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Had an adventure during the last cold snap. Alternator failed. Granted, the alternator, belt, and tensioner were totally original to the 2014 van, at 347,000 miles. So I got my money's worth!

Van broke down 300 miles from home, so I had the driver uber to an Autozone and buy a new battery, and charge the old one. He got mostly home, so when we rescued him we took both batteries with us, charged them overnight, and made it the rest of the way the next day. Then I returned the new battery to Autozone. I don't know about you, but I think they should give awards for stuff like this.

Bet you are dying to know exactly how far/long you've got once your charging system fails. Answer: exactly 75 miles with the headlights on. When it fails, the battery light comes on, and once the power drops to exactly 10v the low tire pressure light comes on. From there, you've got about 5
minutes before the dash lights up like a christmas tree, and eventually it will throw itself into limp mode and limit you to reverse and 2nd gear only.

Things I learned: a brand new alternator is $400! Junkyard ones are $80, and pretty numerous. You can take your junkyard one to a parts store to test before you install it, but the parts store won't have the correct style electrical connector to test it. You could also take this opportunity to upgrade from the 180amp alternator to the 220amp for zero price difference. I assume the 220 is identical with more windings, but I'm not certain, and I don't know if doing this would work, though I expect it would.

You could also downgrade to the 160amp alternator that is common to the minivans, 200, journey, avenger. I'm convinced it would fit, and I was going to do this, but the pn for the belt is unique for the pm. This tells me that maybe the pulleys or something else is different and won't work. I'm planning on taking my old alternator to a junkyard to compare it to a minivan alternator just to find out if they are identical.

Alternator replacement is pretty easy, though I see why it books at over 2 hours. You remove the bottom splash shield, remove much of the grille, remove the radiator core support, and remove the fan assembly, and then it's really easy. You should get familiar with that stuff if you plan to wrench on the pm - once you pull it all EVERYTHING on the engine is right there, and it's easier to work on than a 60s car.

Now the fun stuff: when we had the alternator on the bench, I confirmed that the bearing was sloppy and the pulley was slipping on it's shaft. But then I found out that the new one could slip on it's shaft too. So I started researching! It's designed to do that, and THAT'S why the whole thing lasted 347k, and that's why the belt was still pristine. It's called a Decoupler Pulley, and it's really cool. Check out the following video...
 

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MMXVI - L2H2 in IN
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Good stuff

so did the pulley fail or the alternator?
 

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A decoupler pulley similar to that one (may not be the same manufacturer) was responsible for my no-alternator adventure in my 2006 Jetta TDI. The alternator was OK, but the decoupler pulley started making noise. (this was with well over 200,000 km on the car). I knew something was up because of a rattly noise, then a "bang" and the charging-failure warning lamp came on. I called a mechanic friend of mine, "I'm driving straight there, I'll be there in half an hour" and got there without issue. Opening the hood immediately revealed the problem ... the alternator shaft was bare. No pulley, no belt. Those were sitting on the lower engine cover. The pulley had disintegrated!

Helps to know what accessories are driven by the belt. I knew the coolant pump wasn't on the same belt as the alternator on the VW. The air-con is on the same belt as the alternator, but that's not mission critical ... the coolant pump is.
 

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Discussion Starter #5
Looks to me like the main alternator bearing failed, which wore the brushes. This is a guess, but the alternator failed all at once with no noises or other symptoms.
 

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Thanks, as usuall, Kip! The same type of pulls on my Sprinter alternator failed at about 125k miles.
 

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I didn't know you could rent batteries from Autozone.
 

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Thanks for information.

Some of the new Mercedes engines will have no belts at all, trying to eliminate some of these problems. Should reduce noise too.

Unfortunately it will shift mechanical problems to electrical problems to some degree, but with fewer moving parts it should be more reliable long term once they work out all the bugs.
 

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Mercedes working to bugs out - I had to laugh at that one!

Belts seem archaic, but they are actually good engineering. Simple, cheap, and sacrificial. The pentastar spins 4 devices with one belt (ps, alt, water pump, a/c). If any one device seizes, it probably won't throw the belt but it will make a horrendous noise to warn you. If any one device becomes eccentric, the belt will probably absorb and dampen the vibration. The belt will tolerate a misalignment that would break a chain. The belt is smooth and quiet compared to a chain or a gear or a shaft, and it doesnt need lubricated. The belt has an automatic tensioner. And the belt costs $25 and lasts at least 350k. Belts are good!

Sounds like Benz is trying to engineer a complicated and expensive solution to a problem that never existed.
 
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Mercedes working to bugs out - I had to laugh at that one!

Belts seem archaic, but they are actually good engineering. Simple, cheap, and sacrificial. The pentastar spins 4 devices with one belt (ps, alt, water pump, a/c). If any one device seizes, it probably won't throw the belt but it will make a horrendous noise to warn you. If any one device becomes eccentric, the belt will probably absorb and dampen the vibration. The belt will tolerate a misalignment that would break a chain. The belt is smooth and quiet compared to a chain or a gear or a shaft, and it doesnt need lubricated. The belt has an automatic tensioner. And the belt costs $25 and lasts at least 350k. Belts are good!

