Earlier today, my 2002 ML320 cut out at a stop sign - and locked the gear shift. Then it gave me a hard time getting started. But it did, and I drove it on home. I'd been playing around with the out-of-sync door locks, and I figured I'd just messed something up.

An hour or so later, I went out, started it up, and headed out to meet some friends. I stopped for gas, cut it off, started back up, entered their address in the GPS, and headed off. Everything seemed normal.

Then, maybe 3 or 4 miles and less than 10 minutes later, it didn't want to pull away from a stop sign. It felt like I'd lost the lower gears. After I managed to make the turn there, it did get up to speed (30 or 40) but it took a while to do it. Then I noticed that it was starting to light up some idiot lights. 3 or 4 at first, then 6 or 8, and it still had no kick-down. Then it started to lose more and more power.

I managed to pull off into a school parking lot which sloped down hill from where I entered it, and basically coasted to a place where I could pull over and park (I still had power brakes and steering, but no audio, no NAV, probably no A/C - don't remember). It had now lit up pretty much every warning it has. By the time I got it parked it was dead. Totally. It locked up the shifter again and locked my key into the ignition and there we sat.

We're home now. I love the no-additional-cost roadside assistance I get for a pittance with the insurance!

But getting back to the problem, I think the car lost it's charging system and did that last-ditch effort on the battery. Yeah, it'll probably cost me a $180 battery, or at least the pro-rated piece of that, but my real question is "What went wrong? What caused this? How can I troubleshoot and fix it?"

Alternator? Voltage regulator? Something else?

 

Do you have a voltmeter and know how to use it ?

MB has some amazing wiring and electrical.

First thing is see if the battery is dead. Just because it went dead now doesn't mean it's bad.

 

Phunny man.

I don't know if it's bad or not, but it's deader'n 4 o'clock. I'll start checking it out tomorrow.

Here's the thing, though. All the advice I've seen on checking the alternator says "make sure the battery is fully charged..." and, IIRC, the guys at the parts store swapped in a good battery so they could test the charging system before telling me all I needed was a battery. That was a year or two ago, BTW, not any time recently.

But how do I do that here? It's gonna take a lonnnnnng time to charge that big sucker up with jumpers from my go-to-work beater. (Just realized I gotta jump it anyway to get the roof hatch closed. )

And I still want to know: If I'm right, and it's the charging system that has gone south, is it worthwhile to start with just replacing the voltage regulator, for $60 or so, or should I reduce my time investment by swapping out the whole alternator as the first go, for maybe $200?

 

You'd have to yank the alternator apart to replace the internal regulator. Depending on how many miles is on it it may also need brushes and the commutator may need to be cut.

Sometimes it's just easier to swap it out.

I would definitely get the car running first and check the charging voltage.

 

Your question cannot be answered without some diagnostics. Pull the batt and fully charge it then LOAD test it. If it proves out then install it and check the charging system. And yes an alternator needs some elec. to excite them so they can charge,unlike a generator.

 

Like I said, I had to get some power into the thing to get the roof hatch closed. So I went out and got the old beater running and moved into position and hooked up the cables. I also said I thought it would take a good long while to get any measurable amount of power into the oversize battery in the MB.

It didn't. Before I started charging, the ML wouldn't light up the interior lights or the instrument panel or anything. Zip. Nada. Zilch. I was amazed that I got that level of power back almost immediately. I found out when I opened the door that it had even powered the alarm system! Lots of idiot lights - 14 out of 20. That's part of what it does as self-diagnosis and reporting when you first turn it on, but then they're supposed to go off. Yesterday they came on and stayed on while I was driving.

I closed the hatch and shut down the automatic loads that were still on - wipers, lights, radio and A/C. Then I took a walk and debated whether to put a meter on it. By the time I got back to the cars, maybe 10 minutes later, the precip had gone back to a mist/drizzle. I climbed in and turned the key on. All the idiot lights again. I turned the key to start. It not only started, it turned the idiot lights off! All except one: the battery icon that indicates "improper" charging,

That did it. I grabbed my little pocket-size DVM, set it to DCV, and touched the probes to the posts or the jumper leads - whatever was handy. 12.55 VDC. Nice. I was proud of the old beater. Then I shut everything down and stowed the cables and put the work car back in its usual slot. Back at the ML I tested across the posts again. 11.21 VDC! Wow. I tried cranking it again, and it started. It's lost all memory of time and date and radio presets (I'm sure), but it did start on its own and work all of its checks and reports. And it settled down to just telling me that it wasn't seeing a charge coming in.

OK. How?

OK. Again how?

What do I need to do, beyond what I've already done, to decide that it's time to either replace the VR or swap out the alternator? Check the belt? It's running smooth and silent.

It's always easier! One other thought is that if I spend the $$ for the VR it's probably mine. Not returnable. I'm in for just eating that expense or doing the rebuild then. Right?