Are w123 alternator issues blower motor related? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 03-28-2011, 08:36 AM

Hi,

Driving to work today, I used my 12V volt meter plugged into the cigarette lighter to check system status. I put a 6A charger on the battery last night to bring it up to full SOC (Ive never had a starting issue), just for fun...

Running with fan on low, headlights and fogs on, radio on, my voltage is roughly 13.3V. Take loads off and it will go up to approximately 13.9V at best conditions unloaded and engine at speed.

Stop at a light and be idling, with these loads on, the voltage can drop down to 12.3V! Awful low. However, turn the blower off and the idle voltage will pop back up over 13V.

There are strip fuse retrofits for the blower fuse underhood because it will pull excessive draw. I had a near meltdown on that blower fuse in my old 83 300D. If the blower has an issue and is pulling 20A or some fairly high amount, it is fair to say that the alternator is at pretty close to max capacity. 20A takes away roughly half of the generating output as is.

So I have no proof behind this, but I think that the blower may be the cause of many folks' low voltage conditions.

I have a new 14.1V regulator, BTW, and dont see 14.1V...

funola · #2 03-28-2011, 09:04 AM

The blower draws around 16A on high. That kind of load will drop the charging voltage on any alternator. Where were you measuring? At the cig lighter will be lower. If measured at the battery you'll probably get 14.1 v.

JHZR2 · #3 03-28-2011, 09:08 AM

12.3 v is still unacceptable in my view. There shouldn't be so much contact resistance that the cig lighter shows only 12.3V.

funola · #4 03-28-2011, 09:22 AM

12.3 v = not charging. That is normal for the 65A alternator at idle. Upgrade to the 120A alternator and it will be 13.1V at idle with blower on max.

My question on where you were measuring was in reference to "I have a new 14.1V regulator, BTW, and dont see 14.1V... "

strelnik · #5 03-28-2011, 09:28 AM

Quote:

Originally Posted by funola

12.3 v = not charging. That is normal for the 65A alternator at idle. Upgrade to the 120A alternator and it will be 13.1V at idle with blower on max.

My question on where you were measuring was in reference to "I have a new 14.1V regulator, BTW, and dont see 14.1V... "

Depending on whether you use your car in winter and how cold the winter is, the difference between the two alternators sound pretty significant.

Is the 120A alternator available as a more or less optional, but standard item, or is this an exotic transplant from another vehicle?

JHZR2 · #6 03-28-2011, 09:35 AM

Quote:

Originally Posted by funola

12.3 v = not charging. That is normal for the 65A alternator at idle. Upgrade to the 120A alternator and it will be 13.1V at idle with blower on max.

My question on where you were measuring was in reference to "I have a new 14.1V regulator, BTW, and dont see 14.1V... "

If it is normal then all is well. Inside design range of what was normal in the approach of engineering the system is OK.

Biggest issue is that 12.3V is a low state of charge for a battery... That means it is fairly well loaded to get a terminal voltage that low... One thing for the alternator to be loaded, but the battery...

I wish there was a true drop in 75 or 80a alternator...

funola · #7 03-28-2011, 10:00 AM

Quote:

Originally Posted by strelnik

Depending on whether you use your car in winter and how cold the winter is, the difference between the two alternators sound pretty significant.

Is the 120A alternator available as a more or less optional, but standard item, or is this an exotic transplant from another vehicle?

Many here use Saab alternator ftom JY. I didn't want a used unknown and went with this one

http://www.peachparts.com/shopforum/showthread.php?t=290524

Working great so far.

vstech · #8 03-28-2011, 12:59 PM

Quote:

Originally Posted by JHZR2

If it is normal then all is well. Inside design range of what was normal in the approach of engineering the system is OK.

Biggest issue is that 12.3V is a low state of charge for a battery... That means it is fairly well loaded to get a terminal voltage that low... One thing for the alternator to be loaded, but the battery...

I wish there was a true drop in 75 or 80a alternator...

similar year gasser cars have drop in 80 amp alts... the 280C and 280E have them, the 450SL have them, as do the 420 SEL's
I have several cars with drop in 80 amps in them.

Orv · #9 03-28-2011, 02:42 PM

Quote:

Originally Posted by JHZR2

issue is that 12.3V is a low state of charge for a battery... That means it is fairly well loaded to get a terminal voltage that low... One thing for the alternator to be loaded, but the battery...

