GrizzzlyGoat

Hello all, this is my first time posting a thread, and I wanna say thank you for all the helpful posts from all of you guys for doing work on a W163, but I have a concern about my W203 (2004 C320 4matic). My Valeo alternator that came with the car (120 amp) wasn't charging the battery, so I ordered the voltage regulator for it, and I went to oreillys and the alternator still didn't work, so it seemed to be an internal issue, because it was only charging 11 volts. We went to the junkyard to grab a bosch alternator (120 amp) off of a 2003 SLK 320, which after we tested again at oreillys, passed.The brushes and slip ring were also perfectly fine. We throw the alternator in the car, and it works, and charges the car. We take it for a drive, and then about 15 miles later, the alt/bat visit workshop light comes back on. I had the voltage reading on the screen while we were driving and it was within 13.5-14.5 volts while the car is on, and when its off it reads 12.3-12.8. But when I checked the terminals with a multimeter with the car running i got a 16V reading. I'm not sure if the alternator is overcharging or not, because the car isn't losing power anymore so I'm not sure. The battery is an OEM Dealer battery rated 770 Amps thats 5 years old, which tested well at oreilys. What might be the issue here? I read the W203 manual and there was a column saying that the light may come on if the car was jumpstarted or given a new battery. Should the light go away soon by itself? I'm not sure. Thank you in advance.

slammer111

First, my understanding is that the Bosch and Valeo alternators are not interchangeable in a W203 (hopefully someone can verify this), even with the same engine. Guessing it's some configuration in the SAM. There are definitely differences between the alternators even though both bolt onto the engine the same way. Also did you install the alternator correctly (as opposed to rotated 180 degrees)? Not sure if this would cause your problem, but I am throwing it out there in case it means some wires aren't reinstalled correctly. Not sure about the Bosch, but the Valeo has several variants depending on chassis number. Even the pulleys have different PNs depending on which Valeo was installed. I am referring to the C230 M271 I4 engine here, but a similar scenario may apply to your V6.

Secondly, did you connect the negative terminal (ground) wire AFTER connecting the positive terminal to the battery? My understanding is that not doing so fries the front SAM and possibly other units, and is a common (rookie) mistake when working on these cars. Usually such a frying causes way more errors than just the alternator, but once again, just throwing it out there.

GrizzzlyGoat

I have seen on multiple forums of people doing Valeo to Bosch swaps/upgrades on this forum with success, so I didn't see the issue here, they are both 120 amps and came off of an M112. Secondly, I do not remember putting the ground terminal first, but is there an easy way to check if this module is fried? What are the usual symptoms? The only main issue im having is with the alt/bat light. Thank you for responding and taking you time to help me.

Russell Ormerod

Both alternators should work. Tell me where did you read this 16v?
over charging will also bring the light on.

GrizzzlyGoat

On the battery terminals with a multimeter.

GrizzzlyGoat

UPDATE:
This morning after doing some EGR service (irrelevant to this situation) I measured the battery giving out 16V while running on a multimeter, even though the computer/cluster reads 13.5-14.0 volts with the alternator light coming on. I checked the battery on a 99 ML430 W163 while running, and I got an identical reading, but there is no battery or alternator light coming on in that car. So I feel as if to just ignore the light and check the battery every other day or so to see if the voltage goes way beyond 16V.

Russell Ormerod

I think you need a new battery in your multimeter!
or just buy a decent quality one