617 Idle Speed Adjust Dilemma - PeachParts Mercedes-Benz Forum

JamesDean · #1 02-23-2009, 03:46 PM

Hey everyone,

The problem: The past few years, my SD has been eating throu batteries, around 3 total in the past 5 years. A year ago a new alternator was put on the car, shortly after the battery was dead.

The shop I took my car to determined something weird about the alternator that if you just start the car and never hit the gas pedal (as if you were going to let it sit and warm up a bit) the alternator is never actually engaged, it doesn't start running. There have been a few incidents with this where I would start the car, run it for a while, turn it off and try to start it again and it would simply say no.

The solution: I was playing under the hood with a few things and stuck my voltmeter to the battery and saw that if I pressed down on the linkage leading down to the IP the idle increase marginally and voltage output of the alternator peaked to ~14v.

So the solution is to increase the idle a hair? Its currently idling just above 500rpm, perhaps 550.

Now, here is the problem with that: during the battery killing days, I came to this same conclusion, and in an attempt to increase the idle, I broke the mount for the idle control screw. After some work [an L flange attached to the full load stop with the idle adjust bolt hanging down to its stock position] , I was able to repair it and restore it. However, with my "fix" I cannot increase the idle rpm anymore...

So my question is: How can I increase idle (marginally) without being able to adjust the screw and without having to open up the IP?

Could I machine a new idle adjust screw that is longer?

Anyone have any thoughts?

Thanks
Kris

Brian Carlton · #2 02-23-2009, 03:52 PM

The SD exhibits the exact same behavior. The idle is about 700 warm. If I start it cold with no pedal, I might get the alt light. It will remain until I give it just a touch more fuel.

However, once the alternator is working, it's fine for the remainder of the day. The battery doesn't suffer from this phenomenon.

Biodiesel300TD · #3 02-23-2009, 04:03 PM

You can lengthen the linkage rod that goes to the IP. I'm not sure how much more length you'll get out of it though. You could also look into fitting idle adjust knob from the early 123. This would allow you adjust the idle on a whimsey.

I have the linkages from a early 123 with the idle adjust knob. If you're interested in trying to fit it on your's let me know.

funola · #4 02-23-2009, 04:07 PM

What's happening is the alternator needs a certain RPM to get started, i.e. for the excitation voltage going through the battery light, through the brushes, through the commutator, to do it's thing (rotor and field), before it will develop enough voltage. OK I just made that all up.

The simple solution is to blip the throttle a as Brain does and it should start charging.

#5 02-23-2009, 06:14 PM

Quote:

Originally Posted by funola

The simple solution is to blip the throttle a as Brain does and it should start charging.

But is that the best solution? I really doubt MB would design a car that drains the battery if its started and left to idle.

500rpm is way too low, it should be 750rpm. Biodiesel300TD got it though, lengthen the rod between the stop lever and the pump and it should get you the RPM you need.

kerry · #6 02-23-2009, 06:33 PM

My 409d did the same thing. I don't see a problem with blipping the accelerator to solve it.
I don't see why letting your car idle without the alternator charging would cause a no-start condition. Unless you have tons of accessories running, there's no draw on the battery from the engine running. So I don't think you can blame any battery problems you might have had on this issue.

JamesDean · #7 02-23-2009, 07:08 PM

Quote:

Originally Posted by ForcedInduction

But is that the best solution? I really doubt MB would design a car that drains the battery if its started and left to idle.

500rpm is way too low, it should be 750rpm. Biodiesel300TD got it though, lengthen the rod between the stop lever and the pump and it should get you the RPM you need.

I agree. I did in fact try to lengthen that rod, in fact I switched it out for a longer rod (the cruise control servo rod) and in either case I only succeeding in forcing rod with the black arrow in that direction and forcing the roller in the black circle to move back to that groove ,which makes pedal work difficult and ultimately doesn't help my idle..

I did notice that there is about a 1/4" worth of play when shaking the main rod (in red) back and forth in the direction shown

http://s98.photobucket.com/albums/l250/MafiaNicky/?action=view&current=effect1.jpg

I did notice that if I force the entire mechanism downward (eliminating that play) it hold at what I would approximate at 700...

As to killing the battery: this car did a lot of sitting over the past years after I got my 190E (2006), prior to that I had a few stereo components wired up to it.

funola · #8 02-23-2009, 08:07 PM

I doubt starting the car and being able to idle it for 48 hours was a design criteria for Mercedes. I think the alternator needs to be above 1000 rpm to charge the battery so raising the idle to 750 may not be the solution either. Fortunately most people start and then drive their cars.

Quote:

Originally Posted by ForcedInduction

But is that the best solution? I really doubt MB would design a car that drains the battery if its started and left to idle.

500rpm is way too low, it should be 750rpm. Biodiesel300TD got it though, lengthen the rod between the stop lever and the pump and it should get you the RPM you need.

DIESELVOLVO · #9 02-23-2009, 09:17 PM

I know adjusting the idle spped ought to be the easiest solution but in this case what about the possibility of trying to find a slightly smaller pulley for the Alt.

JamesDean · #10 02-24-2009, 05:09 PM

Quote:

Originally Posted by DIESELVOLVO

I know adjusting the idle spped ought to be the easiest solution but in this case what about the possibility of trying to find a slightly smaller pulley for the Alt.

I haven't thought about that...I was looking/thinkin about a low-cost easy work solution...

My other SD doesnt havent as much play as this one does in that long rod...

I was thinking of somehow machining a new idle speed control rod that is longer and would be able to push down the linkages... thoughts?

argus445 · #11 02-24-2009, 07:57 PM

Quote:

Originally Posted by JamesDean

I haven't thought about that...I was looking/thinkin about a low-cost easy work solution...

My other SD doesnt havent as much play as this one does in that long rod...

I was thinking of somehow machining a new idle speed control rod that is longer and would be able to push down the linkages... thoughts?

How is the battery running down if your engine is idle more 10 min it should be in the off position.

JamesDean · #12 02-24-2009, 10:08 PM

Quote:

Originally Posted by argus445

How is the battery running down if your engine is idle more 10 min it should be in the off position.

The voltage at my idle is ~12.6 Volts, the needle on the tach is 1 or 2 needle widths above 500....

When I pull the linkage up to where i think is 650-700, voltage jumps to a nice 13.6-13.9v

This is where I want it to be.