Weak/dying alternator? Volts dropping slowly at idle.

[efm-7]

Sitting at a red light with my headlights on, I notice that the voltage drops slowly.

While the car has throttle applied, the voltmeter reads between 13.5-15 volts, but as soon as I'm at a red light, it drops to 12.5v and by the time the light turns green, it's dropped to 11.4v - should I be worried? It seems that the alternator can't maintain battery voltage at idle.

[EricB1970]

Take it to your nearest Oreilly's and have them perform a free battery check and alternator check. I'm having the same issue and am upgrading to the Saab 115 amp alternator hopefully tonight.

[cho]

.

belts tighten enough? idle rpm ok?

[efm-7]

Idle RPM is actually on the high side.
Battery is brand new.
Belt tension, haven't checked but I'm not suspecting any slippage.

I don't know what O'Reillys is, but there are a couple of places that can check batteries such as Canadian Tire, unsure about Alternators

[efm-7]

Also, it seem like the PO bypassed the factory wiring harness and wired the alternator to the battery using what looks like 10 gauge wire... I would expect it to be at least 4 gauge..

Another thing is that my volt meter reads from the instrument cluster so there's quite some distance from the battery.

[JamesDean]

I believe 2x 10 gauge wires was the stock setup.

When I put the 115A alternator in the 420SEL I initially connected it to the distribution block.

I found that when measuring voltage at the block and directly at the battery that there was a 0.5 V drop between the two.

I moved the alternator wire so that it connected directly to the battery.

I used 0 gauge wire in that installation. Actually that's what I used in my 300D and 190E as well.

When you are idling, what load do you have on the system? Headlights? Blower? etc?

[efm-7]

Originally Posted by JamesDean

...When you are idling, what load do you have on the system? Headlights? Blower? etc?

Pretty heavy i'm im honest; yesterday I had the Lowbeams + fogs, Heater Fan on 2, and the Radio was on. Lights get brighter when I bring up the RPMs. Maybe it was the fog lights that were doing the system in.

[Waste_Gate]

Have you noticed any odd behavior from your warning lights? When my battery was about dead, my battery light would come on while driving, sometimes it would flicker or glow very dim. Look for a date stamp on your battery. Unless its an Interstate or other battery made by johnson controls, its going to need replacing after 4 years.

Another thing, I always start with the cheapest solution first so I bought a $30 voltage regulator first. That didn't help my problem so I then went with the Interstate MTP-93 (group 49) for $170 and that did the trick.

Willson
85 300CD

[cho]

Originally Posted by Waste_Gate

.... so I bought a $30 voltage regulator first.

Willson
85 300CD

thats crazy..over here VR is like 10 bucks....

.

[Waste_Gate]

Originally Posted by cho

thats crazy..over here VR is like 10 bucks....

.

I paid $28.93 in October of 2012 from that place in AZ. I guess they have to recover shipping expenses from Germany.

Is there a test you can do for the regulator? I've taken the battery out of my car after starting it and was able to have either radio or headlights but when I turned the headlights with the radio on, the radio would cut off. See what accessories you can run with the battery unhooked. I'm not great with electrical work so that could just be a waste of your time.

[efm-7]

Spare voltage regulator... THAT's what I forgot to put on my last R*ckAut* order

Only time I ever had a warning light was when I forgot to tighten the ground cable on the battery and it shook itself loose.

Again, the battery is new.

[SD Blue]

The positive side of the battery circuit was beefed up but what about the negative side? Have you checked and cleaned battery-chassis and chassis-engine grounds?

A charging problem I ran into on mine was poor grounding of the regulator to the alternator case. Cleaned the corrosion and a dab of conductive grease with no further issues in about 3 years.

I would think that with good clean connections, there shouldn't be any problem with a properly operating alternater keeping up with the original system load, without modifications.

[efm-7]

I think I found part of my problem... I don't think the alternator in my car is the right one, it might not even be for a Mercedes..

I was trying to look behind it to see what which replacement VR I should get and couldn't even find one...

The connector isn't the right one and is ghetto-rigged

Also the tensioner nut thing is not in use whatsoever...




I always had a shady feeling about the alternator... I'm amazed it works at all.

[EricB1970]

Originally Posted by efm-7

I think I found part of my problem... I don't think the alternator in my car is the right one, it might not even be for a Mercedes..

I was trying to look behind it to see what which replacement VR I should get and couldn't even find one...

The connector isn't the right one and is ghetto-rigged

Also the tensioner nut thing is not in use whatsoever...

Attachment 121419
Attachment 121420

I always had a shady feeling about the alternator... I'm amazed it works at all.

Someone did a Chevy alternator conversion. Look for late '70s Chevy pickup with a 350 if you want to replace that one. Personally I'd put the proper alternator or the Saab one on there.

[efm-7]

Also is that little section of hose supposed to be on the water pump housing?? I have a feeling that's hiding a crack