Sounds like Benz is trying to engineer a complicated and expensive solution to a problem that never existed.

I think you took my comment out of context in that Mercedes isn't replacing belts with chains, gears, or shafts. At least not as one would think.

1) Power steering has been shifting from hydraulic to electric power steering, hence no need for power steering pump at all.

2) Alternator is being replaced with close-coupled high-capacity unit driven off crankshaft.

3) Water pump is driven by an electric motor so speed can be controlled to what's needed for cooling, hence less energy wasteful.

4) A/C is also powered by an electric motor so speed can be controlled to meet cooling need, thereby make it more efficient. It also allows A/C to run while engine is off at red lights, etc.


I get that belts are simple and cheap -- I designed plenty of drives with them. However, over time we got away from belts and chains and used electric motors controlled by Variable Frequency Drives because while more expensive initially, advantages are too numerous to ignore.

An electric A/C compressor in particular should be of great interest to anyone who is interested in van campers. In a few years we will be able to use the van's factory A/C to air condition without having to put an A/C on roof. What's not to like about that?
 

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Discussion Starter #11
Good stuff, Chance. Yeah, I misunderstood your comment.

My understanding was that most oem's went to electric power steering because it's a cheaper way to do speed weighted assist, and lane keeping autonymous stuff. Correct?

An a/c compressor driven by an electric motor is...err.. just an air conditioner, is it not? But I could see advantages. The biggest being that you a/c module would be generic to gas, hybrid, and electric powertrains.

I don't have the engineering background to support it, but anytime you change the form of energy you lose a lot in efficiency, no? So a mechanical device is likely to be more efficient than generating electricity to power a mechanical device. Could be exceptions of course.

Either way, the decoupling pulley excited me because it makes the belt last, doesnt need any extra effort on my part, and comes with the alternator whether you want it or not. I say that now, until one fails in an otherwise working system. Then I'll rant and rave like "a solid pulley was good enough for Studebaker, so it's good enough for me!"
 

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.....cut.... . Then I'll rant and rave like "a solid pulley was good enough for Studebaker, so it's good enough for me!"

That was my first thought about all of us as we get older -- we often don't want change because we don't see the future as clearly. My Dad was a mechanic and initially hated owning a car without a carburetor. Imagine that. He understood them and could fix them on the fly. The idea of computers scared him because they could go out without warning and leave him stranded. Now his 20-year-old Honda has never left him stranded except when it overheated because he didn't maintain it properly. In time cars became more reliable, not less.


By the way, most of these "improvements" Mercedes is adding are mostly to save fuel and/or cost. Electric power steering can save up to 8% according to a report I read years ago. They are also quieter and don't leak all over your driveway like an old Chevy I had.

Power requirements for the water pump goes up drastically with engine speed (^3), so when going from 2,000 to 4,000 RPM to accelerate for a few seconds, power requirement could be eight times higher. Going from 2,000 to 6,000 RPM could be as much as 3^3, which is way more than needed for additional cooling for a few seconds if just passing, accelerating, or climbing a short hill. An electric water pump should free up much-needed horsepower when the engine is being asked to produce much more power (a lot of the time anyway).

Last point: Belts place a lot of radial load on devices and their bearings. Whether it's an electric water pump or A/C compressor, a closed-coupled motor is a lot easier on the unit than a belt drive. Plus engineers will be able to control the speed independent of engine speed.

I hope that makes sense. When going up a long hill or towing a trailer that requires a lower gear and higher engine RPMs, why would you want to run A/C compressor faster just because you're driving uphill or towing?
 

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Discussion Starter #14
I see your point, but the award wasn't for screwing the man, it was for creative solutions to expensive problems.

At a shop or a dealer, replacing the alternator is close to a $1000 job (1.5x $400, plus 2-3 hours of labor, plus belt). I'm happy to provide a description of how to find one at a junkyard, and put it in yourself for $110. And avoid a $600-800 tow from Maryland to Ohio in -5 degree weather.

More importantly, because I whipped out my meter and stopwatch, we know know exactly how far and how long you can drive a promaster without an alternator. 75 miles, 90 minutes with the lights on but no heater. That info could be extremely valuable to anyone who reads this.

I say this tongue-in-cheek; sometimes heros don't wear capes!
 
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I see your point, but the award wasn't for screwing the man, it was for creative solutions to expensive problems.

At a shop or a dealer, replacing the alternator is close to a $1000 job (1.5x $400, plus 2-3 hours of labor, plus belt). I'm happy to provide a description of how to find one at a junkyard, and put it in yourself for $110. And avoid a $600-800 tow from Maryland to Ohio in -5 degree weather.

More importantly, because I whipped out my meter and stopwatch, we know know exactly how far and how long you can drive a promaster without an alternator. 75 miles, 90 minutes with the lights on but no heater. That info could be extremely valuable to anyone who reads this.

I say this tongue-in-cheek; sometimes heros don't wear capes!


Oh you get full points for everything but returning the battery. That's just low-brow.
 

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Discussion Starter #16
Actually, I agree. And I will admit to using "parts store testing" in the past, where you return a part if it doesn't fix the problem.
 
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