Yes, but it was 12.3 V at the cigarette lighter. That leaves, for me, the question of what the voltage is at the battery. In an older car it's pretty easy for a few corroded connections to produce a volt or more of voltage drop under load between the battery and the interior wiring harness.

Measure at the battery. If it's low there, then maybe you need a bigger alternator. If not, it's time to start checking and cleaning connections.

JHZR2 · #10 03-28-2011, 02:52 PM

The issue with that logic is that I know I see 13.9V at the socket unloaded. I highly doubt the alt puts out more than 14.4V, and I know the regulator is spec at 14.1v... So my losses max are 0.2-0.5V

That only puts me at 12.5-12.8v, which is only 50-109% state of charge... Still not good...

vstech · #11 03-28-2011, 03:33 PM

except voltage drop increases with a lower input voltage. 13v input vd to 12.8 is not the same as 15v to 13.1

funola · #12 03-28-2011, 04:14 PM

In case this is not obvious, if your idle voltage is 12.5-12.8v, it is not charging, which means it is discharging if you have a lot of accessories running, to the point a run down battery if you idle it long enough with the blower and headlights on and your 1000W stereo thumping away.

I think if you put a smaller pulley on your 65A alternator, it may push it over the threashhold where it will charge at idle. Has anyone tried that? I think my 120A alternator has a smaller pulley diameter than the 65A unit it replaced.

funola · #13 03-28-2011, 04:31 PM

Quote:

Originally Posted by JHZR2

The issue with that logic is that I know I see 13.9V at the socket unloaded. I highly doubt the alt puts out more than 14.4V, and I know the regulator is spec at 14.1v... So my losses max are 0.2-0.5V

That only puts me at 12.5-12.8v, which is only 50-109% state of charge... Still not good...

If you want to measure the charging voltage that matters, measure it at the battery, PERIOD!

whunter · #14 03-28-2011, 05:15 PM

Quote:

Originally Posted by JHZR2

Hi,

Driving to work today, I used my 12V volt meter plugged into the cigarette lighter to check system status. I put a 6A charger on the battery last night to bring it up to full SOC (Ive never had a starting issue), just for fun...

Running with fan on low, headlights and fogs on, radio on, my voltage is roughly 13.3V. Take loads off and it will go up to approximately 13.9V at best conditions unloaded and engine at speed.

Stop at a light and be idling, with these loads on, the voltage can drop down to 12.3V! Awful low. However, turn the blower off and the idle voltage will pop back up over 13V.

There are strip fuse retrofits for the blower fuse under hood because it will pull excessive draw. I had a near meltdown on that blower fuse in my old 83 300D. If the blower has an issue and is pulling 20A or some fairly high amount, it is fair to say that the alternator is at pretty close to max capacity. 20A takes away roughly half of the generating output as is.

So I have no proof behind this, but I think that the blower may be the cause of many folks' low voltage conditions.

I have a new 14.1V regulator, BTW, and don't see 14.1V...

Replacing the blower brushes will reduce the amperage draw..

* Is the alternator belt tight = not slipping?
* Is the idle speed is 750 - 850?

Just went through this with a customer who kept reducing idle speed to 450 - 500 RPM..

* Any issues with the instrument cluster?

Printed circuit board repair, instrument cluster

* Has the alternator been replaced = (wrong pulley application) pulley to large = alternator RPM to slow.

.

Orv · #15 03-30-2011, 05:20 PM

Quote:

Originally Posted by JHZR2

The issue with that logic is that I know I see 13.9V at the socket unloaded. I highly doubt the alt puts out more than 14.4V, and I know the regulator is spec at 14.1v... So my losses max are 0.2-0.5V

No, because the higher the load, the more the voltage drop. So if there's a bad connection on a part of the circuit common to both the socket and the blower motor, the drop will increase dramatically with the blower on.

For example, let's say (just as an example) your voltage at the battery is 14 volts, and the normal draw with the blower off is 5 amps. Your 0.5V drop would indicate a 0.1 ohm resistance in the circuit. (5 amps * 0.1 ohm = 0.5 volts). Now you turn on the blower and draw 20A total. Your voltage drop is now 20A * 0.1 ohm = 2V, so you'd see 12 volts at the socket, even though the voltage at the battery is the same.

This is a complicated way of saying you need to measure the voltage at the battery to diagnose charging problems. Let the car idle, and measure at the battery posts with everything off, then again with the blower and headlights on, and tell us what you find.