltage of 12.5, checked the key-on voltage on the small stud on X4/10; it was 11.8V; which I interpret as “some voltage”. This would eliminate the alternator wire and the battery light wire.
2. Renewed the voltage regulator with a Bosch unit, imported from Germany in a Mercedes box with a three week lead time.
3. Removed the battery to alternator wires. The small wire going to the D+ terminal on the alternator is pristine; when I slide the black plastic cover back, it is covered with what appears to be white braided fiberglass. Continuity checks reveal it is solid.
4. Pulled, rebuilt and restrung the three wire harness from the plug (X27, I think) on the driver’s side connection to the violet/white starter wire, the oil pressure and oil level sensors.
5. Pulled the instrument cluster and checked the battery light bulb, it was SAT. Also pulled the cluster again and replaced bulb with a new one. I checked it with 12V and it lit up.

After all this, still no battery light or charging. It starts up just fine, but when the A/C fan & headlights are on voltage went down to 11.8 VDC. I’m thinking it needs a new alternator, is there anything else this could be?
 

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Premium Member
1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
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7,380 Posts
yeah, if the wiring is good, sounds like the alternator.

on one car, tho, everything else was fine but no charging... playing around with my volt meter, I measured a -14V difference from the alternator body and the engine ground. whooops, the alternator ground wire had corroded away under the insulation (it looked fine on the outside). new ground wire, and poof, worked!
 

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W124
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5,420 Posts
This sounded like an alternator issue 8 months ago. The "battery" lamp in the dash is a marginally effective warning light. It is widely known the lamp can illuminate on "key on" but stays out when the alternator is not charging. There is lots of theory on how it should work, but in reality it does not. If you want the peace of mind of a battery lamp, just use a "cigarette lighter"- mounted volt meter.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,467 Posts
Yes, the "battery" light only assesses the voltage output from the rotor windings via a secondary set of rectifiers. It does not assess the status of the main rectifiers nor does it assess whether the alternator is actually putting out current (for example, like in your case, if there is bad connection). And even then, it only lights in the event of total or near total failure of those windings to product voltage. It is definitely not a full diagnostic indicator. Indeed, in old British cars, I used to call it the broken belt light because that was about the only thing it could reliably detect.
 

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'93 300CE, '84 300D, '75 450SL, '82 240D-sold,
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303 Posts
Discussion Starter #11
Thanks again folks for all the sound advice. Being more mechanic than electrician, I appreciate it. A new Bosch alternator is OTW from Pelican Parts.

LeftCoastGeek, I’m reading full battery voltage between the + terminal to the body of the alternator. Also .000 ohms resistance between the alternator body and the engine ground. However, after I re-reading your post, I should have checked VOLTAGE between ground and alternator body. I’ll do that once I get back home.

[U]Floobydust[/U] and [U]tuttebenne[/U], I mostly understand the function of the battery light, but I’ve never heard it put so “technically” :) I have thought about installing a cigarette lighter volt meter, but for now I break out the multi-meter for checking charging. This is my first 124, but I’ve had many 123s and the battery light let me know when the brushes in the regulator were too worn and when the belt broke.
 

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'93 300CE, '84 300D, '75 450SL, '82 240D-sold,
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303 Posts
Discussion Starter #12
Good morning. The Bosche alternator from P.P. is installed and all tests were SAT. Thank you all for all your help with the troubleshooting and with the lower wiring harness rebuild. I couldn't, in good consciousness, sell this to my dear 2nd cousin knowing that eco-junk wiring was still there.

Unfortunately, when we were making the transaction Thursday, the drivers window got stuck on the way up. It seems the glass is hanging up on something inside the door about half-way up. So, I kept the car for a while longer to pull the door panel.
 
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