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Car 2003 S500 Base, had issue one day with car hesitate to start. Was able to start it and noticed the car interior voltage running low at 11ish volts (with interior voltage meter). So my saga to fix this car starts:
1. First charged the battery to full 13+volt and battery holds voltage fine over night (no drain)
2. Upon starting and driving , the voltage will slowly drain itself
3. So it's common next thing I look into is the alternator and VR (voltage regulator) possibly broken and not supplying charging to the battery, so I removed the original alternator to get tested. It tested out fine, but I still end up buying a new alternator just to swap it in anyways (just in case).
4. Swapped in the new alternator and still the same thing, battery will get drained. Then things gets complicated with the diagnosis.
5. Checked the prefuse box at the passenger side floor board, I thought maybe one of the 3 fuses should be blown like what many experienced. Upon crack open the cover and inspect all three fuses are fine. I checked voltage on both end and both ends has continuity, and both sides showing equal battery voltage for all 3 fuses. Which explain why all of my accessories are all working , just battery not getting charged for some reason by the alternator.
6. Did further diagnosis, start the car and measure the B+ power leads going to the prefuse box, and it's only showing 2.75V, which I think the alternator is not being told to release voltage by the car.
7. So, I think to myself could my new alternator be a DOA? I took it out , hooked up an additional thick gauge power wire extending to the engine bay so i can probe it upon starting the car. Put everything back on (again) and start the car. I probe the power leads coming out of the alternator and same thing, was reading 2.0-2.75V, so the alternator is not outputting the 14.5V correctly.
8. Okay, so at this point I told myself my new alternator is bad, I took it out (once again) and get it tested. The test passed, and printed report on the test showed the alternator output is 14.5V. So at this point I am leaning towards that the car is not energizing the alternator to have alternator to give out the proper voltage.
9. I checked all the SAM fuse boxes (left, right, rear) all is fine no blown fuses. I even checked the rear trunk pre-fuse box, and all is fine as well showing continuity on both ends of the uses and voltages are identical on both sides. So at this point I am a bit loss why isn't my alternator when installed in the car not giving out voltage while on the bench testing machine it was able to trigger and output 14.5V on the report.
So lastly where I am at now, I am looking at the passenger side pre-fuse box more carefully I noticed this box is not just a box to hold three fuses and cable, but there is this cylindrical capacity? Anyone know what this cylindrical thing do? I am at the point of checking all the components and the only thing I can think of is perhaps change out the whole pre-fuse box thinking maybe the cylinder thing in the photo is broken? I ordered a new complete pre-fuse box and hope to receive soon and swap everything out and test again.
I want to add now that I swapped the new alternator not sure what happened during the process but now my car will have a battery error light visit work shop, that's when car is still operating at 12.5-12.7V. I didn't have this message in the very beginning even when battery dipped to 10.9V there was no battery error light.
Also my "auto" headlight function is semi working, wont' turn on the head light but only the city driving smaller lights. I can manually turn on the headlight by flicking the switch to headlight position but AUTO will not automatically turn on the head lights now.
I also tested the blue wire plug going into the alternator , with the plug OFF the alternator it's registering battery voltage of 11.7V (at that time the battery was a bit drained), so there is signal going through the plug.
You should find an independent shop to read codes for you or buy the correct tool (it will pay for itself quickly). Throwing parts at these cars is not a good idea unless you have a spare parts car.
You should find an independent shop to read codes for you or buy the correct tool (it will pay for itself quickly). Throwing parts at these cars is not a good idea unless you have a spare parts car.
I have a scanner and already scanned car three times, nothing I see pertaining to battery voltage but just the normal codes that pops up when you reset the battery (bunch of random ones) which goes away after reset.
Just an update car is fixed, now getting full 14v after swapping the new prefuse box.
Finally the saga is over, in my case it wasn't the fuses but something is wrong with the regulator in the box that warrants swapping the whole box out.
Thank you for the update ! Are you now getting 12 volts where it was only showing 2.75v before ?
That is correct, alternator was not giving out any voltage at all (turned on) by the prefuse box circuit, battery warning light was on.
After swapping the entire box all is well upon start up, battery light went away, auto headlight worked, and plugged in the scanner and reset all the usual codes from disconnecting the battery now I can enjoy driving the car again.
Some information for the front prefuse box for everyone's reference:
I saw your post here and I was wondering about my middle cable because my w220 2003 s500 base is burnt just behind the connector. My car will crank but no start. Any ideas on what I should do to fix it. I haven't checked the fuses yet but my friend suggested replacing the entire cable. Thanks
Hi update. All the fuses and the regulator checked out altho now putting everything back, I'm looking at these other diagram that have two red cables attached to the bottom bolt. But I didn't nt see an extra cable. Is that a different model
Last edited by Uhmeleah89; Sep 27, 2023 at 03:24 PM.
Reason: New info
I saw your post here and I was wondering about my middle cable because my w220 2003 s500 base is burnt just behind the connector. My car will crank but no start. Any ideas on what I should do to fix it. I haven't checked the fuses yet but my friend suggested replacing the entire cable. Thanks
I would check the prefuse first, the fact your connector looked burned might have a short somewhere and blew the prefuse on the power distribution box.
A "burnt" connection is almost always due to a poor connection, not to a short circuit (which usually burns the wire, not the connection) or an overload.
A poor connection develops resistance in the connection, which develops heat, which creates more resistance, which develops more heat, which...
What a coincidence seeing this thread. I'm currently going through a battery drain issue, checked the pre-fuse (fuses good), then looked at the alternator. I was skeptical of the alternator because it's not very old, hence checking it last. The plastic cover on the voltage regulator looks a little burned. I'm thinking that or the alternator wiring is my culprit. Now I just need to find the motivation to go out there and dig into it.
I managed to get 18 miles @ 80 mph from a freshly charged battery before everything began shutting down. I monitored the voltage on the dash and watched it drop to 8.5V before the car gave up.
Luckily I made it to an off ramp and pulled into a truck stop before the car completely shut down.
The first time I witnessed the car go through a slow death, the COMAND shut down, AC shut off, windows won't work, and warning lights galore. When when the cluster blanks out, it' a matter of minutes before the car shuts down.
I forgot to follow up on this but I just had a bad alternator. I was in disbelief because it was relatively new. Closer inspection revealed a partially melted voltage regulator. Since replacing the alternator I've been trouble free.
My mechanic disassembled the prefuse box from the top and swapped just the internals . You might need to access from the bottom to remove it completely as it doesn’t seem to be removable from the top. Maybe others will know
What a coincidence seeing this thread. I'm currently going through a battery drain issue, checked the pre-fuse (fuses good), then looked at the alternator. I was skeptical of the alternator because it's not very old, hence checking it last. The plastic cover on the voltage regulator looks a little burned. I'm thinking that or the alternator wiring is my culprit. Now I just need to find the motivation to go out there and dig into it.
I managed to get 18 miles @ 80 mph from a freshly charged battery before everything began shutting down. I monitored the voltage on the dash and watched it drop to 8.5V before the car gave up.
Luckily I made it to an off ramp and pulled into a truck stop before the car completely shut down.
The first time I witnessed the car go through a slow death, the COMAND shut down, AC shut off, windows won't work, and warning lights galore. When when the cluster blanks out, it' a matter of minutes before the car shuts down.
Don't do stupid **** like this, it's not worth risking. The car *should* be fine, but Mercedes *should* have built these cars right.
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