Need a diagnosis with my '79 300D cruise control...

Here are the symptoms:

1. Car accelerates great when lifting the cruise control lever to accelerate.

2. Car immediately does not hold speed after setting CC.

3. If I set CC at 65 mph on highway and let off gas pedal, my speed slowly drops to anywhere between 30-50 mph and stays there.

4. I can cancel CC with brake pedal or CC lever.

Diagnosis: Are these symptoms of a,

A) bad CC Amp?

B) bad CC Actuator (leaky vacuum)?

Also, some of the check valves in the car's vacuum system were found to be disintegrated by oil in the vacuum lines, and I worry some oil might have done in the Actuator. Just a hunch...

I'm trying to avoid re-soldering the Amp unless necessary, but will do it if needed.

Any ideas??

Thanks!

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1979 300D WVO
Boston, MA

Sounds like the amp to me.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Using the great advice on this forum, my cruise control diagnosis and repair was straight forward - a bad cruise control amp. Commonly used re-soldering repair job did the trick. Email me if you want a pictorial.

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1979 300D WVO
Boston, MA

Would love the pictorial! I have about 10 bad amps and only one working good amp. It would be nice to have at least one other working cruise amp.....

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

me too !

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000

brandonmo177@yahoo.com

glenn
Please add me to the list for your repair in pix. minibitz@optonline.net
Mike

Hello everybody! Recently i became not so happy owner of "antique" happiness
Trying to fix the CC now. So the PO sait it malfunctiones so he had to take out the actuator wire from the throttle linkage. I wanted to see if it was mooving at all, and here is what i found:
While driving :
! the wire does not seem to be moving any direction no matter what i do with the CC lever.
! further expection shows that lever is working (tested with a tester)
! the Amp did look and sound like a working one (did all the clicking when turning on or off)

Work done:
Switch checked - seems in working order
Amp seemed working but still resoldered every connection
Going to test now.

And the questions:
! What is this black tube in the pic. It comes out of actuator and goes inside the cabin where ends not being attached to anything. also couldnt find anything to plug it into.
! Two wires (black and brown) coming to actuator is there a difference how to plug the contact, what wire where.
! What voltage or should there even be one over the wires or what resistance shoud be red on a working actuator prongs.

Sorry if there were any grammatical mistakes or unclear statements.

Damn thats quite a long post for a first post

Thanks for help and your answers.

Attached Thumbnails

 

Add me to the pictorial request list. PM sent.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html

I am having the same exact symptoms as you have described. 84 300D turbo. At first I thought that the cruise was working but that smile turned to a frown as the speed dropped off! Please ad me to your list. blackfur@att.net
Thank you,
Tracy

Please let me have that pictorial also. I have 2 amps exhibiting the exact symptoms described by Original Poster (OP). I have determined that it is the amp and not the actuator which is at fault.

If I could get a schematic I could identify the connections and/or the drive transistor(s) which I feel are causing the trouble.

Does anyone out there have a schematic???? We can fix thousands of these things. I do not have the time to reverse engineer the thing eventhough it is a relatively simple single sided printed circuit board. I will have to retire to get the time. So please help. Anybody from VDO....jump in and become a hero

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Dionysius

I have never heard of a schematic around for the cruise control or anyone mention they have one. I believe I read on a euro site the shotgun approach to recondition them. Never could find the site again when searching though. Spent quite a bit of time at it. Not a bad deal really as all the components they mentioned are pretty cheap.

Anyways from what I remember all the electroylitics and several transistors should be changed out with more reliable current types. From what I could tell that gets about ninety percent of the problems out and increasers reliability a lot. Why I wanted the site again is I believed the early ICs could stay as they where not mentioned as being a problem. There may be something to this as the true rebuilders of them out there have claimed they are much more reliable after the reconditioning by them.

Rather than cold joints although a possibility I wondered if the transient heat from resoldering connections reactivates a given component. Yes there is a market for a decent replacement in my opinion.

Now if someone told me the european giant phillips made them for vdo or mercedes in the 1980s cold solder joints are a distinct probability. For it's size and experience that company was a disaster in my opinion.

Someone, somewhere out there must have a schematic for this problem child. In front of me I have a March 1981 board and a June 1978 board. There are visible and substantial differences to the trace layouts between both. The components appear to be the same except for the fact that the 1981 version has about 10% more. There is no visible evidence of heat damage or of cold solder issues on either. The 1981 has an 8 pin DIP chip which the 1979 does not have. Component placement is quite different on both.

There are up to eight capacitors which might be replaced. The memory of the speed is held in one of these caps and is part of the trouble.

Refer to this article:

http://209.85.171.104/translate_c?hl=en&sl=de&u=http://www.sternzeit-107.de/modules.php%3Fname%3DNews%26file%3Darticle%26sid%3D248&prev=/search%3Fq%3D412.203%2Bvdo%26hl%3Den%26lr%3D%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-GBfficial_s%26sa%3DG


There is one electrolytic capacitor (0.1 microFarad, +/- 10%) and it is the same on both boards. It is placed on there as an afterthought and was not designed in when the trace artwork was laid out. Its negative lead is connected to a transistor lead and is hanging in mid air. Very curious about this since it seems to have been added to filter out a transient during board final testing. Interestingly it was not fixed in the 1981 rework. It does look suspicious and probably should be replaced, like Barry suggests, since these are higher failure rate components than anything else on there. If it failed it might have taken out the transistor which should also be replaced.

Both boards exhibit the identical failure as described by the OP. This means that the "redesign" failed to shift any inherent propensity toward this weakness.

There is too much shooting in the dark without a schematic.....so please ......see if you can help.

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Dionysius

Dionnysius, You found yet another article. The one I found was very specific. Included the driver transistors. I have been lurking and using this site for about two years. Never yet has anyone ever mentioned a schematic availability.
I wonder if the mercedes classic car centre might have them? Might be worth a try. I kind of do not think the rebuilders are going to cough any up.

It also might have been the mercedes club of america that had that article. If so it still was of german origin. I will go look. You are of course right on the money. We could easily or some of us at least repair them fairly easy with a schematic.

I still feel most units would benifit from the shotgun approach updating first. Then if it still was down just normal troubleshooting.

Again it's been quite awhile but the shotgun approach vastly improves reliability and gets the vast majority of them up and running again. Thats if I remember right. So even if you are just resoldering them it should also be done as well.

My initial suspicion has not changed. When resoldering it may not be bad solder connections. The transient heat instead has reactivated a component like an open transistor. This is not a particularily good way to go if thats what is happening. Most modern transistors are light years ahead compared to some used in europe at that time. Especially in build quality and reliability.

The design of this cruise amp may not be so bad in itself for all I know at present. It just may have been cursed with some poor quality components from day one. You should have some electronics experience when subbing the transistors to get the lead configurations right. The substitutions will probable have the emitter, collector, and base connections in a different configuration compared to those european transistors. We are still talking very cheap in parts to do what they recommend.

Barry, from my intelligence gathering, the capacitors are the culprits. This is the "memory" device on this 10 pin unit. I do not know which ones however. So I am going to just hit the 8 or so caps (16 terminals) and test. Electrolytic caps will "rejuvenate" with heat. I am not patient enough to go through 100 plus connections. I believe your theory is true since I see no evidence of cold solder joints. If this fails I will then do the transistors and retest. I also believe that the failure mode being exhibited is a "broken" memory for the setpoint speed.

With this approach we can do some isolation.

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Dionysius