Troubleshooting Charging System - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 06-30-2017, 10:41 PM

My '86 300SDL still has it's factory-issue 70A alternator that it left Sindelfingen with 31 years ago. I've not and still don't have issues with the battery running down, but while tinkering with something else earlier, I noticed some trouble signs.

With the engine idling and everything electrical shut off in the car, charging voltage is 13.50V. Revving the engine will not change this, 13.50V is the max charging voltage it ever produces.

Turn the A/C on and the blower on high, the charging voltage drops below 13.25V, and once the aux fan kicks on, it's to 12.75 and dropping quickly. It finally bottoms out around 12.25V at idle. Revving the engine above 800 RPM will bring it back to 13.50V, but at idle with the A/C doing its thing, down it goes. Turn the headlights on and it's into the 11's.

So what is the issue here? Regulator? Alternator going south? Alternator too small? Since it was built this way, I'd expect the alternator to be sized properly for the vehicle load.

The 13.50V maximum output certainly raises an eyebrow towards voltage regulator issues, however I'd expect there to still be problems even if the RPM were raised. Since it seems capable of returning to 13.5V with the RPM elevated, I'm inclined to think that the alternator is low on capacity as well. Is that even possible?

I don't care to waste my money on a voltage regulator for a 31 year old alternator, I'd rather just replace the entire unit at this point. My thoughts being that if the alternator output is questionable anyway, the money spent on a voltage regulator could have been applied towards a new alternator.

Are there any reliable vendors of rebuilt 70A alternators, or is this a case where it's worth it to pony up for a 90A version? I don't run anything that the car didn't come from the factory with, no subwoofers or anything else that draws any sort of power. I'm uninterested in the 115A version with clutched pulley, so please don't suggest that route.

compress ignite · #2 06-30-2017, 10:50 PM

BUT on Boschs the brushes are an integral part of the Regulator.

Reliable Sources (US WEST COAST Family of Re-builders) say Bosch Brush materiel is "Hard"
so always check the "Slip Rings" for excessive wear...
"Slip Rings" are the rotating connection between the Rotor and the Brushes
that the Brushes "Ride" on.

Rebuilding is hit or miss from all the Big Box suppliers.
"Spray and Pray" is the actual rebuilding process.
AND your Alt mos likely Vacationed temporarily in
"Old Mexico" during the process.

Me... I'd go GSXR's 143/150 Amp upgrade route (As I have)
It'd require rewiring the Alt's conductors to a Larger Diameter Wiring.

If you look at the cost of a New Mercedes V.R. as opposed to a Reman Alt,
You'll be all over the "Replace the Voltage Regulator" Bandwagon.

New Regulator is 65 bucks
Re-Man Alt from MB is 306 bucks

URRGHK!

Bosch Reman from "Car House" next to Nevada of AL65X 70Amp is $185.00
With 2 yr warranty and roadside assist.

BUT!
Per your original Inquiry You DO Need At Least a New Regulator.

Your MB Your Pleasure.

tjts1 · #3 06-30-2017, 10:53 PM

Quote:

Originally Posted by Diseasel300

I'm uninterested in the 115A version with clutched pulley, so please don't suggest that route.

If you don't like the 115a alternator you should get a 150a alternator.

Mxfrank · #4 06-30-2017, 11:14 PM

Contrary to what you may think, the regulator isn't designed to maintain constant voltage under all conditions. Alternators are very sensitive to heat, so the prime question is how long the engine had been running, and what the ambient temperature is. On a cold winter day and a light load, your alternator may put out 14.50 volts or more, but that will drop to 13.5 on a hot day. As the alternator heats up under load, the outputvoltage will drop. The attached graphs are from a Bosch technical document.

The current output of the alternator will also vary with RPM, and is lowest at idle. Thing is, low speed operation may not improve much with a "bigger" alternator". If the demand is for more amps than the alternator can deliver, you will have very low voltage. I'd actually suggest you try a couple of left brain things: disassemble your fans and clean & lube the bearings for one. Maybe install new headlight sockets as well. It's also pretty easy to clean the alternator slip rings by removing the regulator and going over them with abrasive. In the end, it will be what it will be.

In answer to your question about reliability, I recently had three rebuilts in a row fail. All seemed to fail for the same reason...the rotor wiring was torn apart by centripetal force. If you take one apart, look at the wiring for the slip rings...out of the factory, the two wires are epoxied to the iron lobes of the alternator. On my rebuilt alternators, the slip rings had been replaced and the wiring left loose. I finally resolved the issue by picking up a good used rotor from a local rebuilder. If you buy a rebuilt, I strongly urge you to open it and epoxy the slip ring wiring.

http://www.peachparts.com/shopforum/diesel-discussion/380379-alternator-mystery.html

optimusprime · #5 07-01-2017, 05:38 AM

With nothing running ,what is the reading from the battery ?

Diseasel300 · #6 07-01-2017, 09:48 AM

I would expect the factory alternator to be able to maintain battery charging under a normal operating condition such as running the A/C. I don't buy the argument that the battery discharging at idle is normal.

As for the voltage regulator - it's entire job is to maintain a set voltage. When the alternator heats up and RPM goes down, the voltage will drop as the regulator goes to full excitation voltage but the RPM is too low to support the load. That's why the setpoint is supposed to be >14V.

