Is there an easy w124 alternator upgrade?
 

So my 80a alternator is struggling and I have replaced the VR already, but it's still only putting out 12.7v and dipping as low as 12.2v sometimes when I have all accessories on. Also replaced battery last week as it was 4yrs old. This is a reman Bosch al66x and it has 105k on it now.

I'm thinking my best option would be to get a bigger alternator off another model.

Are there any bigger alternators that are simply plug and play, or are all of them going to require me to do some wiring? (I have no idea where to start if this is the case)

I want to check on any easy swaps before looking into more modifications.

Thanks,

You could drop a 120 amp alternator from a 1996 S600. It's a direct bolt up with no mods other than a thicker gauge wire. Reason I say 120 amp S600 and not the 143/150 amp S500 is because of the price difference and because you should be fine with the 120 amp. That's what I'm running on my 201

So it's just a matter of buying thicker gauge wire and recrimping the connectors?

You only need a bigger gauge wire that runs from the alternator to the + battery terminal. I sourced a thicker MB wire from the junk yard because I bought one from AutoZone, but the actual end connector ended up breaking. Make sure you keep your pulley.

You can order from AutoZone. P/n 14621
Here is a photo of the terminal where you need to connect a thicker wire.
https://drive.google.com/file/d/0BzESTTCx-sQTYlhhVDRBN2w2bjA/view?usp=drivesdk

https://drive.google.com/file/d/0BzESTTCx-sQTMG5pbHlSeHZFWjA/view?usp=drivesdk

I have had a W140 alt on the M103 powered SEL for about ten years, no issues. Mine is a 150A as it ran the same as the 120A when purchased.

If it matters, the same unit fits the 603 in my SDL. :)

Have you done any mods to the car that would account for excess power consumption? There should be nothing going on in a 124 that would require more than the stock 80A alternator. Upgrading the alternator without solving the root cause can cause more problems than it fixes.

The stock alternator will vary from 12.5 to 14.2v depending on load, battery condition, RPM and temperature. You may briefly have even lower readings under extreme conditions, like having the electric fan engaged while the headlights, a/c, electric fanand blower are on and you are idling for awhile. But if you're not getting at least 13.7 at road speeds, that suggests either a bad alternator or a drain somewhere in the system. I would suggest that you begin by cleaning the ground connections, cleaning the battery terminals, cleaning the main 12V+ bus connections, and checking the engine ground strap. Finally, remove the VR, and dress the slip rings on the rotor with an abrasive. If that doesn't improve things, try pulling fuses to see if any of the circuits are drawing unusual amounts of power.

I have a 143 or 150 amp one I pulled off an ML that's on my M103. You could get a used if you want to go cheaper but there's always a risk. Pulling it cost me like $25

If you're over 100 amps, you should be thinking about an overrun decoupling pulley. These come in various diameters, threads, and offsets, it may take some research to find one that's appropriate.

The OP should do a load/voltage wave form test on the alternator. The basic output is three-phase AC, which is converted to DC via the diode trio and rectifier bridge. If one diode opens up the alternator will still function, but with one of the three phases lost, alternator output is reduced by about a third at all rotor speeds.

An open diode can be seen on the voltage wave form output with a high resolution scope, and any decent electrical shop should be able to quicky diagnose.the problem,

If it has a open diode the least expensive solution would probably be to buy a rebuilt OE spec alternator.

Correctly diagnosing the problem and replacing the failed part or assembly with new or rebuilt OE equivalent is an easier and cheaper way to repair than a back yard re-engineering job, but it happens all the time, usually with less than satisfactory results.

Duke

I did some more diagnosis today and I'm sure its the alternator -- When the engine is cold, the alternator will put out 13.9v and 13.4-13.7v with ALL accessories on. As soon as it heats up and I've driven for 15mins, it plumets to as low as 12.3v averaging around 12.6-12.8v.

To keep things simplistic, can I simply run one additional 4ga wire directly from the alternator to the battery and leave the stock wiring alone with the 143a alternator? (or will I need to change the stock wiring anyway due to the alternator using different connectors?)

I have so many other projects going on right now that I don't have to time to DIY my own cables etc. Looking to buy some premade cables that I can just hook up and be done with it.

I don't see how running a bigger gauge wire is a hassle....You connect one end to the alternator and the other end to the positive battery terminal. It took about 10 minutes plus zip ties to keep it anchored.

How are you measuring this?

These readings aren't awful. It would be be useful if you could measure the temperature of the alternator itself. A diode could be leaking, as mentioned. It's easy enough to pull the alternator and bring it to Autozone (or whatever you have locally) for testing. And I still think you may want to check for an unusually high current drain on some circuit.

The attached graph shows temperature vs expected voltage for a typical alternator. Bear in mind that this is for an lightly loaded alternator and the temperature is measured at the alternator, not ambient temp. A big current drain would affect the reading.

The routing is not in question, its what connector, what size, must it be soldered or is crimping good enough? How do I wire it in with the OE distribution box, or do I bypass it entirely and run the OE wiring in parallel with a new connector soldered/crimped on?

Maybe I'll do that this weekend as I can take it off, get it tested and know for sure that its the alternator and not a current drain.