After the opinions posted here and doing some more late-night reading, I'm starting to lean towards cleaning up the existing alternator and replacing the regulator/brushes and seeing where it gets me. Obviously if I open it up and the slip rings are grooved all to hell it's game-over!

Diesel911 · #7 07-01-2017, 10:05 AM

Was it chaging more then 13.50 volts when you first got the car?

Back to the basics.
As the other member suggested check the Battery voltage by it self after it has sat several hours.

Detach all of the Battery cables at both ends and clean the contact areas so they make good contact. Do the same with the Ground strap.

When I put in a Battery that was not made for the Mercedes I had to change to a new longer ground cable. Once the cable was changed I found I had gained a whole extra volt of changing. So it was likely that the cable had some internal corrosion going on.

funola · #8 07-01-2017, 10:13 AM

Condition of the battery and its state of charge also affects charging voltage. What condition is your battery in? Maybe worthwhile getting it tested or try a known good battery.

Junkman · #9 07-01-2017, 02:19 PM

I went through this on the 84SD. Buy a Bosch regulator. At least 2 others didn't help and I was considering buying an alternator. Also, be 100% certain that the glow plugs aren't drawing current. Autozoo condemned the alternator then I thought to disconnect the glow wiring and the alternator miraculously started working. Oops!!

Charlie Foxtrot · #10 07-01-2017, 03:48 PM

Try a new reg/brush assembly. Be aware that Bosch & the aftermarket make multiple variations in the set voltage of the regulator. If you think you need to nudge the output 'up' you can spec a higher voltage, ie:14v. This family of alternators was/is used in a lot of stationary & industrial applications. I have a client whose 85 w126 w/om617 did lots of idling & low speed w/AC on (hours & hours). The stock alt would not keep up. Switched to a 65/70amp unit, still had issues. Switched the voltage reg to a 14.2VDC unit, problem solved. Also replace or 'sister' (parallel) the main chassis to engine ground. They deteriorate after 30+ years of road salt, grime & water aided corrosion.

BillGrissom · #11 07-01-2017, 04:13 PM

I recall seeing ~14.3 V with engine running, via a cigarette-lighter voltmeter, in my 300D's (and all my vehicles). The factory 70 A should be fine, since the only major load is the radiator fan. But, you won't usually get 70A at idle, and that is probably a rating for some higher rpm. I would try a new brush-Vreg module since cheap ($10) and easy (can do w/ alternator on car, if like my 1985). Re rebuilts, I put one from NAPA on my 1985 in 2001 and no problems since. It wasn't cheap ($120), but lifetime warranty. The original alternator seized and burned up the belts. The bearings had gotten funny where they would usually turn but sometimes jam. I didn't open it up. Before that, get your battery tested free at Autozone (in parking lot). It could be the battery has a bad cell which won't let it charge above the 13.5 V you are seeing.

SD Blue · #12 07-01-2017, 05:20 PM

Before you start swapping out parts and looking at replacements, it might be easiest to check something simple.

On mine, a slight amount of corrosion on the alternator housing caused the regulator to not have a good ground. I attributed it to the low mounting of the alternator on these cars. Cleaned and added a very small dab of conductive grease and nary a problem since.

Diseasel300 · #13 07-01-2017, 09:42 PM

For those following, I intend to take the existing alternator apart and assess the condition inside. If the slip rings aren't all chewed up, I'll install fresh bearings and a new voltage regulator and see where that gets me. If it's beyond hope, new alternator time.

This car has lived its whole life in the Texas Hill Country. There's no rust or corrosion anywhere on the chassis or on the engine, dry climate, distance from the gulf, and the lack of salt on the roads means that corrosion and rust just don't happen around here.

Had the battery tested at 2 different places today, battery is fine. When charged from jumper cables from a different vehicle, it will happily charge at 14V+ Ground straps and cables are all in excellent condition. Assuming the alternator is still mechanically sound, it looks like the regulator has finally had it after 31 years.

funola · #14 07-01-2017, 10:57 PM

Before taking it apart, try cleaning all contacts of the connector. Squeeze the female spade terminals slightly with pliers to increase contact force.

Mxfrank · #15 07-01-2017, 11:14 PM

Quote:

Originally Posted by Diseasel300

As for the voltage regulator - it's entire job is to maintain a set voltage. When the alternator heats up and RPM goes down, the voltage will drop as the regulator goes to full excitation voltage but the RPM is too low to support the load. That's why the setpoint is supposed to be >14V.

There's a lot more going on than you think. The regulator is temperature compensated. The nominal regulation voltage is 14.25-14.50V at STANDARD TEMPERATURE (25C), as can be clearly seen in the chart I posted. But the voltage target decreases .03 degrees for every ten degree increase in regulator temperature. This isn't magic. The zener diode in the regulator has a non-zero temperature coefficient, which is the cause of the voltage drop. This is done deliberately to compensate for internal battery resistance, which changes with ambient temperature.

That said, 11V is too low. This may indicate a problem with the brushes/slip rings, or a problem of excess load due to a substandard component. So checking the slip rings is a good idea, but give some attention to the fans, battery terminals, and engine ground.